Spectrum synthesis of cool stars

Abstract

We present a series of papers with the central theme of molecular band spectrum synthesis of cool stars. These papers present significant new results on the topics of molecular opacities in stellar atmospheres, the importance of extension in luminous cool stars, abundances of luminous and extended stars of the SMC and in the modelling of variable star atmospheres. A full description of the results of these investigation is given in the introduction from which the following is abstracted. We derive new TiO and VO opacities, which when combined with published opacities, provide a significant improvement in the ability to synthesise spectra of late M giants. This opacity data set serves as a basis for the following papers in the series. Construction of model atmospheres shows that the effect of these new opacities upon the atmospheric structure is small but that significant (up to 17%) changes in some molecular band strengths can be expected. An extensive investigation (with an emphasis on observable properties) into model photospheres of extended red giants provides much quantative information upon the effect of temperature, gravity and extension upon red giant photospheres. Photometric indices computed for the grid compare favourably to observations. A system of narrow band filters for separately measuring Teff, gravity and extension is devised and demonstrated. A pioneering effort at modelling mira atmospheres, in which the assumption of hydrostatic equilibrium is removed, reveals significant differences to static models and gives encouraging comparisons to observations. Synthetic spectra for a red giants with a variety of C, N, and O abundances relevant for AGB stars of the SMC are computed. These reveal that N enhancements such as would occur for envelope burning should be detectable from low resolution spectra of hotter M giants. However, an abundance analysis of a sample of the luminous and extended non-carbon upper AGB stars of the SMC is undertaken which reveals no evidence for operation of the hypothetical envelope burning process to the degree considered in our computations.