K–H₂ line shapes for the spectra of cool brown dwarfs^⋆

¹ GEPI, Observatoire de Paris, UMR 8111, CNRS, Université Paris VII, 61 avenue de l’Observatoire, 75014 Paris, France
e-mail: nicole.allard@obspm.fr
² Institut d’Astrophysique de Paris, UMR 7095, CNRS, Université Paris VI, 75014 Paris, France
³ Laboratoire de Chimie et Physique Quantique, UMR 5626, CNRS, Université Paul Sabatier, 118 route de Narbonne, 31400 Toulouse, France
⁴ Department of Physics and Astronomy, University of Louisville, Louisville, Kentucky 40292, USA

Received: 8 February 2016
Accepted: 24 February 2016

Abstract

Observations of cooler and cooler brown dwarfs show that the contribution from broadening at many bars pressure is becoming important. The opacity in the red optical to near-IR region under these conditions is dominated by the extremely pressure-broadened wings of the alkali resonance lines, in particular, the K I resonance doublet at 0.77 μm. Collisions with H₂ are preponderant in brown dwarf atmospheres at an effective temperature of about 1000 K; the H₂ perturber densities reach several 10¹⁹ even in Jupiter-mass planets and exceed 10²⁰ for super-Jupiters and older Y dwarfs. As a consequence, it appears that when the far wing absorption due to alkali atoms in a dense H₂ atmosphere is significant, accurate pressure broadened profiles that are valid at high densities of H₂ should be incorporated into spectral models.