Letter to the Editor

K–H₂ quasi-molecular absorption detected in the T-dwarf  Indi Ba

¹ Centre de Recherche Astrophysique de Lyon, UMR 5574: CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 07, France e-mail: fallard@ens-lyon.fr
² Institut d'Astrophysique de Paris, UMR 7095: CNRS, Université Pierre et Marie Curie-Paris 6, 98bis boulevard Arago, 75014 Paris, France
³ Observatoire de Paris-Meudon, LERMA, UMR 8112, CNRS, 92195 Meudon Principal Cedex, France
⁴ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁵ Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA
⁶ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁷ Laboratoire de Chimie et Physique Quantiques, UMR5626, CNRS and Université Paul Sabatier, 118 route de Narbonne 31062, Toulouse Cedex, France

Received: 27 July 2007
Accepted: 27 August 2007

Abstract

Context.T-type dwarfs present a broad and shallow absorption feature centred around 6950 Å in the blue wing of the K doublet at 0.77 μm which resembles in depth and shape the satellite absorption predicted by detailed collisional broadening profiles. In our previous work, the position of the predicted line satellite was however somewhat too blue compared to the observed feature.

Aims.In this paper, we investigate whether new calculations of the energy surfaces of the potentials in the K–H₂ 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 and at what T_eff these respective opacity sources predominate.

Methods.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–H₂ line profiles using improved interaction potentials.

Results.By comparison with a high signal-to-noise optical spectrum of the T1 dwarf ε 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 observable photosphere in dwarfs of later type than ~L5.

Conclusions.We conclude that previous identification of the feature at this location in the spectra of T dwarfs as well as the latest L dwarfs with CaH was erroneous, as expected on physical grounds: calcium has already condensed onto grains in early L dwarfs and thus should have settled out of the photosphere in cooler brown dwarfs. This finding revokes one of the observational verifications for the cloud-clearing theory assumption: a gradual clearing of the cloud cover in early T dwarfs.