AAS 199th meeting, Washington, DC, January 2002
Session 92. Circumstellar Material and Atmospheres: Cooler
Display, Wednesday, January 9, 2002, 9:20am-6:30pm, Exhibit Hall

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[92.08] Rovibrational Photodissociation of MgH for Advanced Stellar Atmosphere Models

P. F. Weck, A. Schweitzer, P. C. Stancil, P. H. Hauschildt (University of Georgia), K. Kirby (CfA)

For very cool stars (M and later) molecular absorption is the most important opacity source. The lack of accurate and complete molecular data has been a serious limitation to developing atmospheric models of such stars, from which synthetic spectra and important physical parameters, such as surface chemical composition, can be calculated. Models generally include molecular bands with hundreds of millions of spectral lines, but few have considered the effect of molecular photodissociation processes. Photodissociation from a range of vibrational (v) and rotational (R) levels of the ground electronic state of a molecule can provide a significant source of continuum opacity.

We have performed extensive calculations of photodissociation cross sections for MgH, using the most accurate available molecular data. Potential curves and transition moments for the low-lying electronic states of MgH (Saxon et al., 1978), as well as the ground state dipole moment function, were used to generate cross sections for photodissociation through the B'-X and A-X transitions and through the X state, itself. Calculations were performed for the full range of rovibrational levels (v,R) in the ground electronic state.

These photodissociation cross sections have been included in the PHOENIX stellar atmosphere code. The new models, calculated using spherical geometry for all gravities considered, also incorporate our latest database of nearly 670 million molecular lines, and updated equations of state (EOS).

This work was supported in part by NSF grants AST-9720704 and AST-0086246, NASA grants NAG5-8425, NAG5-9222, and NAG5-10551 as well as NASA/JPL grant 961582. Some of the calculations were performed on the IBM SP2 of the UGA UCNS, on the IBM SP ``Blue Horizon'' of the San Diego Supercomputer Center, with support from the NSF, and on the IBM SP of the NERSC with support from the DoE.

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The author(s) of this abstract have provided an email address for comments about the abstract: weck@physast.uga.edu

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