The molecular line opacity of MgH in cool stellar atmospheres

A new, complete, theoretical rotational and vibrational line list for the A (2)Pi<--X (2)Sigma(+) electronic transition in MgH is presented. The list includes transition energies and oscillator strengths for all possible allowed transitions and was computed using the best available theoretical potential energies and dipole transition moment function with the former adjusted to account for experimental data. The A<--X line list as well as new line lists for the B' (2)Sigma(+)<--X (2)Sigma(+) and the X (2)Sigma(+)<--X (2)Sigma(+) (pure rovibrational) transitions were included in comprehensive stellar atmosphere models for M, L, and T dwarfs and solar-type stars. The resulting spectra, when compared to models lacking MgH, show that MgH provides significant opacity in the visible between 4400 and 5600 Angstrom. Further comparison of the spectra obtained with the current line list to spectra obtained using the line list constructed by Kurucz show that the Kurucz list significantly overestimates the opacity due to MgH, particularly for the bands near 5150 and 4800 Angstrom, with the discrepancy increasing with decreasing effective temperature.