Molecular Opacities

Molecules dominate the spectral distribution of brown dwarfs. Young brown dwarfs hotter then 2000K emit flux principally in the near-infrared ZJHK bandpasses from 0.9 to 2.5 $\mu$m with bands of water steam in the infrared and TiO and VO bands in the optical shaping each side of this spectral distribution. Below 2000K, TiO, VO and CaH bands vanish as a result of the condensation of these refractory elements and the collision-induced opacities of H₂ and the growth of CH₄ bands become increasingly important in the H, K and L bandpasses at 1.6, 2.0, and 3.5 $\mu$m. It is this situation that causes the radiation to remain forced to emerge principally from the near-infrared bandpasses rather than emerging redwards as black bodies would do. Several other molecules are also present in brown dwarf spectra; overtones of CO at 2.3 and 4.7 $\mu$m are strong T $_{\rm eff}$ indicators and play an important cooling role upon the upper atmospheric structure, vibrational bands of SiO and hydrides such as OH, SiH and MgH determine the ultraviolet and visual radiation, the Wing-Ford system of FeH at 0.98 $\mu$m is one of the most prominent features in the spectra of late-type stars and young and/or massive brown dwarfs.

Our molecular opacity data base includes (i) a list of 43 million atomic transitions by , (ii) Collision-Induced Absorption (CIA) opacities for H₂, He, H, N₂, Ar, CH₄, and CO₂ by , (iii) ab initio line lists for H₂O and TiO by , (iv) CO from the line list by , , (v) most other molecular systems such as MgH and OH are included from the list by , (v) VO and CrH lines have been calculated by R. D. Freedman for this work, while (vi) FeH lines from have been also included. For the remaining molecular band systems for which no line lists were available to us (CaH) we apply the Just Overlapping Line Approximation (JOLA) as described and utilized by .

We have also included a combination of the HITRAN92 and GEISA databases summing up to about 700,000 lines of 31 molecules with a total of 74 isotopes. The molecules with the largest number of lines which are included in our models are O₃(168,881 lines), CO₂ (60,790), and CH₄ (47,415). This database only includes the strongest lines of these molecules. However, the gf-values and position of the lines have comparatively high accuracy and allow us to diagnose their importance in brown dwarf atmospheres. A comparison of models computed with these opacities to the NextGen models can be found in .

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