K-H_2 Quasi-molecular absorption detected in the T-dwarf epsilon Indi Ba

Abstract

T-type dwarfs present a broad and shallow absorption feature centred around 6950 A in the blue wing of the K doublet at 0.77 micron which resembles in depth and shape the satellite absorption predicted by detailed collisional broadening profiles. In our previous work, the predicted line satellite position was however somewhat too blue compared to the observed feature. In this paper we investigate whether new calculations of the energy surfaces of the potentials in the K-H_2 system, including spin-orbit coupling, result in a closer coincidence of the satellite with the observed position. We also investigate the extent to which CaH absorption bands contribute to the feature. We present model atmospheres and synthetic spectra, including gravitational settling for an improved description of depth-dependent abundances of refractory elements, and based on new K-H_2 line profiles using improved interaction potentials. By comparison with a high signal-to-noise optical spectrum of the T1 dwarf epsilon Indi Ba, we find that these new models do reproduce the observed feature, while CaH does not contribute for the atmospheric parameters considered. We also find that CaH is settled out so deep into the atmosphere that even turbulent vertical mixing would appear insufficient to bring significant amounts of CaH to the photosphere in dwarfs later than ~L5. We conclude that previous identification of the feature at this location in T and late L dwarf spectra with CaH was erroneous, as expected on physical grounds: calcium condenses onto grains in early L dwarfs and thus should have settled out of the photosphere in cooler brown dwarfs. This finding revokes one observational verification for the cloud-clearing theory: a gradual clearing of the cloud cover in early T dwarfs.

arXiv commentaccepted for publication by Astronomy & Astrophysics Letters, 4 pages, 3 colour figures

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