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Abstract: Luhman and collaborators recently discovered an early-T dwarf companion tothe G0 dwarf star HN Peg, using Spitzer Infrared Array Camera IRAC images.Companionship was established on the basis of the common proper motion inferredfrom 1998 Two Micron All Sky Survey images and the 2004 IRAC images. In thispaper we present new near-infrared imaging data which confirms the commonproper motion of the system. We also present new 3 - 4 um spectroscopy of HNPeg B, which provides tighter constraints on both the bolometric luminositydetermination and the comparison to synthetic spectra. New adaptive opticsimaging data are also presented, which shows the T dwarf to be unresolved,providing limits on the multiplicity of the object. We use the age, distanceand luminosity of the solar-metallicity T dwarf to determine its effectivetemperature and gravity, and compare synthetic spectra with these values, and arange of grain properties and vertical mixing, to the observed 0.8 - 4.0 umspectra and mid-infrared photometry. We find that models with temperature andgravity appropriate for the older end of the age range of the system 0.5 Gyrcan do a reasonable job of fitting the data, but only if the photosphericcondensate cloud deck is thin, and if there is significant vertical mixing inthe atmosphere. Dwarfs such as HN Peg B, with well-determined metallicity,radius, gravity and temperature will allow development of dynamical atmospheremodels, leading to the solution of the puzzle of the L to T dwarf transition.

Autor: S. K. Leggett, D. Saumon, Loic Albert, Michael. C. Cushing, Michael C. Liu, K. L. Luhman, M. S. Marley, J. Davy Kirkpatrick, Thom


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