  The opacity due to photodissociation of 24MgH is investigated in the
atmospheres of cool stars. The lowest two electronic transitions A 2Pi -- X
2Sigma+ and B' 2Sigma+ -- X 2 Sigma+ are considered where the cross sections
for the latter were published previously (Weck, Stancil, & Kirby 2002) while
the former are presented in this work. Model atmospheres calculated with the
PHOENIX code are used to investigate the effect of the photodissociation
opacity on spectra of cool stars. The A -- X photodissociation cross sections
are obtained using a combination of ab initio and experimentally derived
potential curves and dipole transition moments. Partial cross sections have
been evaluated over the accessible wavelength range 1770-4560 Angstrom for all
rotational transitions from the vibrational levels v''=0-11. Assuming a
Boltzmann distribution of the rovibrational levels of the X 2Sigma+ state, LTE
photodissociation cross sections are presented for temperatures between 1000
and 5000 K. Shape resonances, arising from rotational predissociation of
quasi-bound levels of the A 2Pi state near threshold, characterize the LTE
photodissociation cross sections. A sum rule is proposed as a check on the
accuracy of the photodissociation calculations.

Astrophysics