* Astronomy

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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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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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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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).


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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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