Archive for brown dwarfs

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.


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.

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.

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.

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.

Formation of brown dwarfs like stars

Posted in astronomy with tags , on August 3, 2015 by Tim Kendall

brown_dwarf_jetAn impression of a young brown dwarf. Evidence appears to be mounting that the star formation process also produces brown dwarfs by the same mechanism, i.e. disk accretion accompanied by energetic, sometimes episodic, outflows.  Even though there is almost certainly an overlap in mass distribution with giant planets, the two classes of bodies are fundamentally different. Article from Scientific American.

If the universe were a fairy tale, the celestial objects called brown dwarfs would be the ugly ducklings. Small and dim as they are, brown dwarfs are informally known as “failed stars.” But some scientists have proposed that brown dwarfs possess unrecognized majesty—that they are in fact gargantuan planets. Alas, a new study suggests that the story is not destined for a happy ending. Astronomers have detected the first direct evidence that these cosmic misfits are forged in a miniature version of star formation. The study was published in The Astrophysical Journal on July 1.

Brown dwarfs have, at most, 8 percent of the mass of our sun, so their interiors lack the high heat and pressure necessary to fuse hydrogen into helium—the thermonuclear process that powers regular stars. But brown dwarfs do not fit comfortably in the planet category, either. They are tens of times more massive than even heavyweights such as Jupiter, and keep much hotter cores by contracting in on themselves and fusing deuterium, explains James Di Francesco, an astrophysicist at the National Research Council Canada who was not involved with the study.

Because brown dwarfs seem to reside in this no-man’s-land between planets and stars, astronomers have long wondered about their origins. On the one hand, brown dwarfs could form like stars, through the collapse of vast clouds of gas and dust. The protostar born in such a condensation wraps itself in a disk of material, and interactions between this accretion disk and the baby star’s magnetic field launch two jets of material from opposite sides of the disk. On the other hand, planets form when bits of material in a disk around a star glom onto one another. Some astronomers even proposed that if a big enough chunk of material broke off the disk, it could seed a brown dwarf.

Over the last 15 years astronomers have found that young brown dwarfs share similar properties with young normal stars, particularly the jets and accretion disks, says Emma Whelan, an astrophysicist at University of Tübingen in Germany. But these surveys primarily examined brown dwarfs whose surrounding envelope of gas and dust had already begun to disperse. In the new study an international team of scientists led by Oscar Morata, an astronomer at the Academia Sinica in Taiwan, examined even younger proto–brown dwarfs to see if they found the jets of material that could only be explained by a process akin to that of normal star formation. “Basically we thought, you know, ‘if it walks like a duck, talks like a duck,’ maybe stars and brown dwarfs are the same kind of thing,” Morata says.

Glimpsing brown dwarfs at such an early stage of formation had previously proved difficult because brown dwarfs are so dim. But Morata’s team scoured data from the Spitzer and Herschel space telescopes for young, faint objects. They eventually settled on 10 proto–brown dwarf candidates and examined them using the Very Large Array (VLA) of radio telescopes in Socorro, New Mexico.

Among their sample, the astronomers spotted four proto–brown dwarfs spitting out the jets characteristic of regular star formation. Moreover, Morata’s team found that the brightness of these jets depended on the brightness of the proto–brown dwarf itself. There is a similar relationship between the brightness of protostars and their associated jets. These results lend the first direct observational evidence to the idea that brown dwarfs are produced by a scaled-down version of the process that forms stars. “I think that this team has done a terrific job of advancing our understanding of how brown dwarfs form,” Di Francesco says.

But what about the other six objects—the majority? According to Morata, the accretion process that drives jets on young stars and brown dwarfs is not continuous, so jets spurt on and off and may have been missed on the other six objects. Or the jets of these objects are simply too faint to see. Morata also allows for the possibility that these objects were not proto–brown dwarfs at all, but rather background galaxies. He says that the team’s next steps will be analyzing data on their proto–brown dwarf candidates that they have collected with the Atacama Large Millimeter/submillimeter Array (ALMA) of radio telescopes and requesting more VLA observation time to reexamine the candidates in better detail. Morata and his colleagues also hope to find other newborn brown dwarfs to study. “Four—well, it’s not that much,” Morata says. “It would be nice to have something more robust.”

Whelan says that many interesting questions arise from the conclusion that brown dwarfs form like stars, particularly regarding whether these objects could host their own planetary systems—and what those systems might be like. If so, the ugly ducklings might be home to great beauty after all.

Note: an updated abstract for the V1309 Sco paper (see sidebar) can be found here.

VLT/SPHERE non-detection of the proposed brown dwarf in the V471 Tauri system

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

Wide-field view of the sky around the unusual binary star V471 TImage and text credit: European Southern Observatory (ESO) science release. A tertiary brown dwarf companion in the V471 Tauri eclipsing binary system has been postulated since at least 2001. The star itself is centred in the above image. Detection of the brown dwarf as a residual in the fitted light curve of the eclipsing primary/secondary pair had been claimed in 2011 and indeed the presence of a fourth, planetary mass body hinted at. However the brown dwarf has not been found by a new instrument which should have been capable of easily detecting it, the SPHERE exoplanet imager at ESO/VLT. A link to the research paper itself is here (via ESO). From the science release:

The new SPHERE instrument on ESO’s Very Large Telescope has been used to search for a brown dwarf expected to be orbiting the unusual double star V471 Tauri. SPHERE has given astronomers the best look so far at the surroundings of this intriguing object and they found — nothing. The surprising absence of this confidently predicted brown dwarf means that the conventional explanation for the odd behaviour of V471 Tauri is wrong. This unexpected result is described in the first science paper based on observations from SPHERE. Some pairs of stars consist of two normal stars with slightly different masses. When the star of slightly higher mass ages and expands to become a red giant, material is transferred to other star and ends up surrounding both stars in a huge gaseous envelope. When this cloud disperses the two move closer together and form a very tight pair with one white dwarf, and one more normal star. [Such pairs are known as post common envelope binaries]. One such stellar pair is called V471 Tauri. It is a member of the Hyades star cluster in the constellation of Taurus and is estimated to be around 600 million years old and about 163 light-years from Earth. The two stars are very close and orbit each other every 12 hours. Twice per orbit one star passes in front of the other — which leads to regular changes in the brightness of the pair observed from Earth as they eclipse each other. A team of astronomers led by Adam Hardy (Universidad Valparaíso, Valparaíso, Chile) first used the ULTRACAM system on ESO’s New Technology Telescope to measure these brightness changes very precisely. The times of the eclipses were measured with an accuracy of better than two seconds — a big improvement on earlier measurements. The eclipse timings were not regular, but could be explained well by assuming that there was a brown dwarf orbiting both stars whose gravitational pull was disturbing the orbits of the stars. They also found hints that there might be a second small companion object. Up to now however, it has been impossible to actually image a faint brown dwarf so close to much brighter stars. But the power of the newly installed SPHERE instrument on ESO’s Very Large Telescope allowed the team to look for the first time exactly where the brown dwarf companion was expected to be. But they saw nothing, even though the very high quality images from SPHERE should have easily revealed it. The SPHERE images are so accurate that they would have been able to reveal a companion such as a brown dwarf that is 70 000 times fainter than the central star, and only 0.26 arcseconds away from it. The expected brown dwarf companion in this case was predicted to be much brighter.

From the paper abstract:

We report that an unprecedented contrast of [12.1 magnitudes (H band)] at a separation of 260 mas was achieved, but resulted in a non- detection. This implies that there is no brown dwarf present in the system unless it is three magnitudes fainter than predicted by evolutionary track models, and provides damaging evidence against the circumbinary interpretation of eclipse timing variations. In the case of V471 Tau, a more consistent explanation is offered with the Applegate mechanism, in which these variations are prescribed to changes in the quadrupole moment within the main-sequence star.

More (on a very different topic): The oldest stars in the universe formed 560 Myr after the Big Bang, 140 Myr later than previously thought (Scientific American). This new Planck result is important as it resolves certain difficulties which had arisen from the earlier date and also implies that the very first stars which formed (often called Population III stars) might be more easily detected in the future. This is one of the prime observational goals of the James Webb Space Telescope (JWST).

A determination of the low mass end of the main sequence and the hydrogen burning limit

Posted in astronomy with tags , , , on August 12, 2014 by Tim Kendall

Image credit: P. Marenfeld & NOAO/AURA/NSF. The ordinate is the stellar radius.

In research accepted for publication in the Astronomical Journal, the RECONS (Research Consortium On Nearby Stars) group from Georgia State University has found clear observational evidence for the theoretically predicted break between very low mass stars and brown dwarfs. The data came from the SOAR (SOuthern Astrophysical Research) 4.1-m telescope and the SMARTS (Small and Moderate Aperture Research Telescope System) 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

For most of their lives, stars obey a relationship referred to as the main sequence, a relation between luminosity and temperature – which is also a relationship between luminosity and radius. Stars behave like balloons in the sense that adding material to the star causes its radius to increase: in a star the material is the element hydrogen, rather than air which is added to a balloon. Brown dwarfs, on the other hand, are described by different physical laws (referred to as electron degeneracy pressure) than stars and have the opposite behavior. The inner layers of a brown dwarf work much like a spring mattress: adding additional weight on them causes them to shrink. Therefore brown dwarfs actually decrease in size with increasing mass.

As Dr. Sergio Dieterich, the lead author, explained, “In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed (see the attached illustration, based on a figure in the publication). We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs. “

Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”

The new paper is S. Dieterich, et al., 2014, in the Astronomical Journal, 147, 94: doi:10.1088/0004-6256/147/5/94. From the abstract:

We find evidence for the local minimum in the radius-temperature and radius-luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T eff ~ 2075 K, log (L/L ☉) ~ –3.9, and (R/R ☉) ~ 0.086. The existence of this local minimum is predicted by evolutionary models, but at temperatures ~400 K cooler. The minimum radius happens near the locus of 2MASS J0523–1403, an L2.5 dwarf with V – K = 9.42.

Praesepe, an open cluster, where brown dwarfs have long been sought observationally. Image credit & copyright: Bob Franke

Record-breaking new brown dwarf: temperature 250 K, distance 2.2 pc

Posted in astronomy with tags , , , , on April 28, 2014 by Tim Kendall


Artistic impression (above), graphic and text credits: State

( —A “brown dwarf” star that appears to be the coldest of its kind—as frosty as Earth’s North Pole—has been discovered by a Penn State University astronomer using NASA’s Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescopes. Images from the space telescopes also pinpointed the object’s distance at 7.2 light-years away, making it the fourth closest system to our Sun.

“It is very exciting to discover a new neighbor of our solar system that is so close,” said Kevin Luhman, an associate professor of astronomy and astrophysics at Penn State and a researcher in the Penn State Center for Exoplanets and Habitable Worlds. “In addition, its extreme temperature should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures.”

Brown dwarfs start their lives like stars, as collapsing balls of gas, but they lack the mass to burn nuclear fuel and radiate starlight. The newfound coldest brown dwarf, named WISE J085510.83-071442.5, has a chilly temperature between minus 54 and 9 degrees Fahrenheit (minus 48 to minus 13 degrees Celsius). Previous record holders for coldest brown dwarfs, also found by WISE and Spitzer, were about room temperature.

Kevin Luhman and Penn State do it again! The preprint is in arXiv today also and the paper already published in the Astrophysical Journal, vol. 786 (2014), L18. Concerning the new object, its parallax is 0.454+/-0.045 arcseconds and the mass is likely to be in the range three to ten Jupiter masses. For more on high proper motion objects in the Wide-field Infrared Survey Explorer dataset, there is another new paper in press at ApJ, Luhman & Sheppard, preprint.


Update: Penn State grad student Benjamin Nelson and collaborators have solved the ongoing mystery of the inner 55 Cancri system, one of the first exoplanet systems found (1997) and itself a close neighbour to the Sun, some 12 parsecs distant. Collecting together the huge dataset, Nelson has now found a dynamically plausible system of orbital resonances. Note the three massive inner planets, all closer to 55 Cnc than Mercury is to the Sun. An accurate mass for the innermost planet e was given in 2012. The outermost planet d was found in 2002 and lies further out in the system, 5.5 AU distant from the sunlike central star.


Young, red, low mass brown dwarf PSO J318.5-22 from Pan-STARRS

Posted in astronomy with tags , on October 11, 2013 by Tim Kendall

Image credit: N. Metcalfe & Pan-STARRS PS1 Science Consortium. The new discovery in arXiv a week ago is now the subject of a press release from the Institute for Astronomy (IfA) at Honolulu, as well as wider publicity. The estimated six Jupiter masses for the object rely on its membership of the beta Pictoris moving group with an age of 12 Myr, but a new estimate today (Binks A.S. & Jeffries R.D., arXiv) of that age is 21 Myr, requiring an upward revision of the mass probably to the deuterium burning limit. nelson2The problem is well illustrated even by very old evolutionary tracks for brown dwarfs and low mass stars (left). Consider an object thought to have age 10 Myr and a luminosity 0.00001 solar; its mass by inspection of these models might be 0.005 solar (a), but if actually nearer 60 Myr old (b) it might have twice the mass. This is quite apart from uncertainty in the luminosity itself due to uncertain distance. Some previous discoveries in the same age/luminosity space are plotted with much more modern models (below, right) in a figure taken from the discovery paper of a recent dusty young L dwarf companion to an M dwarf. apj480297f5_lr

The precise mass apart, the important realisation here is that the infrared spectrum is similar to that of the HR 8799 planets and other young low mass objects such as 2M1207b. From the preprint:

Altogether, PSO J318-22 is the first free-floating object with the colors, magnitudes, spectrum, luminosity, and mass that overlap the young dusty planets around HR 8799 and 2MASS J1207-39.

Evaporating protostar IRAS 20324+4057

Posted in astronomy with tags , on September 4, 2013 by Tim Kendall

Image credit: NASA, ESA, Hubble Heritage Team (STScI/AURA), and IPHAS. We love evaporating protostars in this corner of the internet, so this new APOD image is particularly good especially as it was taken back in 2006, and only released recently. The story is the usual one: stars form in clusters very often and the energetic winds of nearby massive stars are eroding the material from which this protostar is trying to form and blasting it away. We can immediately see that the final mass of the star will be dependent on the effectiveness of this process, and that in general the mass of a clump of material which might form a star will not exactly predict the final main sequence mass outcome.

Elsewhere, more brown dwarfs are being found very nearby in the solar neighbourhood. Following the overview of the diversity of T dwarfs revealed by WISE published earlier this year by Mace et al., and the round-up of a few more by Bihain et al. (2013) – one of which at 5 ± 1 pc happens to be the nearest in the northern hemisphere – there has been renewed interest in the L/T dwarf transition. Seven new such objects within 15 pc – all of which must also be brown dwarfs – have been found by cross-matching the optical Pan-STARRS and infrared WISE data over 30,000 square degrees by Best et al., in arXiv today. Using WISE and 2MASS, Castro et al. have collected three more L dwarfs within 25 pc plus a 10 pc transition object. Lastly, the companion to HD 95086 appears ever more planet-like with its non-detection in the near-infrared H-band (1.6 μm) ruling out confusion with a foreground brown dwarf source.

Update: WISE is of course now NEOWISE, hunting for hazardous near-Earth objects.

Proper motion of the two parsec distant brown dwarf binary WISE J104915.57-531906AB

Posted in astronomy with tags , , , , on May 12, 2013 by Tim Kendall


Image credit: Kevin Luhman/PENN State/Eberly College of Science

The recent discovery of this system, ranking third closest to the Sun after the planet-hosting alpha Centauri system and the red dwarf Barnard’s Star, has not been greeted with as much interest as it merits. Efforts to find the dim red and brown dwarfs close to the Sun over the whole sky including the galactic plane have had some previous successes, notably the record breaking object UGPS 0722-05, which was initially ascribed spectral type T10. The main figure is from the discovery paper and shows the progress of WISE J104915.57-531906 across the sky since being picked up on the DSS-IR plate in 1978. The final frame is the 2010 WISE data. Note that the point spread function of the WISE data is not as narrow as the near-infrared (1999) or red visible (1992) data, as would be expected with imaging in the thermal infrared. With accurate centroiding techniques this does not so much impede astrometric (positional) accuracy. Note also how bright the binary becomes at these increasingly long wavelengths, from the red visible around 8000 Angstroms (0.8 microns) through to WISE wavelengths, 3.6 microns in this case. Above, (lower right panel) significantly higher resolution i-band imaging from the 8 metre Gemini telescope reveals the binary nature of the brown dwarf system itself.

Image credit: Kevin Luhman/PENN State/Eberly College of Science

The binarity is also apparent in this imaging from 1984 (left). The orientation is the same as for the other images, although the scale is different. The position angle very similar to that shown by the Gemini imaging. This suggests the binary has completed one orbit in the ~ 30 years since, in agreement with the initially published estimate. The primary has spectral type L8 and the secondary is likely near the L/T transition. The separation is 1.5 arcsec, or ~ 3 AU.

Update: Further characterisation of the binary has been performed by Kniazev et al., accepted to ApJ, who find a spectral type of T1.5±2 for the secondary using SALT spectroscopy.