Asteroid 2015 BZ509 captured from outside the Solar System ~4.5 Gyr ago

Posted in astronomy with tags , on May 21, 2018 by Tim Kendall

Image credit: C. Veillet/Large Binocular Telescope Observatory and text: Sciencemag: Asteroid 2015 BZ509, orbiting close to Jupiter but in a retrograde direction, has probably been in a stable orbit for almost the whole lifetime of the Solar System, it has been found. Its retrograde orbit means it was highly unlikely to be part of the precursor proto-solar nebula, and must have been captured by gravitational interaction with newly-formed massive Jupiter. This and the age of 2015 BZ509 means it is the first object permanently resident in the Solar System to have been identified as possibly originating by capture from interstellar space:

Astronomers first spotted the object, an asteroid called 2015 BZ509 that is orbiting close to Jupiter, in 2014. They knew it was unusual because it was traveling around the solar system in the opposite direction as almost everything else. (Its motion is shown in the animations above, with 2015 BZ509 circled.) Astronomers have found other objects in “retrograde” orbits, perhaps knocked off course by passing too close to a giant planet, but 2015 BZ509’s orbit was the weirdest of all because it is also elongated and out of alignment with the planets and other bodies.

To find out why, a pair of astronomers ran a series of 1 million simulations of the asteroid’s orbit, each with slightly different parameters. Jupiter’s orbit is a busy part of the solar system where the risk of being knocked off course is high, so eccentric orbits that are stable long term are unlikely. But the researchers found a number of possible orbits that were stable and concluded it is much more likely that 2015 BZ509 is in one of them, rather than that it happened to arrive for a short-term visit. Some of those stable orbits, if wound back in time, would mean that 2015 BZ509 has been with us since the beginning of our solar system, about 4.5 billion years ago.

The story has been covered today in the Guardian: “Asteroid from another star system found orbiting wrong way near Jupiter: For the first time, a permanent member of our solar system has been found to have originated elsewhere”, 21 May 2018.

Update: the story has now been covered in more depth by Scientific American, 22 May 2018, and (24 May) the preprint abstract is now available here.

Advertisements

Star-forming filaments in Orion revealed by ALMA

Posted in astronomy with tags , , on March 29, 2018 by Tim Kendall

This stunning new ALMA image reveals the thermal millimetre-wave emission from extremely cold gas (red) in the region of the Orion nebula. According to the current paradigm of star formation, material flows along the newly discovered filaments, and where filaments intersect, hubs are formed which are likely sites for future massive star formation. In blue is the VLT/HAWK-I near-infrared imaging data, with the famous Trapezium visible at upper left. Four stars are easily visible with a small telescope; larger amateur instruments should allow six Trapezium stars to be visible, in good seeing conditions. From the ESO press release:

This spectacular and unusual image shows part of the famous Orion Nebula, a star formation region lying about 1350 light-years from Earth. It combines a mosaic of millimetre-wavelength images from the Atacama Large Millimeter/submillimeter Array (ALMA) and the IRAM 30-metre telescope, shown in red, with a more familiar infrared view from the HAWK-I instrument on ESO’s Very Large Telescope, shown in blue. The group of bright blue-white stars at the upper-left is the Trapezium Cluster — made up of hot young stars that are only a few million years old. The wispy, fibre-like structures seen in this large image are long filaments of cold gas, only visible to telescopes working in the millimetre wavelength range. They are invisible at both optical and infrared wavelengths, making ALMA one of the only instruments available for astronomers to study them. This gas gives rise to newborn stars — it gradually collapses under the force of its own gravity until it is sufficiently compressed to form a protostar — the precursor to a star. The scientists who gathered the data from which this image was created were studying these filaments to learn more about their structure and make-up. They used ALMA to look for signatures of diazenylium gas, which makes up part of these structures. Through doing this study, the team managed to identify a network of 55 filaments. The Orion Nebula is the nearest region of massive star formation to Earth, and is therefore studied in great detail by astronomers seeking to better understand how stars form and evolve in their first few million years. Earlier mosaics of Orion at millimetre wavelengths had used single-dish telescopes, such as APEX. The new observations from ALMA and IRAM use interferometry to combine the signals from multiple, widely-separated antennas to create images showing much finer detail.

Protoplanetary disk AS 209 imaged by ALMA

Posted in astronomy with tags , , on March 5, 2018 by Tim Kendall

ESO press release: Nestled in the young Ophiuchus star-forming region, 410 light-years from the Sun, a fascinating protoplanetary disc named AS 209 is slowly being carved into shape:

“This wonderful image was captured using the high-resolution ALMA telescope, revealing a curious pattern of rings and gaps in the dust surrounding a young star. Protoplanetary discs are dense, rotating planes of gas and dust that surround newly formed stars; providing the matter that one day becomes orbiting planets, moons and other minor bodies. At less than one million years old, this system is very young, but already two clear gaps are being sculpted from the disc. The outer gap is deep, wide, and largely a dust-free zone, leading astronomers to believe that a giant planet almost the mass of Saturn is orbiting here — around 800 light-minutes from the central star, and more than three times the distance between Neptune and the Sun! As the planet carves out its path, dust piles up at the outer edge of its orbit, creating ever more defined rings in the disc. The thinner, inner dust gap could have been formed by a smaller planet, but astronomers have raised the intriguing possibility that the large and distant circling planet in fact created both paths. This inferred Saturn-like planet so far from its central star raises fascinating questions about planet formation at the edges of protoplanetary discs on particularly short timescales.”

The link to the associated research paper by D. Fedele et al. can be found here. Given the importance of molecules in star formation, and the apparent prevalence of protoplanetary discs, every new image of this sort continues to hint that on astronomical scales, the building blocks of life are everywhere around us. Scientific American magazine have recently given a new overview of the TESS mission, due for launch in April and designed to find transiting exoplanets around stars within ~100 pc of the Sun. Because nearby, these will be the most ready targets for study of their atmospheres, where they are shown to exist, by JWST. For a very different, extragalactic note, I  recommend Astronomy Picture of the Day’s delightful treatment today of the Hubble Ultra-Deep Field.

More: “Planet Nine” has not yet been observed. Scientific American have an excellent new round-up today as to why this is so, with insight from the research teams involved on the practical and scientific difficulties they face in attempting to pin down the hypothetical ten Earth mass body, at least twenty times more distant from the Sun than Neptune, which has been invoked to explain the unusual properties of the orbits of recently discovered small trans-Neptunian objects.

Implications of an upper mass bound on exoplanets

Posted in astronomy with tags , , on January 22, 2018 by Tim Kendall

Image: Illustration of a massive exoplanet orbiting a solar-type star. A new paper, accepted by the Astrophysical Journal, is by K. Schlaufman (Johns Hopkins Univ.), in arXiv today and clarifies a question which I attempted to address in a recent post, here; the findings of an earlier paper (to which I drew attention in that post) are reinforced. Observational evidence is combined with insights from theory to address whether a boundary exists between planets which form like gas-giant planets through core accretion, or like stars, through gravitational instability. From the abstract of today’s paper (abridged):

Celestial bodies with a mass of M ~ 10 M_Jup have been found orbiting nearby stars. It is unknown whether these objects formed like gas-giant planets through core accretion or like stars through gravitational instability. I show that objects with M <~ 4 M_Jup orbit metal-rich solar-type dwarf stars, a property associated with core accretion. Objects with M >~ 10 M_Jup do not share this property. This transition is coincident with a minimum in the occurrence rate of such objects, suggesting that the maximum mass of a celestial body formed through core accretion like a planet is less than 10 M_Jup. Consequently, objects with M >~ 10 M_Jup orbiting solar-type dwarf stars likely formed through gravitational instability and should not be thought of as planets.

These findings apply to solar-type stars, and to how planets around other such stars, much like our Sun, form in general. The findings of both these papers, taken together, strongly point to the 4 – 10 Jupiter mass range as marking a real, distinctive boundary between bona-fide planets and more massive (sub)stellar objects.

Organic matter in extraterrestrial water-bearing salt crystals

Posted in astronomy with tags , on January 11, 2018 by Tim Kendall

(Phys.org) A blue crystal recovered from a meteorite that fell near Morocco in 1998. The scale bar represents 200 microns (millionths of a metre). Credit: Queenie Chan/The Open University, UK.  Two wayward space rocks, which separately crashed to Earth in 1998 after circulating in our solar system’s asteroid belt for billions of years, share something else in common: the ingredients for life. They are the first meteorites found to contain both liquid water and a mix of complex organic compounds such as hydrocarbons and amino acids. A detailed study of the chemical makeup within tiny blue and purple salt crystals sampled from these meteorites, which included results from X-ray experiments at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), also found evidence for the pair’s past intermingling and likely parents. These include Ceres, the dwarf planet that is the largest object in the asteroid belt, and the asteroid Hebe, a major source of meteorites that fall on Earth. The study, published Jan. 10 in the journal Science Advances, provides the first comprehensive chemical exploration of organic matter and liquid water in salt crystals found in Earth-impacting meteorites. The study treads new ground in the narrative of our solar system’s early history and asteroid geology while surfacing exciting possibilities for the existence of life elsewhere in Earth’s neighborhood. From the abstract (abridged):

Direct evidence of complex prebiotic chemistry from a water-rich world in the outer solar system is provided by the 4.5-billion-year-old halite crystals hosted in the Zag and Monahans (1998) meteorites. This study offers the first comprehensive organic analysis of the soluble and insoluble organic compounds found in the millimeter-sized halite crystals containing brine inclusions and sheds light on the nature and activity of aqueous fluids on a primitive parent body. Associated with these trapped brines are organic compounds exhibiting wide chemical variations representing organic precursors, intermediates, and reaction products that make up life’s precursor molecules such as amino acids. The organic compounds also contain a mixture of C-, O-, and N-bearing macromolecular carbon materials exhibiting a wide range of structural order, as well as aromatic, ketone, imine, and/or imidazole compounds. The enrichment in 15N is comparable to the organic matter in pristine Renazzo-type carbonaceous chondrites, which reflects the sources of interstellar 15N, such as ammonia and amino acids. The amino acid content of the Zag halite deviates from the meteorite matrix, supporting an exogenic origin of the halite, and therefore, the Zag meteorite contains organics synthesized on two distinct parent bodies. Our study suggests that the asteroidal parent body where the halite precipitated, potentially asteroid 1 Ceres, shows evidence for a complex combination of biologically and prebiologically relevant molecules.

Blue halite and tweezer tips for scale. Credit: Dr. Queenie Hoi Shan Chan

Onset of ice formation in the Pipe nebula

Posted in astronomy with tags , , , on December 15, 2017 by Tim Kendall

In a recent paper, Goto et al., (preprint) have used infrared absorption spectroscopy of background stars to probe ice formation on grains in the Pipe nebula (APOD image and credits), detecting water ice absorption in seven out of 21 lines of sight investigated. The peak optical depths of the water ice are about half as large as those on the sources in Taurus with similar visual extinctions. One possible explanation (among others) is that the formation of the ice mantle has just started, that is, the Pipe Nebula is in an earlier phase of ice evolution than Taurus is. Alternatively the interstellar radiation field impinging on the Pipe nebula is larger than that on the Taurus molecular cloud, or the visual extinction through the Pipe nebula may have been overestimated. From the abstract:

Spectroscopic studies of ices in nearby star-forming regions indicate that ice mantles form on dust grains in two distinct steps, starting with polar ice formation (water rich) and switching to apolar ice (CO rich). We test how well the picture applies to more diffuse and quiescent clouds where the formation of the first layers of ice mantles can be witnessed. Medium-resolution near-infrared spectra are obtained toward background field stars behind the Pipe Nebula. The water ice absorption is positively detected at 3.0 micron in seven lines of sight out of 21 sources for which observed spectra are successfully reduced. The peak optical depth of the water ice is significantly lower than those in Taurus with the same visual extinction. The source with the highest water-ice optical depth shows CO ice absorption at 4.7 micron as well. The fractional abundance of CO ice with respect to water ice is 16+7-6 %, and about half as much as the values typically seen in low-mass star-forming regions. A small fractional abundance of CO ice is consistent with some of the existing simulations. Observations of CO2 ice in the early diffuse phase of a cloud play a decisive role in understanding the switching mechanism between polar and apolar ice formation.

Does the organic material of comets pre-date our solar system?

Posted in astronomy with tags , , on September 12, 2017 by Tim Kendall

The Rosetta space probe discovered a large amount of organic material in the nucleus of comet 67P Churyumov-Gerasimenko. Researchers now advance the theory that this matter has its origin in interstellar space and before the solar system formed. According to a new paper by Jean-Loup Bertaux and Rosine Lallement – from the Laboratoire Atmosphères, Milieux, Observations Spatiales (CNRS / UPMC / Université de Versailles Saint-Quentin-en-Yvelines) and the Galaxies, Étoiles, Physique et Instrumentation department of the Paris Observatory (Observatoire de Paris / CNRS / Université Paris Diderot), respectively – these organic molecules were produced in interstellar space, well before the formation of the Solar System. Bertaux and Lallement further assert that astronomers are already familiar with much of this matter. They refer to the diffuse interstellar bands, discovered nearly 100 years ago. For some time now there has been strong evidence that these absorption bands, observed in the spectra of stars seen through and dimmed by dense interstellar clouds, originate from large molecular carriers. Science Daily continues the story:

For [more than] seventy years, scientists have known that analysis of stellar spectra indicates unknown absorptions, throughout interstellar space, at specific wavelengths called the diffuse interstellar bands (DIBs). DIBs are attributed to complex organic molecules that US astrophysicist Theodore Snow believes may constitute the largest known reservoir of organic matter in the Universe. This interstellar organic material is usually found in the same proportions. However, very dense clouds of matter [such as nebulae which are or may go on to forming stars] are exceptions. In the middle of these nebulae, where matter is very dense, DIB absorptions plateau or even drop. This is because the organic molecules responsible for DIBs clump together there. The clumped matter absorbs less radiation than when it floated freely in space. Such primitive nebulae end up contracting to form a solar system like our own, with planets, and comets. The Rosetta mission taught us that comet nuclei form by gentle accretion of grains progressively greater in size. First, small particles stick together into larger grains. These in turn combine into larger chunks, and so on, until they form a comet nucleus a few kilometers wide. Thus, the organic molecules that formerly populated the primitive nebulae — and that are responsible for DIBs — were probably not destroyed, but instead incorporated into the grains making up cometary nuclei. And they may have remained there for longer than the 4.6 billion year age of the solar system. A sample-return mission would allow laboratory analysis of cometary organic material [to identify and date this material] and finally reveal the identity of the mysterious interstellar matter underlying observed absorption lines in stellar spectra. If cometary organic molecules were indeed produced in interstellar space — and if they played a role in the emergence of life on our planet, as scientists believe today — might they not also have seeded life on many other planets of our galaxy?

Imaging, spectroscopy and statistics of exoplanets

Posted in astronomy on August 30, 2017 by Tim Kendall

Gemini/GPI (Gemini Planet Imager) image of 51 Eridani b, discovered in 2015, courtesy www.sci-news.com. The object has been spectroscopically characterised this year by Samland et al. who note it is outside the normal colour-colour relations for brown dwarfs. The authors employ the Markov Chain Monte Carlo technique to find that the exoplanet has a high metallicity; in fact, 51 Eri b is as metal-rich as Saturn. (We may recall that here metals mean any element heavier than helium, which is sensible, as lithium and for that matter beryllium are generally classed as metals). The most likely estimate for the mass is about 9 Jupiter masses. Image courtesy A&A highlights:Statistical treatment of measurements of the metallicity of large ensembles of exoplanets can also yield interesting results which are quite readily visualized. In the case of a recent paper by Santos et al., a division is seen at around 4 Jupiter masses (see image below) with less massive planets being found most often around high metallicity stars while more massive exoplanets are found around stars with a wider range of [Fe/H] values. (The metallicity can be thought of most simply as [Fe/H], since iron, being at the peak of the binding energy curve of nuclei, is the heaviest element forged by nuclear fusion in stars). From the abstract (abridged):

Analysis of the statistical properties of exoplanets, together with those of their host stars, are providing a unique view into the process of planet formation and evolution. In this paper we explore the properties of the mass distribution of giant planet companions to solar-type stars, in a quest for clues about their formation process. With this goal in mind we studied, with the help of standard statistical tests, the mass distribution of giant planets using data from the exoplanet.eu catalog and the SWEET-Cat database of stellar parameters for stars with planets. We show that the mass distribution of giant planet companions is likely to present more than one population with a change in regime around 4 MJup. Above this value host stars tend to be more metal poor and more massive and have [Fe/H] distributions that are statistically similar to those observed in field stars of similar mass. On the other hand, stars that host planets below this limit show the well-known metallicity-giant planet frequency correlation. We discuss these results in light of various planet formation models and explore the implications they may have on our understanding of the formation of giant planets. In particular, we discuss the possibility that the existence of two separate populations of giant planets indicates that two different processes of formation are at play.

In general giant planets are thought to form in circumstellar disks, while brown dwarfs and stars are thought to form directly by gravitational collapse and fragmentation of a gas cloud and this analysis may be evidence pointing to a value of 4 Jupiter masses as being an approximate but typical maximum mass for an object formed around a star, i.e., in a disk. Of course this does not rule out much more massive objects forming in some circumstellar disks, and a lot more evidence will be needed before we can be certain of any conclusion of this nature. 4 Jupiter masses, though, is rather less than the deuterium-burning limit which has historically been invoked as a kind of delimiter between brown dwarfs and massive giant planets; brown dwarfs burn deuterium, giant planets do not. The new statistical finding could be showing us which among the giant exoplanets are the least “planet-like”. Image courtesy A&A highlights:

A wide-field skyscape of the Virgo cluster of galaxies

Posted in astronomy with tags , on August 24, 2017 by Tim Kendall

Image Credit & Copyright: Rogelio Bernal Andreo (Deep Sky Colors) and APOD where an annotated version of this image is available. This colorful and broad telescopic mosaic links Markarian’s Chain of galaxies across the core of the Virgo Cluster to dusty spiral galaxy Messier 64. Galaxies are scattered through the field of view that spans some 20 full moons across a gorgeous night sky. The cosmic frame is also filled with foreground stars from constellations Virgo and  Coma Berenices, and faint, dusty nebulae drifting above the plane of the Milky Way. Look carefully for Markarian’s eyes. The famous pair of interacting galaxies is near the top, not far from M87, the Virgo cluster’s giant elliptical galaxy. Toward bottom left is Messier 64, also known as the Black Eye Galaxy. The Virgo Cluster is the closest large galaxy cluster to our own local galaxy group. Virgo Cluster galaxies are about 50 million light-years distant, but M64 lies a mere 17 million light-years away.

Saturn near opposition, observed from Earth

Posted in astronomy with tags on June 24, 2017 by Tim Kendall

This view of Saturn from Earth has been taken at the Pic du Midi Observatory, France, which is known for episodes of truly excellent seeing. This is why features can be discerned such as inner faint rings and Saturn’s polar hexagon, which are normally visible only from space probes such as Cassini. Image Credit & Copyright: D. Peach, E. Kraaikamp, F. Colas, M. Delcroix, R. Hueso, G. Therin, C. Sprianu, S2P, IMCCE, OMP and APOD:

Saturn reached its 2017 opposition on June 16. Of course, opposition means opposite the Sun in Earth’s sky and near opposition Saturn is up all night, at its closest and brightest for the year. This remarkably sharp image of the ringed planet was taken only days before, on June 11, with a 1-meter telescope from the mountain top Pic du Midi observatory. North is at the top with the giant planet’s north polar storm and curious hexagon clearly seen bathed in sunlight. But Saturn’s spectacular ring system is also shown in stunning detail. The narrow Encke division is visible around the entire outer A ring, small ringlets can be traced within the fainter inner C ring, and Saturn’s southern hemisphere can be glimpsed through the wider Cassini division. Near opposition Saturn’s rings also appear exceptionally bright, known as the opposition surge or Seeliger Effect. Directly illuminated from Earth’s perspective, the ring’s icy particles cast no shadows and strongly backscatter sunlight creating the dramatic increase in brightness. Still, the best views of the ringed planet are currently from the Saturn-orbiting Cassini spacecraft. Diving close, Cassini’s Grand Finale orbit number 9 is in progress.

Young stars and dusty nebulae in Taurus

Posted in astronomy with tags , , on April 22, 2017 by Tim Kendall

Image and text credit: APOD: Lloyd L. Smith, Deep Sky West. This complex of dusty nebulae lingers along the edge of the Taurus molecular cloud, a mere 450 light-years distant. Stars are forming on the cosmic scene. Composed from almost 40 hours of image data, the 2 degree wide telescopic field of view includes some youthful T-Tauri class stars embedded in the remnants of their natal clouds at the top. Millions of years old and still going through stellar adolescence, the stars are variable in brightness and in the late phases of their gravitational collapse. Their core temperatures will rise to sustain nuclear fusion as they grow into stable, low mass, main sequence stars, a stage of stellar evolution achieved by our middle-aged Sun about 4.5 billion years ago. Another youthful variable star, V1023 Tauri, can be spotted in the lower part of the image. Within its yellowish dust cloud, it lies next to the striking blue reflection nebula Cederblad 30, also known as LBN 782. Above the bright bluish reflection nebula is dusty dark nebula Barnard 7.

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 http://dx.doi.org/10.1038/nature21360 (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.

Update: more accurate and precise eccentricities are found, < 0.02 for the six innermost planets, together with a more constrained mass for the seventh, h, in a new preprint by S. Wang et al., 2017 Apr 17. Overall, derived masses for the outer planets d, e, f and g decrease compared to the estimations from the discovery paper.

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: palereddot.org. 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 palereddot.org, 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

N159_Legacy

(phys.org) 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: http://za-neptunie.livejournal.com/55097.html. 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 (www.recons.org) 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, www.recons.org) 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
TW Hydrae: ESO/ALMA
(phys.org) 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

vlashowsearl

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

27160Meingast

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; astrosurf.com, 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

45-p9_kbo_extras_orbits_2_

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 wired.com) 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

PIA15254_LMC2048
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)

LMC_HaOIIILRGB_lorenzi2000c
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