Seven temperate rocky planets orbiting the red dwarf TRAPPIST-1

Posted in astronomy with tags , , on February 23, 2017 by Tim Kendall

pia21421The TRAPPIST-1 star, an ultracool red dwarf located 12 parsecs from the Sun, has seven Earth-size planets orbiting it. This artist’s concept appeared on the cover of the journal Nature on Feb. 23, 2017.  It shows the expected physical state of water at the probable temperature of each planet: (courtesy: NASA)

This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. In May 2016, researchers using TRAPPIST announced they had discovered three planets in the system. Assisted by several ground-based telescopes, including the European Southern Observatory’s Very Large Telescope, Spitzer confirmed the existence of two of these planets and discovered five additional ones, increasing the number of known planets in the system to seven.The new results were published Wednesday in the journal Nature, and announced at a news briefing at NASA Headquarters in Washington. Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated. Based on their densities, all of the TRAPPIST-1 planets are likely to be rocky. Further observations will not only help determine whether they are rich in water, but also possibly reveal whether any could have liquid water on their surfaces. The mass of the seventh and farthest exoplanet has not yet been estimated – scientists believe it could be an icy, “snowball-like” world, but further observations are needed. “The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Michael Gillon, lead author of the paper and the principal investigator of the TRAPPIST exoplanet survey at the University of Liege, Belgium. “It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds.”

The paper in Nature is available to subscribers: Gillon, M. et al. Nature (2017) and a summary at Nature News here. The planets have radii between about three-quarters that of Earth up to 1.13 times that of Earth, and their derived masses yield densities ranging between 0.6 and 1.2 times that of Earth (image courtesy NASA):

This chart shows, on the top row, artist conceptions of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars.

T Tauri and Hind’s variable nebula

Posted in astronomy with tags , on December 31, 2016 by Tim Kendall

hindsvariable_goldmanT Tauri stars exist often in association with OB stars, whose short lifetimes mean the lower-mass T Tauri stars must also be young. The history of how this came to be known is recounted in a 2008 paper by Scott J. Kenyon et al., here, and an excerpt is below. Image: Optical image of T Tauri and surroundings (courtesy D. Goldman, APOD). T Tau is the bright yellow star near the centre. Barnard’s nebula is visible as faint nebulosity immediately surrounding T Tau. Hind’s nebula is the bright, arc-shaped cloud that covers some of the lower-right pair of diffraction spikes from the T Tau image. Fainter nebulosity, mostly ionized gas powered by a weak ultraviolet radiation field, covers the rest of the image. Burnham (1894) and Barnard (1895) discuss the relationship between Burnham’s nebula and the more distant Hind’s and Struve’s nebulae.

In October 1852, J. R. Hind ‘noticed a very small nebulous looking object’ roughly 18′′ west of a tenth magnitude star in Taurus. Over the next 15 years, the nebula slowly faded in brightness and in 1868 vanished completely from the view of the largest telescopes. O. Struve then found a new, smaller and fainter, nebulosity roughly 4′ west of Hind’s nebula. While trying to recover these nebulae, Burnham (1890, 1894) discovered a small elliptical nebula surrounding T Tau (above image). In the 1940’s, A. Joy compiled the first lists of ‘T Tauri stars,’ irregular variable stars associated with dark or bright nebulosity, with F5-G5 spectral types and low luminosity (Joy 1945, 1949; Herbig 1962). Intense searches for other T Tauri stars revealed many stars associated with dark clouds and bright nebulae, including a class with A- type spectra (e.g. Herbig 1950a, 1960). Most of these stars were in loose groups, the T associations, or in dense clusters, the O associations (e.g., Herbig 1950b, 1957; Kholopov 1958; Dolidze & Arakelyan 1959). Because O stars have short lifetimes, both types of associations have to be composed of young stars, with ages of 10 Myr or less (Ambartsumian 1957). This realization – now 50 years old – initiated the study of star formation in dark [molecular] clouds.

Earliest stages of star formation in Perseus

Posted in astronomy with tags , on November 13, 2016 by Tim Kendall

perseuscloud_hilborn2048Stardust in Perseus: image APOD, credit & copyright: Lynn Hilborn. The Perseus star-forming clouds contain objects which are candidates to be a first hydrostatic core (FHSC), theorized by Larson (1969) to be the very first stage of star formation, consisting mostly of molecular hydrogen and intermediate in evolutionary status between dense molecular cores known as “starless” and “pre-stellar”. Its lifetime is short, less than 30,000 yr. The subject was treated by Scientific American in 2010, the year one such object was found, Per-Bolo 58 (see abstract below). The region is the subject of much ongoing work with new surveys at (sub)-mm and longer wavelengths: see new work by Storm et al. (2016) for a very recent example.

The first hydrostatic core (FHSC) represents a very early phase in the low-mass star formation process, after collapse of the parent core has begun but before a true protostar has formed. This large (few AU), cool (100 K), pressure-supported core of molecular hydrogen is expected from theory, but has yet to be observationally verified. Here, we present observations of an excellent candidate for the FHSC phase: Per-Bolo 58, a dense core in Perseus that was previously believed to be starless. The 70 μm flux of 65 mJy, from new deep Spitzer MIPS observations, is consistent with that expected for the FHSC. A low signal-to-noise detection at 24 μm leaves open the possibility that Per-Bolo 58 could be a very low luminosity protostar, however. We utilize radiative transfer models to determine the best-fitting FHSC and protostar models to the spectral energy distribution and 2.9 mm visibilities of Per-Bolo 58. The source is consistent with an FHSC with some source of lower opacity through the envelope allowing 24 μm emission to escape; a small outflow cavity and a cavity in the envelope are both possible. While we are unable to rule out the presence of a protostar, if present it would be one of the lowest luminosity protostellar objects yet observed, with an internal luminosity of ~0.01 L .

JAXA/Akatsuki imaging of the nightside of Venus

Posted in astronomy with tags on October 23, 2016 by Tim Kendall

ir2_25mar2016a_226um-rgb2colorImage courtesy JAXA. Recent newspaper coverage has drawn attention to the Japanese space mission and results have been published in the Geophysical Research Letters:

Present-day Venus is an inhospitable place with surface temperatures approaching 750 K and an atmosphere 90 times as thick as Earth’s. Billions of years ago the picture may have been very different. We have created a suite of 3-D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 Gya, present-day Venus orbital parameters, an ocean volume consistent with current theory, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a prograde rotation period slower than ~16 Earth days, despite an incident solar flux 46–70% higher than Earth receives. At its current rotation period, Venus’s climate could have remained habitable until at least 0.715 Gya. These results demonstrate the role rotation and topography play in understanding the climatic history of Venus-like exoplanets discovered in the present epoch.

Radial velocity discovery of Proxima Centauri b

Posted in astronomy with tags , on August 26, 2016 by Tim Kendall

The sky around Alpha Centauri and Proxima Centauri

“It is true. We are convinced that there is a planet orbiting Proxima now. The evidence goes as follows: a signal was spotted back in 2013 on previous surveys (UVES and HARPS). The preliminary detection was first done by Mikko Tuomi, our in-house applied mathematician and his Bayesian codes. However, the signal was not convincing as the data were really sparse and the period was ambiguous (other possible solutions at 20 and 40 days, plus a long period signal of unknown origin). We followed up Proxima in the next years but our two observing runs were 12 days, barely sufficient to secure a signal which ended up being 11.2 days. So the Pale Red Dot was designed with the sole purpose of confirming or refuting its strict periodicity, plus carefully monitor the star for activity induced variability. We got very lucky with the weather so we obtained 54 out of 60 observations. The photometric monitoring telescopes (ASH2 and several units of Las Cumbres Observatory Global Telescope network), worked flawlessly so we could see the effect of spots, flares and rotation of the star, which also had a footprint on the spectra. However, nothing indicated that spurious variability would be happening at 11.2 days. So that’s basically it: the Pale Red Dot campaign also detects the same period, and confirms that the signal has been in phase for the 16 years of accumulated observations. This is a requirement for a proper Keplerian orbit. Features like starspots are more short lived plus affect the velocities in the time-scales of the rotation of the star, which is now confirmed at ~83 days.”

Image [section, Proxima Centauri is the orange-red star in center of this image]: ESO press release. The alpha Centauri AB pair are off to the upper left of this image and of course exceedingly bright. Here is link to the Nature paper. Text: Huge and heartfelt congratulations to the Pale Red Dot Team. Also, the website contains an interview by Guillem Anglada-Escudé, who led this work, with Didier Queloz, co-discoverer of 51 Peg b back in 1995, and it is well worth reading to compare the stories of the two discoveries, as well as everything else on, for a sense of the field.

Gemini adaptive optics imaging of shocked star formation in the Large Magellanic Cloud

Posted in astronomy with tags , on July 5, 2016 by Tim Kendall


( Gemini South GeMS/GSAOI near-infrared image of the N159W field in the Large Magellanic Cloud. The image spans 1.5 arcminutes across, resolves stars to about 0.09 arcseconds, and is a composite of three filters (J, H, and Ks). Integration (exposure) time for each filter was 25 minutes. Color composite image by Travis Rector, University of Alaska Anchorage. Image credit: Gemini Observatory/AURA

An unprecedented view from the Gemini South telescope in Chile probes a swarm of young and forming stars that appear to have been shocked into existence. The group, known as N159W, is located some 158,000 light years away in the Large Magellanic Cloud (LMC), a satellite to our Milky Way Galaxy. Despite the group’s distance beyond our galaxy the extreme resolution of the image presents researchers with a fresh perspective on how prior generations of stars can trigger, or shock, the formation of a new generation of stars. “Because of the remarkable amount of detail, sensitivity, and depth in this image we identified about 100 new Young Stellar Objects, our YSOs, in this region,” says Benoit Neichel of the Laboratoire d’Astrophysique de Marseille, who worked with PhD student Anais Bernard on the research. Bernard expects to complete her PhD based upon this work in 2017.

Bernard adds that YSO’s are very red objects, often still enshrouded in a cocoon of the natal material from which they were born. “What we are seeing appears to be groups of YSOs forming at the edge of a bubble containing ionized gas expanding from an older generation of stars within the bubble.” Astronomers refer to these areas of expanding gas as HII regions due to the abundance of ionized (energized) hydrogen gas. “In a very real sense these young stars are being shocked into existence by the expanding gas from these more mature stars,” said Bernard. “Without this advanced adaptive optics technology on Gemini we wouldn’t be able push our observations out to the distance of the LMC,” said Neichel. “This gives us a unique chance to explore star formation in a different environment.” He adds that part of the challenge is differentiating between “boring field stars” and the YSOs, which, he describes as “…the gems that make this research possible.”

The research team, led by Neichel and Bernard, published their work in the journal Astronomy and Astrophysics. The team used the Gemini South telescope with the Gemini Multi-conjugate adaptive optics System (GeMS) combined with the Gemini South Adaptive Optics Imager (GSAOI). The Gemini South adaptive optics system uses a multi-conjugated configuration that samples turbulence in several layers in our atmosphere using a “constellation” of five laser guide stars. This system provides exceptionally large adaptive optics fields of view and high levels of correction to minimize the blurring effect of atmospheric distortions uniformly across the image (essentially to the theoretical, or “diffraction limit”).

Complex structure at sub-arcsecond resolution in the gas-rich debris disk HD 141569A

Posted in astronomy with tags , , , on June 6, 2016 by Tim Kendall

28396PerrotHD 141569A is a transition disk that is still gas-rich but contains significant amounts of dust. In a new paper using the SPHERE coronagraph on the ESO VLT, Perrot et al. reveal that the inner 100 astronomical units (au) contains a series of concentric ringlets at physical separation of 47 au, 64 au, and 93 au. The paper is “Discovery of concentric broken rings at sub-arcsec separations in the HD 141569A gas-rich, debris disk with VLT/SPHERE”, 2016, A&A 590, L7. From the abstract:

Transition disks correspond to a short stage between the young protoplanetary phase and older debris phase. Along this evolutionary sequence, the gas component disappears leaving room for a dust-dominated environment where already-formed planets signpost their gravitational perturbations. We endeavor to study the very inner region of the well-known and complex debris, but still gas-rich disk, around HD 141569A using the exquisite high-contrast capability of SPHERE at the VLT. Recent near-infrared (IR) images suggest a relatively depleted cavity within ~200 au, while former mid-IR data indicate the presence of dust at separations shorter than ~100 au. We obtained multi-wavelength images in the near-IR in J, H2, H3 and Ks-bands with the IRDIS camera and a 0.95–1.35 μm spectral data cube with the IFS. Data were acquired in pupil-tracking mode, thus allowing for angular differential imaging. We discovered several new structures inside 1′′, of which the most prominent is a bright ring with sharp edges (semi-major axis: 0.4′′) featuring a strong north-south brightness asymmetry. Other faint structures are also detected from 0.4′′ to 1′′ in the form of concentric ringlets and at least one spiral arm. Finally, the VISIR data at 8.6 μm suggests the presence of an additional dust population closer in. Besides, we do not detect companions more massive than 1–3 mass of Jupiter. The performance of SPHERE allows us to resolve the extended dust component, which was previously detected at thermal and visible wavelengths, into very complex patterns with strong asymmetries; the nature of these asymmetries remains to be understood. Scenarios involving shepherding by planets or dust-gas interactions will have to be tested against these observations.

Local solar space: the very nearby and coolest known brown dwarf, WISE 0855-0714

Posted in astronomy with tags , on May 19, 2016 by Tim Kendall

432836_900The above image is found by google and taken courtesy of a website located in Siberia: credit: It shows the detections of WISE 0855-0714 by the WISE and Spitzer infrared space telescopes. Each field of view is about an arcminute on a side. At top left (first red circle) the object is detected by WISE in 2010. At a third epoch (top right and third red circle) is the Spitzer detection, shown at three further epochs following (lower panels). The object is far too faint to be seen in the optical Digitized Sky Survey data. The status of this object, with such a low effective surface temperature and lying only 2.31 pc distant, makes it unique, for now. Today new Hubble photometry has been reported in arXiv marginally detecting the object at visible magnitude >26 and also giving an H-band (F160W) magnitude of 23.90±0.02. Spectroscopically, it is only possible to say it is later than Y2, and a very recent report on first spectroscopy, only possible so far in the range 4.5 – 5.2µm, shows thermal emission strikingly like Jupiter.

At the observational frontier of local solar space: RECONS and the ten parsec census

Posted in astronomy with tags , , on April 28, 2016 by Tim Kendall

RECONS-grabImage credit: A. Riedel and the RECONS group, P.I. Todd J. Henry taken from a new visualization by the RECONS consortium ( which can be seen on youtube. This is my final post (for now) on the subject of the completeness of our knowledge of stellar and planetary systems near the Sun. Stars here are plotted coloured broadly by spectral type and sized approximately by luminosity class (dwarfs or giants). The horizontal blue circle is the galactic plane crossed by the equatorial plane (grey circle) plotted at 2, 5, 10 and 25 parsecs distance. RECONS is preparing a 10 parsec census for publication and leads the field in both observation and visualization. The latest data were presented last year by Henry to the 227th meeting of the American Astronomical Society, and I reproduce the abstract in part below. In the meantime, Bihain & Scholz (2016) have investigated the projected distribution of brown dwarfs around the Sun and listed 26 brown dwarfs within 6.5 parsecs distance (as compared to 136 stars) in their Table 1 (also below):

The sample of stars, brown dwarfs, and exoplanets known within 10 parsecs of our Solar System as of January 1, 2015 is presented. All systems have trigonometric parallaxes of 100 mas or more with errors of 10 mas or less. Included in the sample are 12 systems in the southern sky added to the sample via new parallaxes from the RECONS (REsearch Consortium On Nearby Stars, effort at the CTIO/SMARTS 0.9m.The census consists of 366 stars (including the Sun and white dwarfs), 37 brown dwarfs, and 34 planets (eight in our Solar System and 26 exoplanets). Red dwarfs clearly dominate the sample, accounting for 75% of all stars known within 10 pc, while brown dwarfs are currently outnumbered 10 to 1 by stars. The completeness of the sample is assessed, indicating that additional discoveries of red, brown, and white dwarfs within 10 pc, both as primaries and secondaries, are likely, although we estimate that roughly 90% of the stellar systems have been identified. The evolution of the 10 pc sample over the past 70 years is outlined to illustrate the growth of the sample. The luminosity and mass functions are described. In contrast to many studies, once all known close multiples are resolved into individual components, the true stellar mass function rises to the end of the main sequence. With far fewer brown dwarfs than stars, different formation scenarios for objects that fuse hydrogen and those that do not are likely. Of 270 stellar primaries, 28% have companion stars, only 2% have brown dwarf companions, and 6% have detected planets. The planetary rate so far is low but climbing, while searches for brown dwarf companions to stars within 10 pc have been quite rigorous, so the brown dwarf companion rate is unlikely to rise noticeably. Overall, the solar neighborhood is dominated by small stars that are potentially orbited by many small, as yet unseen, planets.

bihain-grabBrown dwarfs near the Sun. Red dwarfs like Barnard’s star are missing. The coldest known brown dwarf WISE J0855-0714 (~250K) is third on the list following Luhman 16AB, given its WISE designation in the table. WISE J0720-0846B is the mid-T companion to Scholz’s star, a 6 pc distant M9 dwarf.

High precision radial velocity planet searches in the near-infrared

Posted in astronomy with tags , , on April 23, 2016 by Tim Kendall

Spinvelocity-vs-mass-BetaPicb-and-solarsystemSystem along with the recently measured spin rate of the planet Beta Pictoris b. Credit: ESO/I. Snellen (Leiden University). Spin rates are a different matter, but even radial velocities (RV) are notoriously hard to measure for the majority of nearby stars, which are M dwarfs, and this extends to brown dwarfs as well. Very new instrumental developments are beginning to allow RV to be measured in the near-infrared, where the spectra of these stars are less crowded with lines, allowing line widths to be measured properly, with RV and spin rates also much easier to determine. A natural extension into the planet-seeking arena follows. A new paper by Jonathan Gagné, Peter Plavchan (who is a pioneer in this field) and many co-authors has been accepted to the Astrophysical Journal and appears on arXiv:

We present the results of a precise near-infrared (NIR) radial velocity (RV) survey of 32 low-mass stars with spectral types K2-M4 using CSHELL at the NASA IRTF in the K-band with an isotopologue methane gas cell to achieve wavelength calibration and a novel iterative RV extraction method. We surveyed 14 members of young ( 25-150 Myr) moving groups, the young field star ε Eridani as well as 18 nearby (< 25 pc) low-mass stars and achieved typical single-measurement precisions of 8-15 m/sec with a long-term stability of 15-50 m/sec. We obtain the best NIR RV constraints to date on 27 targets in our sample, 19 of which were never followed by high-precision RV surveys. Our results indicate that very active stars can display long-term RV variations as low as 25-50 m/sec at 2.3125 μm, thus constraining the effect of jitter at these wavelengths. We provide the first multi-wavelength confirmation of GJ 876 bc and independently retrieve orbital parameters consistent with previous studies. We recovered RV variability for HD 160934 AB and GJ 725 AB that are consistent with their known binary orbits, and nine other targets are candidate RV variables with a statistical significance of 3-5σ. Our method combined with the new iSHELL spectrograph will yield long-term RV precisions of 5 m/sec in the NIR, which will allow the detection of Super-Earths near the habitable zone of mid-M dwarfs.

Parallaxes and proper motions of 134 southern late M, L and T dwarfs

Posted in astronomy with tags , , on April 22, 2016 by Tim Kendall

groombridge1830_720x956Stars in nearby solar space exhibit large proper motions because they are moving against a background of more distant stars. All stars have their own motion through space and most stars in the solar vicinity share approximately the velocity and direction of motion of the Sun. In the case where a parallax as well as the transverse velocity are known, the knowledge of the exact distance together with the radial velocity yields the precise motion of the star relative to the Sun. The image shows the motion of the nearby star Groombridge 1830, using two images taken a year apart. The ability to determine these basic facts is especially interesting for investigating young brown dwarfs near the Sun, as  a new paper by Weinberger et al., accepted to the Astronomical Journal, elaborates:

We report trigonometric parallaxes for 134 low mass stars and brown dwarfs, of which 38 have no previously published measurement and 79 more have improved uncertainties. Our survey targeted nearby targets, so 119 are closer than 30 pc. Of the 38 stars with new parallaxes, 14 are within 20 pc and seven are likely brown dwarfs (spectral types later than L0). These parallaxes are useful for studies of kinematics, multiplicity, and spectrophotometric calibration. Two objects with new parallaxes are confirmed as young stars with membership in nearby young moving groups: LP 870-65 in AB Doradus and G 161-71 in Argus. We also report the first parallax for the planet-hosting star GJ 3470; this allows us to refine the density of its Neptune-mass planet. One T-dwarf, 2MASS J12590470-4336243, previously thought to lie within 4 pc, is found to be at 7.8 pc, and the M-type star 2MASS J01392170-3936088 joins the ranks of nearby stars as it is found to be within 10 pc. Five stars that are over-luminous and/or too red for their spectral types are identified and deserve further study as possible young stars.

In other news the location of the still putative Planet Nine has been refined to within a twenty degree area centred on RA 2h 40m and declination -15°.

New nearby high proper motion objects from the AllWISE survey

Posted in astronomy with tags , , on April 14, 2016 by Tim Kendall

Many years of work have gone into the quest to find the nearest objects outside the Solar System. The holy grail of a proper motion search based on (thermal) infrared data where low mass stars and brown dwarfs emit most of their radiation and where extinction by dust in the galactic plane is low has been achieved by the latest AllWISE survey (Kirkpatrick et al. 2016). The third nearest L dwarf was found only recently, in the galactic plane, and was fully investigated last year by Valentin Ivanov et al. The new research, led by J Davy Kirkpatrick, has found several new nearby systems of note as well as confirming a host of known objects. From the abstract:

We use the AllWISE Data Release to continue our search for WISE-detected motions. In this paper, we publish another 27,846 motion objects, bringing the total number to 48,000 when objects found during our original AllWISE motion survey are included. We use this list, along with the lists of confirmed WISE-based motion objects from the recent papers by Luhman and by Schneider et al. and candidate motion objects from the recent paper by Gagné et al. to search for widely separated, common-proper-motion systems. We identify 1,039 such candidate systems. All 48,000 objects are further analyzed using color-color and color-mag plots to provide possible characterizations prior to spectroscopic follow-up. We present spectra of 172 of these, supplemented with new spectra of 23 comparison objects from the literature, and provide classifications and physical interpretations of interesting sources. Highlights include: (1) the identification of three G/K dwarfs that can be used as standard candles to study clumpiness and grain size in nearby molecular clouds because these objects are currently moving behind the clouds, (2) the confirmation/discovery of several M, L, and T dwarfs and one white dwarf whose spectrophotometric distance estimates place them 5-20 pc from the Sun, (3) the suggestion that the Na ‘D’ line be used as a diagnostic tool for interpreting and classifying metal-poor late-M and L dwarfs, (4) the recognition of a triple system including a carbon dwarf and late-M subdwarf, for which model fits of the late-M subdwarf (giving [Fe/H] ~ -1.0) provide a measured metallicity for the carbon star, and (5) a possible 24-pc-distant K5 dwarf + peculiar red L5 system with an apparent physical separation of 0.1 pc.

Image: In this huge image of part of the southern constellation of Norma wisps of crimson gas are illuminated by rare, massive stars that have only recently ignited and are still buried deep in thick dust clouds. These scorching-hot, very young stars are only fleeting characters on the cosmic stage and their origins remain mysterious. The vast nebula where these giants were born, known as RCW 106, is captured here in fine detail by ESO’s VLT Survey Telescope (VST), at the Paranal Observatory in Chile.

ESO/ALMA imaging of planet formation in an Earth-like orbit

Posted in astronomy with tags , , , on April 2, 2016 by Tim Kendall
( ALMA‘s best image of a protoplanetary disc to date. This picture of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are in formation in this system. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO). The star TW Hydrae is a popular target of study for astronomers because of its proximity to Earth and its status as an infant (or T Tauri) star about 10 million years old. Its distance has been recently re-calculated to be as close as 38 pc. The star itself is slightly less massive than the Sun, spectral type K8IVe (as given in an excellent recent review of young stars in nearby stellar associations here). It also has a face-on orientation as seen from Earth, giving astronomers a rare view of the complete protoplanetary disc around the star.

ALMA TW Hya central regions

This is the inner region of the TW Hydrae protoplanetary disk as imaged by ALMA. The image has a resolution of 1 AU (Astronomical Unit, the distance from the Earth to the Sun in our own Solar System). This new ALMA image reveals a gap in the disk at 1 AU, suggesting that a planet with the same orbit as Earth is forming there. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO). The paper “Ringed Substructure and a Gap at 1 AU in the Nearest Protoplanetary Disk”, by S.M. Andrews et al., appearing in the Astrophysical Journal Letters (pdf copy via ESO). These recent observations represent a huge breakthrough in direct imaging at the resolutions required and are very suggestive evidence for the existence of Earth-like planets in nearby interstellar space. From the abstract:
We present long-baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870 μm continuum emission from the nearest gas-rich protoplanetary disk, around TW Hya, that trace millimeter-sized particles down to spatial scales as small as 1 AU (20 milliarcseconds). These data reveal a series of concentric ring-shaped substructures in the form of bright zones and narrow dark annuli (1-6 AU) with modest contrasts (5-30%). We associate these features with concentrations of solids that have had their inward radial drift slowed or stopped, presumably at local gas pressure maxima. No significant non-axisymmetric structures are detected. Some of the observed features occur near temperatures that may be associated with the condensation fronts of major volatile species, but the relatively small brightness contrasts may also be a consequence of magnetized disk evolution (the so-called zonal flows). Other features, particularly a narrow dark annulus located only 1 AU from the star, could indicate interactions between the disk and young planets. These data signal that ordered substructures on ~AU scales can be common, fundamental factors in disk evolution, and that high resolution microwave imaging can help characterize them during the epoch of planet formation.

Update on the ongoing search for the proposed “Planet Nine”, from Scientific American: The article highlights the research of Fienga et al., (2016) using the Cassini spacecraft data to pinpoint the planet. The planet is likely sub-Jovian, ten Earth masses, eccentric, e ~ 0.6, distant but not that distant, ~ 700 AU, and possibly located in the region of the sky in the direction of the southern constellation of Cetus, with true anomaly 117.8°±11°. I predict that it will be found soon, and there is as good a chance of finding it by its own internal heat, in millimeter data, as by reflected light in the visible part of the spectrum.

Combined VLA/ALMA view of the forming planetary system HL Tauri

Posted in astronomy with tags , on March 26, 2016 by Tim Kendall


Image: Combined ALMA (red) and VLA image of HL Tau. Credit: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF. The paper is Carrasco-Gonzalez et al., “The VLA view of the HL Tau Disk – Disk Mass, Grain Evolution, and Early Planet Formation,” accepted by Astrophysical Journal Letters (preprint). The image above is certainly ground-breaking, and has been noted extensively elsewhere. The age of the system is thought to be less than 100,000 (105) years and HL Tau itself is quite Sun-like, spectral type K5. It was already known to host a protoplanet (the bright clump in the yellow VLA data above) about 14 times as massive as Jupiter and about twice as far from HL Tau as Neptune is from our Sun. From the abstract:

The first long-baseline ALMA campaign resolved the disk around the young star HL Tau into a number of axisymmetric bright and dark rings. Despite the very young age of HL Tau these structures have been interpreted as signatures for the presence of (proto)planets. The ALMA images triggered numerous theoretical studies based on disk-planet interactions, magnetically driven disk structures, and grain evolution. Of special interest are the inner parts of disks, where terrestrial planets are expected to form. However, the emission from these regions in HL Tau turned out to be optically thick at all ALMA wavelengths, preventing the derivation of surface density profiles and grain size distributions. Here, we present the most sensitive images of HL Tau obtained to date with the Karl G. Jansky Very Large Array at 7.0 mm wavelength with a spatial resolution comparable to the ALMA images. At this long wavelength the dust emission from HL Tau is optically thin, allowing a comprehensive study of the inner disk. We obtain a total disk dust mass of 0.001 – 0.003 Msun, depending on the assumed opacity and disk temperature. Our optically thin data also indicate fast grain growth, fragmentation, and formation of dense clumps in the inner densest parts of the disk. Our results suggest that the HL Tau disk may be actually in a very early stage of planetary formation, with planets not already formed in the gaps but in the process of future formation in the bright rings.

A new observational basis for star formation studies in Orion

Posted in astronomy with tags , , on March 8, 2016 by Tim Kendall


A new paper entitled “VISION – Vienna Survey in Orion. I. VISTA Orion A Survey” is the first looking at the closest region of massive star formation – Orion. At the moment the complete paper by S. Meingast, J. Alves, D. Mardones, et al. (2016) is available here:

The Orion nebula cluster (ONC), the nearest region of massive star formation, is embedded in the much larger Orion A molecular cloud. The ONC has been studied much more extensively than other parts of Orion A, in spite of the opportunity that this region offers to understand the processes connected with the formation of both low- and high-mass stars. Using the ESO Visible and Infrared Survey Telescope for Astronomy (VISTA), the authors have surveyed the entire Orion A molecular cloud in the J, H, and K (short) near-infrared bands, covering a total of around 18.3 square degrees, and present the most detailed and sensitive near-infrared (NIR) observations of the entire molecular cloud to date. They find about 2500 embedded objects in Orion A and confirm the existence of a recently discovered foreground population above the Galactic field. The Orion A VISTA catalog contains 799 995 sources, which increases the source counts by about an order of magnitude compared to the 2MASS survey. It provides a basis for future studies of star formation processes toward Orion.

A new route for the formation of black hole binaries and a prediction for their mass distribution

Posted in astronomy with tags , , on February 15, 2016 by Tim Kendall

hs-2007-04-a-full-jpgIn a new Oxford University press release astronomers have revealed an evolutionary route to the formation of stellar-mass black hole binaries such as the one recently seen by Advanced LIGO. While the observed mass deficit of three solar masses exactly fitted Einstein’s predictions, the actual masses, around 30 MSun each, are unexpectedly large. However, tidally induced internal mixing often occurs in massive close binary stars, and careful modelling of this process has revealed new insights into exactly how this affects the masses of the final black holes:

One complication to theories around the formation of tight pairs of compact stars stems from the fact that stars are generally thought to expand when they age. This new theory avoids such an expansion by invoking a mechanism that keeps the interior of very close and sufficiently massive stars completely chemically mixed. The reason for this is that tidal forces keep the stars in bound rotation – that is, they continue to show the same side to each other, similar to how the same side of the Moon always faces the Earth. The fast orbital motion involved in this process leads to extremely rapid rotation of the stars, which triggers internal chemical mixing inside the star. Marchant and colleagues show, through a large number of detailed evolutionary calculations, that these stars never expand and that the binaries remain compact up to the collapse phase, when the stars turn into relatively massive black holes.

An online preprint is available and the abstract (which I reproduce in part) yields the bigger picture:

With recent advances in gravitational-wave astronomy, the direct detection of gravitational waves from the merger of two stellar-mass compact objects has become a realistic prospect. Evolutionary scenarios towards mergers of various double compact objects generally invoke so-called common-envelope evolution, which is poorly understood and leads to large uncertainties in the predicted merger rates. Here we explore, as an alternative, the scenario of massive overcontact binary (MOB) evolution, which involves two very massive stars in a very tight binary that remain fully mixed as a result of their tidally induced high spin. While many of these systems merge early on, we find many MOBs that swap mass several times, but survive as a close binary until the stars collapse. The simplicity of the MOB scenario allows us to use the effcient public stellar-evolution code MESA to explore it systematically by means of detailed numerical calculations. We find that, at low metallicity, MOBs produce double-black-hole (BH+BH) systems that will merge within a Hubble time with mass-ratios close to one, in two mass ranges, about 25 to 60 MSun and > 130 M, with pair- instability supernovae (PISNe) [no remnant at all] being produced at intermediate masses.

Image: Massive stars in NGC 602 (Hubble 25th Anniversary). Updates: News from the Fermi gamma-ray observatories in orbit: LAT finds no counterpart but GBM data contains a transient event on the same date, and the discoverers themselves write on the implications of GW150914 for a large stochastic gravitational wave background from merging black hole binaries. Now there is a further paper dealing with the weak GBM transient which arrived 0.4 sec after the GW event.

VLT/MUSE spectroscopy suggests a central intermediate mass black hole in globular cluster NGC 6397

Posted in astronomy with tags , on February 5, 2016 by Tim Kendall

NGC6397-NRGBhiTwo new papers appearing today on the astro-ph preprint server have highlighted the capabilities of a new instrument at ESO/VLT, the Multi Unit Spectroscopic Explorer, MUSE. Studying the velocity dispersion of stars in the globular cluster NGC 6397, astronomers infer a central black hole of  some six hundred solar masses. They have also been able to construct the first complete spectroscopic HR diagram for a globular cluster, using nearly 19000 stellar spectra. Image: Antilhue/Chile;, 14.5″ mirror, prime focus f/9. From the abstracts: (Paper I/Paper II)

We demonstrate the high multiplex advantage of crowded field 3D spectroscopy using the new integral field spectrograph MUSE by means of a spectroscopic analysis of more than 12,000 individual stars in the globular cluster NGC 6397. The stars are deblended with a PSF (point spread function) fitting technique, using a photometric reference catalogue from HST as prior, including relative positions and brightnesses. This catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis using a full-spectrum fitting method based on a large grid of PHOENIX [theoretical model] spectra. With 18,932 spectra from 12,307 stars in NGC 6397 we have analysed the largest sample so far available for a single globular cluster. We derived a mean radial velocity of 17.84 ± 0.07 km/s and a mean metallicity of [Fe/H]= −2.120 ± 0.002, with the latter seemingly varying with temperature for stars on the RGB. We determine effective temperature and [Fe/H] from the spectra, and surface gravity from HST photometry. This is the first very comprehensive HRD for a globular cluster based on the analysis of several thousands of stellar spectra. Furthermore, two interesting objects were identified with one being a post-AGB star and the other a possible millisecond-pulsar companion.

We present a detailed analysis of the kinematics of the galactic globular cluster NGC 6397 based on more than ~18,000 spectra obtained with the novel integral field spectrograph MUSE. While NGC 6397 is often considered a core collapse cluster, our analysis suggests a flattening of the surface brightness profile at the smallest radii. Although it is among the nearest globular clusters, the low velocity dispersion of NGC 6397 of <5 km/s imposes heavy demands on the quality of the kinematical data. We show that despite its limited spectral resolution, MUSE reaches an accuracy of 1 km/s in the analysis of stellar spectra. We find slight evidence for a rotational component in the cluster and the velocity dispersion profile that we obtain shows a mild central cusp. To investigate the nature of this feature, we calculate spherical Jeans models and compare these models to our kinematical data. This comparison shows that if a constant mass-to-light ratio is assumed, the addition of an intermediate-mass black hole with a mass of 600 M_sun brings the model predictions into agreement with our data, and therefore could be at the origin of the velocity dispersion profile. We further investigate cases with varying mass-to-light ratios and find that a compact dark stellar component can also explain our observations. However, such a component would closely resemble the black hole from the constant mass-to-light ratio models as this component must be confined to the central ~5 arcsec of the cluster and must have a similar mass. Independent constraints on the distribution of stellar remnants in the cluster or kinematic measurements at the highest possible spatial resolution should be able to distinguish the two alternatives.

IDL TIFF fileHubble image of the central regions of NGC 6397 (Wikipedia). In 2006, a study using such data was published that showed a clear lower limit in the intrinsic brightness of the cluster population of faint stars at around visual magnitude 26. The authors therefore were able to deduce observationally the lower limit for the mass necessary for stars to develop a core capable of fusion: roughly 0.083 times the mass of the Sun.

Strong evidence suggests a super-Earth lies beyond Pluto

Posted in astronomy with tags , , , on January 24, 2016 by Tim Kendall


Burdick-Discovering-Planet-Nine-chart-1200Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun. A consequence of Planet Nine is that six distant Kuiper belt objects (magenta) all follow elliptical orbits that point in the same direction in physical space; they have the same argument of perihelion. That is particularly surprising because the outermost points of their orbits move around the solar system, and they travel at different rates. A second predicted consequence of Planet Nine is that a second set of confined objects should also exist. These objects are forced into positions at right angles to Planet Nine and into orbits that are perpendicular to the plane of the solar system. Five known objects (cyan, upper figure) fit this prediction precisely. The Sun is at centre in both plots. Credit: Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.]

The researchers, Konstantin Batygin and Mike Brown, discovered the planet’s existence through mathematical modeling and computer simulations but have not yet observed the object directly. “This would be a real ninth planet,” says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. “There have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our solar system that’s still out there to be found, which is pretty exciting.” Brown notes that the putative ninth planet—at 5,000 times the mass of Pluto—is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. In fact, it dominates a region larger than any of the other known planets—a fact that Brown says makes it “the most planet-y of the planets in the whole solar system.” Batygin and Brown describe their work in the current issue of the Astronomical Journal and show how Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt. “Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there,” says Batygin, an assistant professor of planetary science. “For the first time in over 150 years, there is solid evidence that the solar system’s planetary census is incomplete.” [more]

Further reading: could you live on Planet Nine? (at and also see this article at the New Yorker, from which is taken the second graphic (above).

Narrowband optical and infrared views of the Large Magellanic Cloud

Posted in astronomy with tags , , on January 16, 2016 by Tim Kendall

Cosmic dust clouds ripple across this infrared portrait of our Milky Way’s satellite galaxy, the Large Magellanic Cloud. In fact, the remarkable composite image from the Herschel Space Observatory and the Spitzer Space Telescope show that dust clouds fill this neighboring dwarf galaxy, much like dust along the plane of the Milky Way itself. The dust temperatures tend to trace star forming activity. Spitzer data in blue hues indicate warm dust heated by young stars. Herschel’s instruments contributed the image data shown in red and green, revealing dust emission from cooler and intermediate regions where star formation is just beginning or has stopped. Dominated by dust emission, the Large Magellanic Cloud’s infrared appearance is different from views in optical images. But this galaxy’s well-known Tarantula Nebula still stands out, easily seen here as the brightest region to the left of center. A mere 160,000 light-years distant, the Large Cloud of Magellan is about 30,000 light-years across. (Credits: APOD)

The LMC is seen in amazing detail in this very deep 4 frame mosaic of telescopic images, a view that reveals the Milky Way’s satellite to have the appearance of a fledgling barred spiral galaxy. The mosaic includes image data taken through a narrow filter that transmits only the red light of hydrogen atoms. Ionized by energetic starlight, a hydrogen atom emits the characteristic red H-alpha light as its single electron is recaptured and transitions to lower energy states. As a result, this mosaic seems spattered with pinkish clouds of hydrogen gas surrounding massive, young stars. Sculpted by the strong stellar winds and ultraviolet radiation, the glowing hydrogen clouds are known as H II (ionized hydrogen) regions. Composed of many overlapping clouds, the sprawling Tarantula Nebula left of center, is by far the LMC’s largest star forming region. (Credits: APOD). In other interesting news, Bradley Schaefer of Louisiana State University, an expert in the analysis of archival photographic plates, has uncovered a long-term fading of ~20% in the visual brightness of KIC 8462852 between 1890 and 1989. New Scientist reports:

Known as KIC 8462852, or Tabby’s star, it has been baffling astronomers for the past few months after a team of researchers noticed its light seemed to be dipping in brightness in bizarre ways. Proposed explanations ranged from a cloud of comets to orbiting “alien megastructures”. Now an analysis of historical observations reveals the star has been gradually dimming for over a century, leaving everyone scratching their heads as to the cause. The first signs of this space oddity came from NASA’s planet-hunting Kepler space telescope, which continually watched the star’s region of the sky between 2009 and 2013. Most planet-hosting stars show small, regular dips in light when their planets pass in front of them. But Tabby’s star dipped erratically throughout the four years, sometimes losing as much as 20 per cent of its brightness. In September, a team led by Tabetha Boyajian of Yale University, who lends the star its informal name, tried to make sense of this unusual signal. Ultimately they determined that dust from a large cloud of comets was the best explanation. A month later, the star made headlines across the globe thanks to a paper by Jason Wright of Pennsylvania State University and his colleagues, who suggested that “alien megastructures”, such as satellites designed to collect light from the star, could be responsible for the signal. Now Bradley Schaefer of Louisiana State University has discovered that the mystery goes even further. When Boyajian’s team studied the star, they looked at data from a Harvard University archive of digitally scanned photographic plates of the sky from the past century or so to see if the star had behaved unusually in the past, but found nothing. Schaefer decided this unusual star deserved a second look. He averaged the data in five-year bins to look for slow, long-term trends, and found that the star faded by about 20 per cent between 1890 and 1989. “The basic effect is small and not obvious,” he says. To confirm the fade was real, Schaefer went to Harvard to look at the original photographic plates and inspected them by eye for changes, a skill few astronomers possess these days. “Since no one uses photographic plates any more, it’s basically a lost art,” says Wright. “Schaefer is an expert at this stuff.” Schaefer saw the same century-long dimming in his manual readings, and calculated that it would require 648,000 comets, each 200 kilometres wide, to have passed by the star – completely implausible, he says. “The comet-family idea was reasonably put forth as the best of the proposals, even while acknowledging that they all were a poor lot,” he says. “But now we have a refutation of the idea, and indeed, of all published ideas. This presents some trouble for the comet hypothesis,” says Boyajian. “We need more data through continuous monitoring to figure out what is going on.” What about those alien megastructures? Schafer is unconvinced. “The alien-megastructure idea runs wrong with my new observations,” he says, as he thinks even advanced aliens wouldn’t be able to build something capable of covering a fifth of a star in just a century. What’s more, such an object should radiate light absorbed from the star as heat, but the infrared signal from Tabby’s star appears normal, he says. “I don’t know how the dimming affects the megastructure hypothesis, except that it would seem to exclude a lot of natural explanations, including comets,” says Wright. “It could be that there were just more dimming events in the past, or that astronomers were less lucky in the past and caught more dimming events in the 1980s than in the 1900s. But that seems unlikely.” There’s no doubt KIC 8462852 is behaving strangely, so something must be responsible, says Schaefer. “Either one of our refutations has some hidden loophole, or some theorist needs to come up with some other proposal.”

HST/ACS stellar archaeology of the Galactic bulge: a population of relic white dwarfs

Posted in astronomy with tags , on December 19, 2015 by Tim Kendall

hs-2015-38-c-xlarge_webSmall section of Hubble’s view of the dense collection of stars crammed together in the galactic bulge. The region surveyed is part of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field and is located 26,000 light-years away. Credits: NASA/ESA/STScI/SWEEPS Science Team.

Using data from the Hubble Space Telescope to conduct a “cosmic archaeological dig” at the very heart of our Milky Way galaxy, astronomers have uncovered the blueprints of our galaxy’s early construction phase. Peering deep into the Milky Way’s crowded central hub of stars, Hubble researchers have uncovered for the first time a population of ancient white dwarfs — smoldering remnants of once-vibrant stars that inhabited the core. Finding these relics at last can yield clues to how our galaxy was built, long before Earth and our sun formed. The observations are the deepest, most detailed study of the galaxy’s foundational city structure — its vast central bulge that lies in the middle of a pancake-shaped disk of stars, where our solar system dwells. As with any archaeological relic, the white dwarfs contain the history of a bygone era. They contain information about the stars that existed about 12 billion years ago that burned out to form the white dwarfs. As these dying embers of once-radiant stars cool, they serve as multi-billion-year-old time pieces that tell astronomers about the Milky Way’s groundbreaking years. An analysis of the Hubble data supports the idea that the Milky Way’s bulge formed first and that its stellar inhabitants were born very quickly—in less than roughly 2 billion years. The rest of the galaxy’s sprawling disk of second- and third-generation stars grew more slowly in the suburbs, encircling the central bulge like a giant sombrero.

hs-2015-38-g-xlarge_webHubble uncovered extremely faint and hot white dwarfs. This is a sample of 16 out of the 70 brightest white dwarfs spied by Hubble in the Milky Way’s bulge. The unusually dust-free location on the sky offers a unique keyhole view into the “downtown” bulge. Hubble’s Advanced Camera for Surveys (ACS) made the observations in 2004 and 2011 –  2013. Astronomers picked them out based on their faintness, blue-white color, and motion relative to our sun. The white dwarfs are blue and exactly centred in the individual images above. Credits: NASA/ESA/STScI/SWEEPS Science Team.

“It is important to observe the Milky Way’s bulge because it is the only bulge we can study in detail,” explained Annalisa Calamida of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, the science paper’s lead author. “You can see bulges in distant galaxies, but you cannot resolve the very faint stars, such as the white dwarfs. The Milky Way’s bulge includes almost a quarter of the galaxy’s stellar mass. Characterizing the properties of the bulge stars can then provide important information to understanding the formation of the entire Milky Way galaxy and that of similar, more distant galaxies.” The Hubble survey also found slightly more low-mass stars in the bulge, compared to those in the galaxy’s disk population. “This result suggests that the environment in the bulge may have been different than the one in the disk, resulting in a different star-formation mechanism,” Calamida said. The observations were so sensitive that the astronomers also used the data to pick out the feeble glow of white dwarfs. The team based its results on an analysis of 70 of the hottest white dwarfs detectable by Hubble in a small region of the bulge among tens of thousands of stars.

These stellar relics are small and extremely dense. They are about the size of Earth but 200,000 times denser. A teaspoon of white dwarf material would weigh about 15 tons. Their tiny stature makes them so dim that it would be as challenging as looking for the glow of a pocket flashlight located on the moon. Astronomers used the sharp Hubble images to separate the bulge stars from the myriad stars in the foreground of our galaxy’s disk by tracking their movements over time. The team accomplished this task by analyzing Hubble images of the same field of 240,000 stars, taken 10 years apart. The long timespan allowed the astronomers to make very precise measurements of the stars’ motion and pick out 70,000 bulge stars. The bulge’s stellar inhabitants move at a different rate than stars in the disk, allowing the astronomers to identify them.

“Comparing the positions of the stars from now and 10 years ago we were able to measure accurate motions of the stars,” said Kailash Sahu of STScI, and the study’s leader. “The motions allowed us to tell if they were disk stars, bulge stars, or halo stars.” The astronomers identified the white dwarfs by analyzing the colors of the bulge stars and comparing them with theoretical models. The extremely hot white dwarfs appear bluer relative to sun-like stars. As white dwarfs age, they become cooler and fainter, becoming difficult even for sharp-eyed Hubble to detect. “These 70 white dwarfs represent the peak of the iceberg,” Sahu said. “We estimate that the total number of white dwarfs is about 100,000 in this tiny Hubble view of the bulge. Future telescopes such as NASA’s James Webb Space Telescope will allow us to count almost all of the stars in the bulge down to the faintest ones, which today’s telescopes, even Hubble, cannot see.” The team next plans to increase their sample of white dwarfs by analyzing other portions of the SWEEPS field. This should ultimately lead to a more precise estimate of the age of the galactic bulge. They might also determine if star formation processes in the bulge billions of years ago were different from what’s seen in the younger disk of our galaxy.

The team’s results appeared in the September 1, 2015, issue of The Astrophysical Journal. A companion paper appeared in The Astrophysical Journal in 2014.

Cosmography of nearby OB associations with HIPPARCOS

Posted in astronomy with tags , , on November 29, 2015 by Tim Kendall

ESO’s VLT reveals the Carina Nebula's hidden secrets(ESA press release:) Astronomers have used modern techniques to visualise data from ESA’s Hipparcos space astrometry mission in three dimensions. The treatment of the data has offered insights into the distribution of nearby stars and uncovered new groupings of stars in the solar neighbourhood, shedding light on the origins of the stars in Orion and calling into question the existence of the Gould Belt – an iconic ring-shaped structure of stars in the Milky Way. The results show the potential of 3D visualisation of the solar neighbourhood, an approach which is of particular relevance to ESA’s Gaia mission which will map the Milky Way and Local Group in 3D with unprecedented sensitivity and accuracy. The above image is the Carina Nebula, imaged using VLT/HAWK-I and roughly centred on the extremely young (~0.3 – 0.5 Myr) cluster Trumpler 14 (right), part of the Carina OB1 association. The paper is “Cosmography of OB stars in the solar neighbourhood” H. Bouy & J. Alves, 2015 A&A, 584, 13 and I reproduce in part the abstract. The must-view visualisation is here (screenshot below).ESO-Trumpler14-cluster

We construct a 3D map of the spatial density of OB stars within 500 pc from the Sun using the Hipparcos  catalogue and find three large-scale stream-like structures that allow a new view on the solar neighbourhood. The spatial coherence of these blue streams and the monotonic age sequence over hundreds of parsecs suggest that they are made of young stars, similar to the young streams that are conspicuous in nearby spiral galaxies. The three streams are 1) the Scorpius to Canis Majoris stream, covering 350 pc and 65 Myr of star formation history; 2) the Vela stream, encompassing at least 150 pc and 25 Myr of star formation history; and 3) the Orion stream, including not only the well-known Orion OB1abcd associations, but also a large previously unreported foreground stellar group lying only 200 pc from the Sun. The map also reveals a remarkable and previously unknown nearby OB association, between the Orion stream and the Taurus molecular clouds, which might be responsible for the observed structure and star formation activity in this cloud complex. This new association also appears to be the birthplace of Betelgeuse, as indicated by the proximity and velocity of the red giant. If this is confirmed, it would solve the long-standing puzzle of the origin of Betelgeuse. The well-known nearby star-forming low-mass clouds, including the nearby T and R associations Lupus, Cha, Oph, CrA, Taurus, Vela R1, and various low-mass cometary clouds in Vela and Orion, appear in this new view of the local neighbourhood to be secondary star formation episodes that most likely were triggered by the feedback from the massive stars in the streams. We also recover well-known star clusters of various ages that are currently cruising through the solar neighbourhood. Finally, we find no evidence of an elliptical structure such as the Gould belt, a structure we suggest is a 2D projection effect, and not a physical ring.Screen Shot 2015-11-28 at 10.16.01

Chandra observes the “Cheshire Cat” group of galaxies

Posted in astronomy with tags , on November 27, 2015 by Tim Kendall

Chandra-Views-the-Cheshire-CatThis new image from NASA’s Chandra X-ray Observatory shows the group of galaxies nicknamed the “Cheshire Cat.” X-rays from Chandra show that the two “eye” galaxies and the smaller galaxies associated with them are slamming into one another in a giant galactic collision. Courtesy NASA/Chandra X-ray Observatory.

One hundred years ago this month, Albert Einstein published his theory of general relativity, one of the most important scientific achievements in the last century. A key result of Einstein’s theory is that matter warps space-time, and thus a massive object can cause an observable bending of light from a background object. Astronomers have since found many examples of this phenomenon, known as “gravitational lensing.” More than just a cosmic illusion, gravitational lensing provides astronomers with a way of probing extremely distant galaxies and groups of galaxies in ways that would otherwise be impossible even with the most powerful telescopes. The latest results from the “Cheshire Cat” group of galaxies show how manifestations of Einstein’s 100-year-old theory can lead to new discoveries today. Astronomers have given the group this name because of the smiling cat-like appearance. Some of the feline features are actually distant galaxies whose light has been stretched and bent by the large amounts of mass, most of which is in the form of dark matter detectable only through its gravitational effect, found in the system. More specifically, the mass that distorts the faraway galactic light is found surrounding the two giant “eye” galaxies and a “nose” galaxy. The multiple arcs of the circular “face” arise from gravitational lensing of four different background galaxies well behind the “eye” galaxies. The individual galaxies of the system, as well as the gravitationally lensed arcs, are seen in optical light from NASA’s Hubble Space Telescope.Each “eye” galaxy is the brightest member of its own group of galaxies and these two groups are racing toward one another at over 300,000 miles per hour. Data from NASA’s Chandra X-ray Observatory (purple) show hot gas that has been heated to millions of degrees, which is evidence that the galaxy groups are slamming into one another. Chandra’s X-ray data also reveal that the left “eye” of the Cheshire Cat group contains an actively feeding supermassive black hole at the center of the galaxy. Astronomers think the Cheshire Cat group will become what is known as a fossil group, defined as a gathering of galaxies that contains one giant elliptical galaxy and other much smaller, fainter ones. Fossil groups may represent a temporary stage that nearly all galaxy groups pass through at some point in their evolution. Therefore, astronomers are eager to better understand the properties and behavior of these groups.

A wide-field view of the Sculptor dwarf galaxy

Posted in astronomy with tags on November 14, 2015 by Tim Kendall

Wide-field image of the sky around the Sculptor Dwarf GalaxyThis image of the sky around the Sculptor Dwarf Galaxy, a close neighbour of our galaxy, the Milky Way, was created from pictures from the Digitized Sky Survey 2. Despite their close proximity, both galaxies have very distinct histories and characters. Credit: ESO/Digitized Sky Survey 2

This galaxy is much smaller and older than the Milky Way, making it a valuable subject for studying both star and galaxy formation in the early Universe. However, due to its faintness, studying this object is no easy task. The Sculptor Dwarf Galaxy — also known as the Sculptor Dwarf Elliptical or the Sculptor Dwarf Spheroidal — is a dwarf spheroidal galaxy, and is one of the fourteen (and counting) known satellite galaxies orbiting the Milky Way. These galactic hitchhikers are located close by in the Milky Way’s extensive halo, a spherical region extending far beyond our galaxy’s spiral arms. As indicated by its name, this galaxy is located in the southern constellation of Sculptor and lies about 280 000 light-years away from Earth. Despite its proximity, the galaxy was only discovered in 1937, as its stars are faint and spread thinly across the sky. Although difficult to pick out, the Sculptor Dwarf Galaxy was among the first faint dwarf galaxies found orbiting the Milky Way. The tiny galaxy’s shape intrigued astronomers at the time of its discovery, but nowadays dwarf spheroidal galaxies play a more important role in allowing astronomers to dig deeply into the Universe’s past. The Milky Way, like all large galaxies, is thought to have formed from the build-up of smaller galaxies during the early days of the Universe. If some of these small galaxies still remain today, they should now contain many extremely old stars. The Sculptor Dwarf Galaxy fits the bill as a primordial galaxy, thanks to a large number of ancient stars, visible in this image taken by the Wide Field Imager camera, installed on the 2.2-metre MPG/ESO telescope at ESO’s La Silla Observatory. Astronomers can determine the age of stars in the galaxy because their light carries the signatures of only a small quantity of heavy chemical elements. These heavy elements accumulate in galaxies with successive generations of stars. A low level of heavy elements thus indicates that the average age of the stars in the Sculptor Dwarf Galaxy is high. This quantity of old stars makes the Sculptor Dwarf Galaxy a prime target for studying the earliest history of star formation. In a recent study, astronomers combined all the data available for the galaxy to create the most accurate star formation history ever determined for a dwarf spheroidal galaxy. This analysis revealed two distinct groups of stars in the galaxy. The first, predominant group is the older population, which is lacking in heavier elements. The second, smaller population, in contrast, is rich with heavy elements: this youthful stellar population is concentrated toward the galaxy’s core.

There has been a great abundance of interesting solar system news lately so it is time for a round-up. Recent investigations of the extreme trans-Neptunian object 2012 VP113 and its orbital alignment with that of Sedna and certain other, nearer objects have sparked new speculations concerning a putative large planet in the very outer reaches of the solar system. The difference this time is that the postulated planet is thought to be rocky and too cold to have been observed by the WISE survey. Moving sunward, it is now thought that the abundance of water on Earth is primordial, rather than having been delivered by the bombardment of comets and asteroids at a somewhat later epoch:

Fragments of Earth’s earliest rock, preserved unchanged deep in the mantle until they were coughed up by volcanic eruptions, suggest that our planet has had water from the very beginning. If so, that raises the likelihood that water – one of the key prerequisites for life – could be native to other planets, too. The origin of Earth’s water has long been a mystery to planetary scientists, because the young sun would have burned hot enough to vaporise any ice that was present as dust coalesced to form our planet. Scientists therefore assumed that newborn Earth must have formed from dry material and acquired its water through bombardment by objects from more distant, icy reaches of the solar system.

Young multiple system DI Chamaeleontis observed by Hubble/ACS

Posted in astronomy with tags , , on November 6, 2015 by Tim Kendall

Smoke ring for a halo

Credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt

( Two stars shine through the centre of a ring of cascading dust in this image taken by the NASA/ESA Hubble Space Telescope. The star system is named DI Cha, and while only two stars are apparent, it is actually a quadruple system containing two sets of binary stars. As this is a relatively young star system it is surrounded by dust. The young stars are molding the dust into a wispy wrap. The host of this alluring interaction between dust and star is the Chamaeleon I dark cloud—one of three such clouds that comprise a large star-forming region known as the Chamaeleon Complex. DI Cha’s juvenility is not remarkable within this region. In fact, the entire system is among not only the youngest but also the closest collections of newly formed stars to be found and so provides an ideal target for studies of star formation.

Iron droplet clouds and hot silicates in the atmosphere of lone planetary mass object PSO J318.5-22

Posted in astronomy with tags , , , on November 2, 2015 by Tim Kendall

ps1-lonely_planet-3x3in300dpiRGBimageOnly Deep multi-colour image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. The exoplanet, or low mass brown dwarf, is extremely cold and faint, about 100 billion times fainter in optical light than the planet Venus. Most of its energy is emitted at infrared wavelengths, hence the very red colour. The image is 125 arcseconds on a side. An update on this object from New Scientist: the paper by Beth A. Biller et al., (2015) “Variability in a Young, L/T Transition Planetary-Mass Object” is accepted to the Astrophysical Journal Letters [preprint]. From the abstract:

As part of our ongoing NTT SoFI survey for variability in young free-floating planets and low mass brown dwarfs, we detect significant variability in the young, free-floating planetary mass object PSO J318.5-22, likely due to rotational modulation of inhomogeneous cloud cover. A member of the 23±3 Myr β Pic moving group, PSO J318.5-22 has Teff = 1160+3040 K and a mass estimate of 8.3±0.5 MJup for a 23±3 Myr age. PSO J318.5-22 is intermediate in mass between 51 Eri b and β Pic b, the two known exoplanet companions in the β Pic moving group. With variability amplitudes from 7-10% in JS at two separate epochs over 3-5 hour observations, we constrain the rotational period of this object to >5 hours. In KS, we marginally detect a variability trend of up to 3% over a 3 hour observation. This is the first detection of weather on an extrasolar planetary mass object. Among L dwarfs surveyed at high-photometric precision (<3%) this is the highest amplitude variability detection. Given the low surface gravity of this object, the high amplitude preliminarily suggests that such objects may be more variable than their high mass counterparts, although observations of a larger sample is necessary to confirm this. Measuring similar variability for directly imaged planetary companions is possible with instruments such as SPHERE and GPI and will provide important constraints on formation.

New Scientist gives a glimpse as to the hellish nature of the atmosphere of this bizarre object:

The starless planet, PSO J318.5-22, was discovered in the Pan-STARRS survey in 2013. At about eight times the mass of Jupiter, it’s much more like the giant planets we see orbiting other stars than the small, failed stars called brown dwarfs. That means it probably formed around a star and was somehow shot out of its orbit into lonely deep space. That also makes this planet much easier to study than those that are almost lost in the dazzle from the stars they circle. “You have to work really hard to even see them, whereas this object is just by itself,” says Beth Biller at the University of Edinburgh, UK. Biller’s team measured the planet’s brightness and found that it could vary by up to 10 per cent in just a few hours. The explanation, they say, could lie in its weather systems. “If you think about the Great Red Spot on Jupiter, it would be stormy spots like that,” Biller says. Both worlds have similar rotation periods: 10 hours for Jupiter, and between 5 and 10 hours for the lone planet. But unlike Jupiter, which has cooled from a hot start over the long life of our solar system, this planet retains a scorching surface temperature of about 1100 kelvin – maintained by internal heat since it has no star. Those conditions mean that any clouds it has should be molten, containing liquid metals where on Earth we would have water. “These are likely hot silicates and iron droplet clouds,” Biller says. “This makes Venus look like a nice place.” Caroline Morley, who models exoplanet atmospheres at the University of California, Santa Cruz, thinks the finding may mean that similar planets – whether orbiting stars or not – might show the same behaviour. “It strongly suggests that these objects should be variable [in brightness],” Morley says. “We really want to be able to look at this variability and then connect it to storm systems.” Biller’s team is already trying to tease out a similar analysis from observations of a star called HR 8799, which has planets closely resembling this lone world.

VLT/SPHERE imaging of the circumbinary disk and massive exoplanet around HD 106906

Posted in astronomy with tags , , , , on October 28, 2015 by Tim Kendall

Planet-hunting SPHERE Images First Circumbinary Planet System wi

Image and text credit: ESO, A. M. Lagrange (Université Grenoble Alpes) Observations by ESO’s planet-finding instrument, SPHERE, a high-contrast adaptive optics system installed on the third Unit Telescope of ESO’s Very Large Telescope, have revealed the edge-on disc of gas and dust present around the binary star system HD 106906AB.

HD 106906AB is a double star located in the constellation of Crux (The Southern Cross). Astronomers had long suspected that this 13 million-year-old stellar duo was encircled by a debris disc, due to the system’s youth and characteristic radiation. However, this disc had remained unseen — until now. The system’s spectacular debris disc can be seen towards the lower left area of this image. It is surrounding both stars, hence its name of circumbinary disc. The stars themselves are hidden behind a mask which prevent their glare from blinding the instrument.These stars and the disc are also accompanied by an exoplanet, visible in the upper right, named HD 106906 b, which orbits around the binary star and its disc at a distance greater than any other exoplanet discovered to date — 650 times the average Earth–Sun distance, or nearly 97 billion kilometres. HD 106906 b has a mammoth mass of up to 11 times that of Jupiter, and a scorching surface temperature of 1500 degrees Celsius. Thanks to SPHERE, HD 106906AB has become the first binary star system to have both an exoplanet and a debris disc successfully imaged, providing astronomers with a unique opportunity to study the complex process of circumbinary planet formation.

HD_106906_b_imageImage: This is a discovery image of planet HD 106906 b in thermal infrared light from MagAO/Clio2, processed to remove the bright light from its host star, HD 106906 AB. The planet is more than 20 times farther away from its star than Neptune is from our Sun. This is one of the most extreme separations known and it may be more appropriate to consider HD 106906 b a low mass brown dwarf companion. (Image: Vanessa Bailey).

Massive overcontact binary VFTS 352 in the Tarantula Nebula

Posted in astronomy with tags , , on October 21, 2015 by Tim Kendall

Location of VFTS 352 in the Large Magellanic Cloud

This image shows the location of  VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing system could be heading for a dramatic end, either merging to form a single giant star or forming a binary black hole. This view of the Tarantula star-forming region includes visible-light images from the Wide Field Imager at the MPG/ESO 2.2-metre telescope at La Silla and infrared images from the 4.1-metre infrared VISTA telescope at Paranal. Image and text courtesy ESO Science Release eso1540. The discovery at the ESO/VLT of this remarkable binary is reported in “Discovery of the massive overcontact binary VFTS 352: Evidence for enhanced internal mixing”, L.M. Almeida et al., in Astrophysical Journal, vol 812, 2, 102 (2015) (DOI: 10.1088/0004-637X/812/2/102) and I reproduce here the abstract:

The contact phase expected to precede the coalescence of two massive stars is poorly characterized due to the paucity of observational constraints. Here we report on the discovery of VFTS 352, an O-type binary in the 30 Doradus region, as the most massive and earliest spectral type overcontact system known to date. We derived the 3D geometry of the system, its orbital period  Porb = 1.1241452(4) day, components’ effective temperatures — T1 = 42 540 ± 280 K and T2 = 41 120 ± 290 K — and dynamical masses M1 = 28.63 ± 0.30 M⊙ and M2 = 28.85 ± 0.30 M⊙. Compared to single-star evolutionary models, the VFTS 352 components are too hot for their dynamical masses by about 2700 and 1100 K, respectively. These results can be explained naturally as a result of enhanced mixing, theoretically predicted to occur in very short-period tidally locked systems. The VFTS 352 components are two of the best candidates identified so far to undergo this so-called chemically homogeneous evolution. The future of VFTS 352 is uncertain. If the two stars merge, a very rapidly rotating star will be produced. Instead, if the stars continue to evolve homogeneously and keep shrinking within their Roche Lobes, coalescence can be avoided. In this case, tides may counteract the spin down by winds such that the VFTS 352 components may, at the end of their life, fulfill the requirements for long gamma-ray burst (GRB) progenitors in the collapsar scenario. Independently of whether the VFTS 352 components become GRB progenitors, this scenario makes VFTS 352 interesting as a progenitor of a black hole binary, hence as a potential gravitational wave source through black hole–black hole merger.

Light from Universe’s first galaxies observed

Posted in astronomy with tags , on October 16, 2015 by Tim Kendall

image_3222e-Early-GalaxiesThe three panels show the different components of extragalactic background light detected with the Hubble Space Telescope. Image Credit: Ketron Mitchell-Wynne, UC Irvine. Text: Scientific American.

In between the thousands of bright galaxies that populate many Hubble Space Telescope photos of the distant cosmos are empty dark spots—tantalizing patches that could be chock-full of more galaxies if only we could see them. Now, astronomers have taken another look at those empty patches and spotted faint light streaming from stars formed only 500 million years after the Big Bang. The new results (pdf) suggest this light came from some of the first galaxies ever formed, which could be 10 times more numerous than previously thought.

This so-called “extragalactic background light” likely dates from roughly 250 million years after the Big Bang. Shortly after the birth of the universe, space was filled with a hot, dense fog of ionized gas. But over hundreds of thousands of years, the gas expanded and cooled, allowing giant clouds of hydrogen and helium to collapse and form the first stars. Ever since these stars first ignited, their light—and all the light from successive generations of stars—has been filling the universe, creating a pervasive glow throughout the darkest depths of space.

Although the extragalactic background radiation has proved arduous to conclusively detect, the light seen in the Hubble photos looks to be the most distant background light yet. Using data from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) and the Great Observatories Origins Deep Survey (GOODS), the team was able to separate out the light from later stars and galaxies, isolating the contribution from the first stars.

Graduate Student Ketron Mitchell-Wynne from the University of California, Irvine, and his colleagues looked for fluctuations in the intensity of the seemingly dark and empty pixels in Hubble photos taken from 2002 to 2012 to measure the elusive first light. The fluctuations helped them statistically determine that they were seeing a faint signal associated with the first stars and not simply noise. They then subtracted any light added by the stars within our galaxy, light added by the nearby galaxies, and even light added by the rogue stars that have been torn from their host galaxies and now occupy intergalactic space, until they were left with light solely from the early universe. [continues]

Gemini views star formation in the Herbig-Haro object HH24

Posted in astronomy with tags , on October 2, 2015 by Tim Kendall

Gemini HH24Image and text credits: The HH 24 jet complex emanates from a dense cloud core that hosts a small multiple protostellar system known as SSV63. The nebulous star to the south is the visible T Tauri star SSV59. Color image based on the following filters with composite image color assignments in parenthesis: g (blue), r (cyan), I (orange), hydrogen-alpha (red), sulphur II (blue) images obtained with GMOS on Gemini North in 0.5 arcsecond seeing, and NIRI. Field of view is 4.2×5.1 arcminutes, orientation: north up, east left. Credit: Gemini Observatory/AURA/B. Reipurth, C. Aspin, T. Rector

A new Gemini Observatory image reveals the remarkable “fireworks” that accompany the birth of stars. The image captures in unprecedented clarity the fascinating structures of a gas jet complex emanating from a stellar nursery at supersonic speeds. The striking new image hints at the dynamic (and messy) process of star birth. Researchers believe they have also found a collection of runaway (orphan) stars that result from all this activity. Gemini Observatory has released one of the most detailed images ever obtained of emerging gas jets streaming from a region of newborn stars. The region, known as the Herbig-Haro 24 (HH 24) Complex, contains no less than six jets streaming from a small cluster of embedded in a molecular cloud in the direction of the constellation of Orion.

“This is the highest concentration of jets known anywhere,” says Principal Investigator Bo Reipurth of the University of Hawaii’s Institute for Astronomy (IfA), who adds, “We also think the very dynamic environment causes some of the lowest mass stars in the area to be expelled, and our Gemini data are supporting that idea.” Reipurth along with co-researcher, Colin Aspin, also at the IfA, are using the Gemini North data from the Gemini Multi-Object Spectrograph (GMOS), as well as the Gemini Near-Infrared Imager, to study the region which was discovered in 1963 by George Herbig and Len Kuhi. Located in the Orion B cloud, at a distance of about 400 parsecs, or about 1,300 light-years from our Solar System, this region is rich in young stars and has been extensively studied in all types of light, from radio waves to X-rays.

“The Gemini data are the best ever obtained from the ground of this remarkable jet complex and are showing us striking new detail,” says Aspin. Reipurth and Aspin add that they are particularly interested in the fine structure and “excitation distribution” of these jets. “One jet is highly disturbed, suggesting that the source may be a close binary whose orbit perturbs the jet body,” says Reipurth. The researchers report that the jet complex emanates from what is called a Class I protostar, SSV63, which high-resolution infrared imaging reveals to have at least five components. More sources are found in this region, but only at longer, submillimeter wavelengths of light, suggesting that there are even younger, and more deeply embedded sources in the region. All of these embedded sources are located within the dense molecular cloud core.

A search for dim optical and infrared young stars has revealed several faint optical stars located well outside the star-forming core. In particular, a halo of five faint Hydrogen-alpha emission stars (which emit large amounts of red light) has been found with GMOS surrounding the HH 24 Complex well outside the dense cloud core. Gemini spectroscopy of the hydrogen alpha emission stars show that they are early or mid-M dwarfs (very low-mass stars), with at least one of which being a borderline brown dwarf. The presence of these five very low-mass stars well outside the star-forming cloud core is puzzling, because in their present location the gas is far too tenuous for the stars to have formed there. Instead they are likely orphaned protostars ejected shortly after birth from the nearby star-forming core. Such ejections occur when many stars are formed closely together within the same cloud core. The crowded stars start moving around each other in a chaotic dance, ultimately leading to the ejection of the smallest ones. A consequence of such ejections is that pairs of the remaining stars bind together gravitationally. The dense gas that surrounds the newly formed pairs brakes their motion, so they gradually spiral together to form tight binary systems with highly eccentric orbits. Each time the two components are closest in their orbits they disturb each other, leading to accretion of gas, and an outflow event that we see as supersonic jets. The many knots in the jets thus represent a series of such perturbations.

A new T2 dwarf within 15 pc, WISE J2121-6239

Posted in astronomy with tags , on September 28, 2015 by Tim Kendall

2M2121Images: NASA/IPAC Infrared Science Archive. These near-infrared (2MASS) images clearly show the object, now identified as WISE J212100.87-623921.6. It was discovered as part of a new study of high proper motion sources from the WISE survey, published today (preprint) and accepted to MNRAS. It is interesting to note that the object is clearly defined in 2MASS with magnitudes J = 15.43, H = 14.54 and K = 14.27, ± 0.05 – 0.07 (2MASS PSC). Both because it is faint – there is no optical counterpart in the SuperCosmos catalogue, preventing a proper motion measurement over a suitably long timeline – and because T dwarfs have quite blue near-infrared colours, it was missed in the previous epoch of surveys. (Moreover, the object is in a region of the sky not observed by the Sloan Digital Sky Survey). Despite the fact that the T dwarfs populate a locus somewhat blueward of the main sequence, there is perhaps more potential for contamination by background stars than in the very red L dwarf locus, as can be seen in the figures below:jhkl1 Above: Example colour selection criteria for ultracool dwarfs in the (J-H)/(H-K) two-colour diagram: blue triangles are nearby main-sequence stars; green points ultracool M and L dwarfs; red stars are T dwarfs; deep blue circles are M subdwarfs; and purple crosses are giants. The box outlines the (J-H)/(H-K) selection limits for much redder L dwarfs. Gliese 229B is the prototypical and first to be discovered brown dwarf, with spectral type T6.

Tcomp2Above: Illustrative comparison of T dwarf JHK colours (right) compared to a selection of main sequence and other cluster, association or field dwarfs of various ages. Data for main sequence stars are from Straižys & Lazauskaitė (Baltic Astronomy, 18, 19, 2009) and are on the 2MASS system. Data for T dwarfs from S. Leggett (link here) are on the MKO system. However for these roughly comparative purposes it is sufficient to see that T dwarfs can potentially occupy the same JHK colour space as any number of background interlopers. Conversion between the two systems (and many others) are here. T dwarf data were plotted using TopCat. Note the position of WISE  J2121 and the direction of reddening.

From the preprint abstract:

The census of the solar neighborhood is almost complete for stars and becoming more complete in the brown dwarf regime. Spectroscopic, photometric and kinematic characterization of nearby objects helps us to understand the local mass function, the binary fraction, and provides new targets for sensitive planet searches. We aim to derive spectral types and spectro-photometric distances of a sample of new high proper motion sources found with the WISE satellite, and obtain parallaxes for those objects that fall within the area observed by the Vista Variables in the Via Lactea survey (VVV). We used low resolution spectroscopy and template fitting to derive spectral types, multiwavelength photometry to characterize the companion candidates and obtain photometric distances. Multi-epoch imaging from the VVV survey was used to measure the parallaxes and proper motions for three sources. We confirm a new T2 brown dwarf within 15 pc. We derived optical spectral types for twenty four sources, mostly M dwarfs within 50 pc. We addressed the wide binary nature of sixteen objects found by the WISE mission and previously known high proper motion sources. Six of these are probably members of wide binaries, two of those are new, and present evidence against the physical binary nature of two candidate binary stars found in the literature, and eight that we selected as possible binary systems.

The spectrum of the object is compared to T1, T2 and T3 spectral standards below (taken from the same paper).

WISE 2121-62 spec.

Layers of haze in Pluto’s nitrogen atmosphere

Posted in astronomy with tags , on September 18, 2015 by Tim Kendall

Pluto-Mountains-Plains 9-17-15Credit: NASA/Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

Just 15 minutes after its closest approach to Pluto on July 14, 2015, NASA’s New Horizons spacecraft looked back toward the sun and captured a near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. The smooth expanse of the informally named Sputnik Planum (right) is flanked to the west (left) by rugged mountains up to 11,000 feet (3,500 meters) high, including the informally named Norgay Montes in the foreground and Hillary Montes on the skyline. The backlighting highlights more than a dozen layers of haze in Pluto’s tenuous but distended atmosphere. The image was taken from a distance of 11,000 miles (18,000 kilometers) to Pluto; the scene is 230 miles (380 kilometers) across.

Text credit: Johns Hopkins Applied Physics Laboratory. All the science photos from New Horizons are indexed here.

Using stellar ‘twins’ to improve distance measurements in the outer galaxy

Posted in astronomy with tags , on September 17, 2015 by Tim Kendall


Image: Two ‘twin’ stars with identical spectra observed by the La Silla Telescope. Since it is known that one star is 40 parsecs away, the difference in their apparent brightnesses allows calculation of the second star’s distance. Credit: Carolina Jofré.

Astronomers from the University of Cambridge have developed a new, highly accurate method of measuring the distances between stars, which could be used to measure the size of the galaxy, enabling greater understanding of how it evolved.

Using a technique which searches out stellar ‘twins’, the researchers have been able to measure distances between stars with far greater precision than is possible using typical model-dependent methods. The technique could be a valuable complement to the Gaia satellite – which is creating a three-dimensional map of the sky over five years – and could aid in the understanding of fundamental astrophysical processes at work in the furthest reaches of our galaxy. Details of the new technique are published in the Monthly Notices of the Royal Astronomical Society.

“Determining distances is a key problem in astronomy, because unless we know how far away a star or group of stars is, it is impossible to know the size of the galaxy or understand how it formed and evolved,” said Dr Paula Jofre Pfeil of Cambridge’s Institute of Astronomy, the paper’s lead author. “Every time we make an accurate distance measurement, we take another step on the cosmic distance ladder.”

The best way to directly measure a star’s distance is by an effect known as parallax, which is the apparent displacement of an object when viewed along two different lines of sight – for example, if you hold out your hand in front of you and look at it with your left eye closed and then with your right eye closed, your hand will appear to move against the background. The same effect can be used to calculate the distance to stars, by measuring the apparent motion of a nearby star compared to more distant background stars. By measuring the angle of inclination between the two observations, astronomers can use the parallax to determine the distance to a particular star.

However, the parallax method can only be applied for stars which are reasonably close to us, since beyond distances of 1600 light years, the angles of inclination are too small to be measured by the Hipparcos satellite, a precursor to Gaia. Consequently, of the 100 billion stars in the Milky Way, we have accurate measurements for just 100,000.

Gaia will be able to measure the angles of inclination with far greater precision than ever before, for stars up to 30,000 light years away. Scientists will soon have precise distance measurements for the one billion stars that Gaia is mapping – but that’s still only one percent of the stars in the Milky Way.

For even more distant stars, astronomers will still need to rely on models which look at a star’s temperature, surface gravity and chemical composition, and use the information from the resulting spectrum, together with an evolutionary model, to infer its intrinsic brightness and to determine its distance. However, these models can be off by as much as 30 percent. “Using a model also means using a number of simplifying assumptions – like for example assuming stars don’t rotate, which of course they do,” said Dr Thomas Mädler, one of the study’s co-authors. “Therefore stellar distances obtained by such indirect methods should be taken with a pinch of salt.”

The Cambridge researchers have developed a novel method to determine distances between stars by relying on stellar ‘twins’: two stars with identical spectra. Using a set of around 600 stars for which high-resolution spectra are available, the researchers found 175 pairs of twins. For each set of twins, a parallax measurement was available for one of the stars.

The researchers found that the difference of the distances of the twin stars is directly related to the difference in their apparent brightness in the sky, meaning that distances can be accurately measured without having to rely on models. Their method showed just an eight percent difference with known parallax measurements, and the accuracy does not decrease when measuring more distant stars.

“It’s a remarkably simple idea – so simple that it’s hard to believe no one thought of it before,” said Jofre Pfeil. “The further away a star is, the fainter it appears in the sky, and so if two stars have identical spectra, we can use the difference in brightness to calculate the distance.”

Since a utilised spectrum for a single star contains as many as 280,000 data points, comparing entire spectra for different stars would be both time and data-consuming, so the researchers chose just 400 spectral lines to make their comparisons. These particular lines are those which give the most distinguishing information about the star – similar to comparing photographs of individuals and looking at a single defining characteristic to tell them apart.

The next step for the researchers is to compile a ‘catalogue’ of stars for which accurate distances are available, and then search for twins among other stellar catalogues for which no distances are available. While only looking at stars which have twins restricts the method somewhat, thanks to the new generation of high-powered telescopes, high-resolution spectra are available for millions of stars. With even more powerful telescopes under development, spectra may soon be available for stars which are beyond even the reach of Gaia, so the researchers say their method is a powerful complement to Gaia.

“This method provides a robust way to extend the crucially-important cosmic distance ladder in a new special way,” said Professor Gerry Gilmore, the Principal Investigator for UK involvement in the Gaia mission. “It has the promise to become extremely important as new very large telescopes are under construction, allowing the necessary detailed observations of stars at large distances in galaxies far from our Milky Way, building on our local detailed studies from Gaia.”

Text courtesy The paper is P. Jofré, et al., 2015, in Monthly Notices of the Royal Astronomical Society vol. 453, 2, p. 1428.