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Posts: 131433
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T Dwarfs
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Title: The Kinematic Age of the Coolest T Dwarfs
Authors: Leigh Smith, Philip Lucas, Ben Burningham, Hugh Jones, Ricky Smart, David Pinfield, Federico Marocco, James Clarke

Surprisingly, current atmospheric models suggest that the coolest T dwarfs (T8.5 to T10) are young and very low mass (0.06-2Gyr, 5-20Mjup, Leggett et al.2009, 2010, 2012). Studies of population kinematics offer an independent constraint on the age of the population. We present kinematic data of a sample of 75 mid to late T dwarfs drawn from a variety of sources. We define our samples, T5.5 to T8 and T8.5 to T10, as mid and late T respectively. UKIDSS LAS kinematics were derived from our automated LAS proper motion pipeline and distance estimates derived from spectral types and photometry for the minority of sources that lack parallaxes. Our results show that the mid and late T populations do not have distinctly separate tangential velocity distributions to 95% probability. They also give an approximate mean kinematic age equal to that of a population with B-V colour 0.51-0.54, and a spectral type late F, which corresponds to an age of about 2 Gyr. However the median and modal ages are greater. This indicates that while model atmospheres correctly predict some trends in colour with gravity and age, reliable ages cannot yet be inferred from them. More benchmark objects are needed to anchor the models.

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Title: The sub-stellar birth rate from UKIDSS
Authors: A. C. Day-Jones, F. Marocco, D. J. Pinfield, Z. H. Zhang, B. Burningham, N. Deacon, M. T. Ruiz, J. Gallardo, H. R. A. Jones, P. W. L. Lucas, J. S. Jenkins, J. Gomes, S. L. Folkes, J. R. A. Clarke

We present a new sample of mid L to mid T dwarfs with effective temperatures of 1100 to 1700K selected from the UKIDSS Large Area Survey and confirmed with infrared spectra from X-Shooter/VLT. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate, and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unresolved binaries, which form the first complete sub-set from our program, covering 495 sq degrees of sky, complete to J=18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of mass 0.10-0.03Msol and ages 1-10Gyrs. We find that the more extreme birth rates (e.g. a halo type form) can likely be excluded as the true form of the birth rate. In addition we find that although there is substantial scatter we find a preference for a mass function, with a power-law index, alpha in the range -1 < alpha < 0 that is consistent (within the errors) with the studies of late T dwarfs.

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Title: A Study of the Diverse T Dwarf Population Revealed by WISE
Authors: Gregory N. Mace (UCLA, IPAC), J. Davy Kirkpatrick (IPAC), Michael C. Cushing (U. of Toledo), Christopher R. Gelino (IPAC), Roger L. Griffith (IPAC), Michael F. Skrutskie (U. of Virginia), Kenneth A. Marsh (Cardiff University), Edward L. Wright (UCLA), Peter R. Eisenhardt (NASA JPL), Ian S. McLean (UCLA), Maggie A. Thompson (Princeton University), Katholeen Mix (IPAC), Vanessa Bailey (Steward Observatory), Charles A. Beichman (IPAC), Joshua S. Bloom (UC Berkeley), Adam J. Burgasser (UCSD, MIT), Jonathan J. Fortney (UCSC), Philip M. Hinz (Steward Observatory), Russell P. Knox (Steward Observatory), Patrick J. Lowrance (IPAC), Mark S. Marley (NASA Ames), Caroline V. Morley (UCSC), Timothy J. Rodigas (Steward Observatory), Didier Saumon (LANL), Scott S. Sheppard (Carnegie Institution of Washington), et al. (1 additional author not shown)

We report the discovery of 87 new T dwarfs uncovered with the Wide-field Infrared Survey Explorer (WISE) and three brown dwarfs with extremely red near-infrared colors that exhibit characteristics of both L and T dwarfs. Two of the new T dwarfs are likely binaries with L71 primaries and mid-type T secondaries. In addition, our follow-up program has confirmed 10 previously identified T dwarfs and four photometrically-selected L and T dwarf candidates in the literature. This sample, along with the previous WISE discoveries, triples the number of known brown dwarfs with spectral types later than T5. Using the WISE All-Sky Source Catalogue we present updated colour-colour and colour-type diagrams for all the WISE-discovered T and Y dwarfs. Near-infrared spectra of the new discoveries are presented, along with spectral classifications. To accommodate later T dwarfs we have modified the integrated flux method of determining spectral indices to instead use the median flux. Furthermore, a newly defined J-narrow index differentiates the early-type Y dwarfs from late-type T dwarfs based on the J-band continuum slope. The K/J indices for this expanded sample show that 32% of late-type T dwarfs have suppressed K-band flux and are blue relative to the spectral standards, while only 11% are redder than the standards. Comparison of the Y/J and K/J index to models suggests diverse atmospheric conditions and supports the possible re-emergence of clouds after the L/T transition. We also discuss peculiar brown dwarfs and candidates that were found not to be substellar, including two Young Stellar Objects and two Active Galactic Nuclei. The coolest WISE-discovered brown dwarfs are the closest of their type and will remain the only sample of their kind for many years to come.

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Posts: 131433
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T and Y Dwarfs
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Title: Neglected Clouds in T and Y Dwarf Atmospheres
Authors: Caroline V. Morley, Jonathan J. Fortney, Mark S. Marley, Channon Visscher, Didier Saumon, S. K. Leggett

As brown dwarfs cool, a variety of species condense in their atmospheres, forming clouds. Iron and silicate clouds shape the emergent spectra of L dwarfs, but these clouds dissipate at the L/T transition. A variety of other condensates are expected to form in cooler T dwarf atmospheres. These include Cr, MnS, Na2S, ZnS, and KCl, but the opacity of these optically thinner clouds has not been included in previous atmosphere models. Here, we examine their effect on model T and Y dwarf atmospheres. The cloud structures and opacities are calculated using the Ackerman & Marley (2001) cloud model, which is coupled to an atmosphere model to produce atmospheric pressure-temperature profiles in radiative-convective equilibrium. We generate a suite of models between Teff = 400 and 1300 K, log g=4.0 and 5.5, and condensate sedimentation efficiencies from fsed=2 to 5. Model spectra are compared to two red T dwarfs, Ross 458C and UGPS 0722-05; models that include clouds are found to match observed spectra significantly better than cloudless models. The emergence of sulphide clouds in cool atmospheres, particularly Na2S, may be a more natural explanation for the "cloudy" spectra of these objects, rather than the re-emergence of silicate clouds that wane at the L-to-T transition. We find that sulphide clouds provide a mechanism to match the near- and mid-infrared colours of observed T dwarfs. Our results indicate that including the opacity of condensates in T dwarf atmospheres is necessary to accurately determine the physical characteristics of many of the observed objects.

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Posts: 131433
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T4 brown dwarfs
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Title: Discovery of Three Distant, Cold Brown Dwarfs in the WFC3 Infrared Spectroscopic Parallels Survey
Authors: Daniel Masters, Patrick McCarthy, Adam J. Burgasser, Nimish P. Hathi, Matthew Malkan, Nathaniel R. Ross, Brian Siana, Claudia Scarlata, Alaina Henry, James Colbert, Hakim Atek, Marc Rafelski, Harry Teplitz, Andrew Bunker, Alan Dressler

We present the discovery of three late type (>T4) brown dwarfs, including a probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) Survey. We use the G141 grism spectra to determine the spectral types of the dwarfs and derive distance estimates based on a comparison with nearby T dwarfs with known parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs, with the farthest at an estimated distance of ~400 pc. We compare the number of cold dwarfs found in the WISP survey with simulations of the brown dwarf mass function. The number found is generally consistent with an initial stellar mass function dN/dM \propto M^{-\alpha} with \alpha = 0.0--0.5, although the identification of a Y dwarf is somewhat surprising and may be indicative of either a flatter absolute magnitude/spectral type relation than previously reported or an upturn in the number of very late type brown dwarfs in the observed volume.

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RE: T dwarfs
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Title: Four new T dwarfs identified in PanSTARRS 1 commissioning data
Authors: Niall R. Deacon (1), Michael C. Liu (1), Eugene A. Magnier (1), Brendan P. Bowler (1), Bertrand Goldman (2), Joshua A. Redstone (3), W. S. Burgett (1), K. C. Chambers (1), H. Flewelling (1), N. Kaiser (1), R.H. Lupton (4), J.S. Morgan (1), P.A. Price (4), W.E. Sweeney (1), J.L. Tonry (1), R.J. Wainscoat (1), C. Waters (1) ((1) IfA, University of Hawai`i, (2) Max Planck Institute for Astronomy, Heidelberg, (3) Facebook, (4) Princeton University Observatory)

A complete well-defined sample of ultracool dwarfs is one of the key science programs of the Pan-STARRS 1 optical survey telescope (PS1). Here we combine PS1 commissioning data with 2MASS to conduct a proper motion search (0.1--2.0\arcsec/yr) for nearby T dwarfs, using optical+near-IR colours to select objects for spectroscopic followup. The addition of sensitive far-red optical imaging from PS1 enables discovery of nearby ultracool dwarfs that cannot be identified from 2MASS data alone. We have searched 3700 sq. deg. of PS1 y-band (0.95--1.03 um) data to y\approx 19.5 mag (AB) and J\approx 16.5 mag (Vega) and discovered four previously unknown bright T dwarfs. Three of the objects (with spectral types T1.5, T2 and T3.5) have photometric distances within 25 pc and were missed by previous 2MASS searches due to more restrictive colour selection criteria. The fourth object (spectral type T4.5) is more distant than 25 pc and is only a single-band detection in 2MASS. We also examine the potential for completing the census of nearby ultracool objects with the PS1 3 \pi survey.

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Posts: 131433
Date:
T dwarf star
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An international team of astronomers has discovered a unique and exotic star system comprised of a very cool, methane-rich (or T-) dwarf star and a "dead" white dwarf star in orbit around each other, the European Southern Observatory (ESO) said on Tuesday.
The system, which is the first of its type to be found, is a "Rosetta Stone" for T-dwarf stars and gives scientists a way of finding the mass and age of this old methane dwarf.

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Posts: 131433
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RE: T dwarfs
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Title: Hip 63510C, Hip 73786B, and nine new isolated high proper motion T dwarf candidates from UKIDSS DR6 and SDSS DR7
Authors: R.-D. Scholz

Aims: Completing the poorly known substellar census of the solar neighbourhood, especially with respect to the coolest brown dwarfs, will lead to a better understanding of failed star formation processes and binary statistics with different environmental conditions.
Methods: Using UKIDSS data and their cross-correlation with the SDSS, we searched for high proper motion mid- to late-T dwarf candidates with extremely blue near-infrared (J-K<0) and very red optical-to-near-infrared (z-J>+2.5) colours.
Results: With 11 newly found T dwarf candidates, the proper motions of which range between 100 and 800 mas/yr, we increased the number of UKIDSS T dwarf discoveries by about 30 percent. Large proper motions were also measured for six of eight previously known T4.5-T9 dwarfs detected in our survey. All new candidates can be classified as T5-T9 dwarfs based on their colours. Two of these objects were found to be common proper motion companions of Hipparcos stars with accurate parallaxes. The latter allow us to determine absolute magnitudes from which we classify Hip 63510C as T7.5-T8 and Hip 73786B as T6.5-T7 dwarfs. The projected physical separation from their low-mass (M0.5 and K5) primaries is in both cases about 1200 AU. One of the Hipparcos stars has already a known brown dwarf companion on a close astrometric orbit (Hip 63510B = Gl 494B). With distances of only 11.7 and 18.6 pc, deduced from their primaries respectively for Hip 63510C and Hip 73786B, various follow-up observations can easily be carried out to study these cool brown dwarfs in more detail and to compare their properties with those of the already well-investigated primaries.

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Title: The Physical Properties of Four ~600K T Dwarfs
Authors: S. K. Leggett, M. C. Cushing, D. Saumon, M. S. Marley, T. L. Roellig, S. J. Warren, B. Burningham, H. R. A. Jones, J. D. Kirkpatrick, N. Lodieu, P. W. Lucas, A. K. Mainzer, E. L. Martin, M. J. McCaughrean, D. J. Pinfield, G. C. Sloan, R. L. Smart, M. Tamura, J. Van Cleve

We present Spitzer 7.6-14.5um spectra of ULAS J003402.77-005206.7 and ULAS J133553.45+113005.2, two T9 dwarfs with the latest spectral types currently known. We fit synthetic spectra and photometry to the near- through mid-infrared energy distributions of these dwarfs and that of the T8 dwarf 2MASS J09393548-2448279. We also analyse near-infrared data for another T9, CFBD J005910.82-011401.3. We find that the ratio of the mid- to near-infrared fluxes is very sensitive to effective temperature at these low temperatures, and that the 2.2 and 4.5um fluxes are sensitive to metallicity and gravity; there is a degeneracy between these parameters. The 4.5 and 10um fluxes are also sensitive to vertical transport of gas through the atmosphere, which we find to be significant for these dwarfs. The full near- through mid-infrared spectral energy distribution allows us to constrain the effective temperature (K)/gravity (m/s)/metallicity ([m/H] dex) of ULAS J0034-00 and ULAS J1335+11 to 550-600/ 100-300/ 0.0-0.3 and 500-550/ 100-300/ 0.0-0.3, respectively. These fits imply low masses and young ages for the dwarfs of 5-20 M(Jup) and 0.1-2 Gyr. The fits to 2MASS J0939-24 are in good agreement with the measured distance, the observational data, and the earlier T8 near-infrared spectral type if it is a slightly metal-poor 4-10 Gyr old system consisting of a 500 and 700K, ~25 and ~40 M(Jup), pair, although it is also possible that it is an identical pair of 600K, 30 M(Jup), dwarfs. As no mid-infrared data are available for CFBD J0059-01 its properties are less well constrained; nevertheless it appears to be a 550-600K dwarf with g= 300-2000 m/s and [m/H]= 0-0.3 dex. These properties correspond to mass and age ranges of 10-50 M(Jup) and 0.5-10 Gyr for this dwarf.

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Title: Identifying nearby field T dwarfs in the UKIDSS Galactic Clusters Survey
Authors: N. Lodieu (1), B. Burningham (2), N. C. Hambly (3), D. J. Pinfield (2) ((1) IAC, Tenerife, Spain (2) University of Hertfordshire, UK, (3) University of Edinburgh, Royal Observatory, UK)

We present the discovery of two new late-T dwarfs identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) Data Release 2 (DR2). These T dwarfs are nearby old T dwarfs along the line of sight to star-forming regions and open clusters targeted by the UKIDSS GCS. They are found towards the Alpha Per cluster and Orion complex, respectively, from a search in 54 square degrees surveyed in five filters. Photometric candidates were picked up in two-colour diagrams, in a very similar manner to candidates extracted from the UKIDSS Large Area Survey (LAS) but taking advantage of the Z filter employed by the GCS. Both candidates exhibit near-infrared J-band spectra with strong methane and water absorption bands characteristic of late-T dwarfs. We derive spectral types of T6.50.5 and T71 and estimate photometric distances less than 50 pc for UGCS J030013.86+490142.5 and UGCS J053022.52-052447.4, respectively. The space density of T dwarfs found in the GCS seems consistent with discoveries in the larger area coverage of the UKIDSS Large Area Survey, indicating one T dwarf in 6-11 square degrees. The final area surveyed by the GCS, 1000 square degrees in five passbands, will allow expansion of the LAS search area by 25%, increase the probability of finding ultracool brown dwarfs, and provide optimal estimates of contamination by old field brown dwarfs in deep surveys to identify such objects in open clusters and star-forming regions.

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