Spectrum synthesis of cool stars
Date
1987
Authors
Brett, John Marshall
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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.
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