Line Intensities and Molecular Opacities of The F ⁴Δ _i-X ⁴Δ_i Transition

We calculate new line lists and opacities for the F⁴Δ _i-X⁴Δ_i transition of FeH. The 0-0 band of this transition is responsible for the Wing-Ford band seen in M-type stars, sunspots, and brown dwarfs. The new Einstein A-values for each line are based on a high-level ab initio calculation of the electronic transition dipole moment. The necessary rotational line strength factors (Hönl-London factors) are derived for both the Hund's case (a) and (b) coupling limits. A new set of spectroscopic constants was derived from the existing FeH term values for v = 0, 1, and 2 levels of the X and F states. Using these constants, extrapolated term values were generated for v = 3 and 4 and for J-values up to 50.5. The line lists (including Einstein A-values) for the 25 vibrational bands with v ≤ 4 were generated using a merged list of experimental and extrapolated term values. The FeH line lists were used to compute the molecular opacities for a range of temperatures and pressures encountered in L and M dwarf atmospheres. Good agreement was found between the computed and observed spectral energy distribution of the L5 dwarf 2MASS-1507.