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TOPIC: Brown Dwarfs


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Posts: 131433
Date:
11 New T Dwarfs
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Title: Discovery of 11 New T Dwarfs in the Two Micron All-Sky Survey, Including a Possible L/T Transition Binary
Authors: Dagny L. Looper (Institute for Astronomy, UH), J. Davy Kirkpatrick (IPAC/Caltech), Adam J. Burgasser (MIT)
(Version v2)

We present the discovery of 11 new T dwarfs, found during the course of a photometric survey for mid-to-late T dwarfs in the 2MASS Point Source Catalogue and from a proper motion selected sample of ultracool dwarfs in the 2MASS Working Database. Using the NASA Infrared Telescope Facility SpeX spectrograph, we obtained low-resolution (R~150) spectroscopy, allowing us to derive near-infrared spectral types of T2-T8. One of these new T dwarfs, 2MASS J13243559+6358284, was also discovered independently by Metchev et al., in prep. This object is spectroscopically peculiar and possibly a binary and/or very young (<300 Myr). We specifically attempted to model the spectrum of this source as a composite binary to reproduce its peculiar spectral characteristics. The latest-type object in our sample is a T8 dwarf, 2MASS J07290002-3954043, now one of the four latest-type T dwarfs known. All 11 T dwarfs are nearby given their spectrophotometric distance estimates, with 1 T dwarf within 10 pc and 8 additional T dwarfs within 25 pc, if single. These new additions increase the 25 pc census of T dwarfs by ~14%. Their proximity offers an excellent opportunity to probe for companions at closer separations than are possible for more distant T dwarfs.

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Posts: 131433
Date:
Lithium in LP 944-20
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Title: Lithium in LP 944-20
Authors: Ya.V.Pavlenko (UH, MAO NASU), H.R.A.Jones (UH), E.L.Martin (IAC, Univ. Centr. Florida), E.Guenther (Turinger Sternwarte), M.A. Kenworthy (CAAO), M.R.Zapatero Osorio (IAC)

We present a new estimate of the lithium abundance in the atmosphere of the brown dwarf LP 944-20. Our analysis is based on a self-consistent analysis of low, intermediate and high resolution optical and near-infrared spectra. We obtain log N(Li) = 3.25 ±0.25 using fits of our synthetic spectra to the Li I resonance line doublet profiles observed with VLT/UVES and AAT/SPIRAL. This lithium abundance is over two orders of magnitude larger than previous estimates in the literature. In order to obtain good fits of the resonance lines of K I and Rb I and better fits to the TiO molecular absorption around the Li I resonance line, we invoke a semi-empirical model atmosphere with the dusty clouds located above the photosphere. The lithium abundance, however, is not changed by the effects of the dusty clouds. We discuss the implications of our estimate of the lithium abundance in LP 944-20 for the understanding of the properties of this benchmark brown dwarf.

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Posts: 131433
Date:
SCR 1845-6357
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Title: The very nearby M/T dwarf binary SCR 1845-6357
Authors: Markus Kasper (ESO), Beth A. Biller (Steward Observatory), Adam Burrows (Steward Observatory), Wolfgang Brandner (MPIA), Jano Budaj (Steward Observatory), Laird M. Close (Steward Observatory)

The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary discovered. SCR 1845 is a particular object due to its tight orbit (currently around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to the primary SCR 1845A, orbital motion is evident even within a year. Following the orbital motion, the binary's mass can be measured accurately within a decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR spectra allow for accurate determination of spectral type and also for rough estimates of the object's physical parameters. The spectral type of SCR 1845B is determined by direct comparison of the flux calibrated JHK spectra with T dwarf standard template spectra and also by NIR spectral indices obtained from synthetic photometry. Constrained values for surface gravity, effective temperature and metallicity are derived by comparison with model spectra. Our data prove that SCR 1845B is a brown dwarf of spectral type T6 that is co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting the NIR spectrum of SCR 1845B to model spectra yields an effective temperature of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses and 1.8 to 3.1 Gyr.

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Posts: 131433
Date:
Ultracool Dwarfs and Extrasolar Planets
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Title: Line and Mean Opacities for Ultracool Dwarfs and Extrasolar Planets
Authors: Richard S. Freedman, Mark S. Marley, Katharina Lodders

Opacities and chemical abundance data are crucial ingredients of ultracool dwarf and extrasolar giant planet atmosphere models. We report here on the detailed sources of molecular opacity data employed by our group for this application. We also present tables of Rosseland and Planck mean opacities which are of use in some studies of the atmospheres, interiors, and evolution of planets and brown dwarfs. For the tables presented here, we have calculated the opacities of atomic and molecular species important in the atmosphere including the effect of pressure induced absorption due to H2-H2, H2-He, and H2-H collisions, and other important sources such as H- but neglecting opacity from condensates. Our opacity database includes line opacity from the most important molecular species, and atomic lines due to the strongest transitions of the alkali atoms Na, K, Cs, Rb and Li. We report for each species how we have assembled molecular line data from a combination of public databases, laboratory data that is not yet in the public databases, and our own numerical calculations. We combine these opacities with abundances computed from a chemical equilibrium model using recently revised solar abundances to compute mean opacities. The chemical equilibrium calculation accounts for the settling of condensates in a gravitational field, and is applicable to ultracool dwarf and extrasolar planetary atmospheres, but not circumstellar disks. We find that the inclusion of alkali atomic opacity substantially increases the mean opacities over those currently in the literature at densities relevant to the atmospheres and interiors of giant planets and brown dwarfs. We provide our opacity tables for public use and discuss their limitations.

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Posts: 131433
Date:
SDSS J080531.84+481233.0
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Title: SDSS J080531.84+481233.0: An Unresolved L Dwarf/T Dwarf Binary
Authors: Adam J. Burgasser (MIT)

SDSS J080531.84+481233.0 is a peculiar L-type dwarf that exhibits unusually blue near-infrared and mid-infrared colours and divergent optical (L4) and near-infrared (L9.5) spectral classifications. These peculiar spectral traits have been variously attributed to condensate cloud effects or subsolar metallicity. Here I present an improved near-infrared spectrum of this source which further demonstrates the presence of weak CH4 absorption at 1.6 micron but no corresponding band at 2.2 micron. It is shown that these features can be collectively reproduced by the combined light spectrum of a binary with L4.5 and T5 components, as deduced by spectral template matching. Thus, SDSS J080531.84+481233.0 appears to be a new low-mass binary straddling the L dwarf/T dwarf transition, an evolutionary phase for brown dwarfs that remains poorly understood by current theoretical models. The case of SDSS J080531.84+481233.0 further illustrates how a select range of L dwarf/T dwarf binaries could be identified and characterised without the need for high angular resolution imaging or radial velocity monitoring, potentially alleviating some of the detection biases and limitations inherent to such techniques.

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Posts: 131433
Date:
2MASS J04381486+2611399
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Title: Hubble and Spitzer Observations of an Edge-on Circumstellar Disk around a Brown Dwarf
Authors: K. L. Luhman, Lucia Adame, Paola D'Alessio, Nuria Calvet, Kim K. McLeod, C. J. Bohac, William J. Forrest, Lee Hartmann, B. Sargent, Dan M. Watson

We present observations of a circumstellar disk that is inclined close to edge-on around a young brown dwarf in the Taurus star-forming region. Using data obtained with SpeX at the NASA Infrared Telescope Facility, we find that the slope of the 0.8-2.5 um spectrum of the brown dwarf 2MASS J04381486+2611399 cannot be reproduced with a photosphere reddened by normal extinction. Instead, the slope is consistent with scattered light, indicating that circumstellar material is occulting the brown dwarf. By combining the SpeX data with mid-IR photometry and spectroscopy from the Spitzer Space Telescope and previously published millimetre data from Scholz and coworkers, we construct the spectral energy distribution for 2MASS J04381486+2611399 and model it in terms of a young brown dwarf surrounded by an irradiated accretion disk. The presence of both silicate absorption at 10 um and silicate emission at 11 um constrains the inclination of the disk to be ~70 deg, i.e. ~20 deg from edge-on. Additional evidence of the high inclination of this disk is provided by our detection of asymmetric bipolar extended emission surrounding 2MASS J04381486+2611399 in high-resolution optical images obtained with the Hubble Space Telescope. According to our modelling for the SED and images of this system, the disk contains a large inner hole that is indicative of a transition disk (R_in~58 R_star~0.275 AU) and is somewhat larger than expected from embryo ejection models (R_out=20-40 AU vs. R_out<10-20 AU).

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Posts: 131433
Date:
Brown dwarf ULAS J0034-00
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Astrophysicists have found a star-like object with a surface temperature just one tenth that of the Sun.
The cold object is known as a brown dwarf: a "failed" star that never achieved the mass required to begin nuclear fusion reactions in its core.
This one - called J0034-00 - is thought to have a surface temperature of just 600-700 Kelvin (up to 430C/800F).
It is the coldest solitary brown dwarf ever seen, according to the British team that discovered it.
This find further tests the boundary between high-mass gas planets and the smallest brown dwarfs.

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Posts: 131433
Date:
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The coolest-known star-like object beyond the solar system is giving astronomers a new look at the differences between massive planets and the smallest brown dwarfs. This newly discovered object, called ULAS J0034-00 and located in the constellation Cetus, has a record-setting surface temperature of 600-700 K, cooler than any known solitary brown dwarf. In addition, its a relative lightweight, with an estimated mass of only 15-30 times that of Jupiter (although they both have about the same diameter).

ULAS J0034-00
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UKIDSS image of the coolest known brown dwarf showing more distant stars around it. The image was made by combining three infrared images.
Credit: Gemini Observatory/AURA

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Posts: 131433
Date:
2MASS1207-3932
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VLT Finds Smallest Galactic Object with Jets
Jets of matter have been discovered around a very low mass 'failed star', mimicking a process seen in young stars. This suggests that these 'brown dwarfs' form in a similar manner to normal stars but also that outflows are driven out by objects as massive as hundreds of millions of solar masses down to Jupiter-sized objects.
The brown dwarf with the name 2MASS1207-3932 is full of surprises. Its companion, a 5 Jupiter-mass giant, was the first confirmed exoplanet for which astronomers could obtain an image, thereby opening a new field of research - the direct detection of alien worlds. It was then later found  that the brown dwarf has a disc surrounding it, not unlike very young stars.
Now, astronomers using ESO's Very Large Telescope (VLT) have found that the young brown dwarf is also spewing jets, a behaviour again quite similar to young stars.
The mass of the brown dwarf is only 24 Jupiter-masses. Hence, it is by far the smallest object known to drive an outflow.

"This leads us to the tantalising prospect that young giant planets could also be associated with outflows" - Emma Whelan, the lead-author of the paper reporting the results.

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Posts: 131433
Date:
Brown Dwarfs emits beams of radiation
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A class of "failed" star called a brown dwarf emits beams of radiation that are thousands of times brighter than any released by the Sun.
The brown dwarfs are behaving like an altogether different and exotic cosmic object called a pulsar.
Pulsars are rotating neutron stars that emit a flashing radio signal.
When the rotating beams sweep Earth, astronomers detect the radio pulse, which has been likened to the rotating beacon of a lighthouse.
Greg Hallinan from the National University of Ireland in Galway and his colleagues used the Very Large Array radio telescope in New Mexico to observe a very cool, rapidly rotating brown dwarf called TVLM 513-46546.

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