Philippe F. Weck

The Molecular Line Opacity of MgH in Cool Stellar Atmospheres

A new, complete, theoretical rotational and vibrational line list for the A 2Π ← X 2Σ+ 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 2Σ+ ← X 2Σ+ and the X 2Σ+ ← X 2Σ+ (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 A. 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 A, with the discrepancy increasing with decreasing effective temperature.

Theoretical study of the rovibrationally resolved transitions of CaH

Comprehensive absorption line lists for 40CaH have been calculated for electronic transitions from the Xu200a2Σ+ ground state to the Au200a2Π, B/B′u200a2Σ+, and Eu200a2Π low-lying excited states. The lists include transition energies and oscillator strengths for all possible allowed transitions and were computed using the most recent set of theoretical potential energy curves and dipole transition moment functions, with adjustments to account for experimental dissociation energies and asymptotic limits. Good agreement with previous calculations and available experimental data has been obtained. Oscillator strengths for the transition from the Xu200a2Σ+ state to the Du200a2Σ+ state are also given, but due to the large uncertainty of the available dipole transition moment function the line list is highly uncertain. For the Cu200a2Σ+←Xu200a2Σ+ transition, a Franck–Condon approximation has been used.

New Theoretical Line List for the B′ 2Σ+←X 2Σ+ System of 24MgH

Fully quantum-mechanical techniques have been applied to compute the complete line list for the B 2Σ+ ← X 2Σ+ system of 24MgH. The list includes transition energies and oscillator strengths over the wavelength range 11,850-32,130 cm-1, for all possible allowed transitions from the ground electronic state vibrational levels v = 0-11. This list was computed using the best available ab initio potential energies and dipole transition moment function, with the former adjusted to account for experimental data. The agreement of the current calculations with previous theoretical results and the line list of Kurucz is discussed. Although spectroscopic accuracy for a particular line cannot be claimed for our calculations, comparison is made with the recent astronomical measurements of Wallace et al. A line list for pure rovibrational transitions in the X 2Σ+ state is also presented.

The Molecular Continuum Opacity of 24 MgH in Cool Stellar Atmospheres

The opacity due to photodissociation of 24MgH in the atmospheres of cool stars is investigated. The lowest two electronic transitions A 2Π ← X 2Σ+ and B 2Σ+ ← X 2Σ+ are considered, where the cross sections for the latter were published previously (Weck, Stancil, & Kirby) and 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 2Π ← X 2Σ+ 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 A for all rotational transitions from the vibrational levels v = 0-11. Assuming a Boltzmann distribution of the rovibrational levels of the X 2Σ+ 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 2Π state near threshold, characterize the LTE photodissociation cross sections. A sum rule is proposed as a check on the accuracy of the photodissociation calculations.

B'← X Rovibrationally Resolved Photodissociation of 24MgH

New accurate photodissociation cross sections have been obtained for the B 2Σ+ ← X 2Σ+ transition of 24MgH using a combination of ab initio and experimentally derived potential curves and dipole transition moments. The partial cross sections have been evaluated for all rotational transitions from the vibrational levels v = 0-11 and over the entire accessible wavelength range λλ1770-4560 A. In particular, all shape resonances arising from rotational predissociation of quasi-bound levels of the B state have been obtained near threshold. Assuming a Boltzmann distribution of the rovibrational levels of the X 2Σ+, LTE photodissociation cross sections are presented for temperatures between 1000 and 10,000 K.

Ab initio configuration interaction study of the low-lying 1Σ+ electronic states of LiCl

Ab initio configuration interaction calculations have been performed for the X 1Sigma+ and B 1Sigma+ electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R = 2.5a0 and 50a0. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R = 16.2a0, in close agreement with previous calculations using a semiempirical covalent-ionic resonance model. X 1Sigma+ state spectroscopic constants are in excellent agreement with experimental values.