Astron. Astrophys. 323, 202-210 (1997)

SiO rotation-vibration bands in cool giants

I. A grid of model spectra for different stellar parameters

B. Aringer ¹, U.G. Jorgensen ² and S.R. Langhoff <sup>3

1</sup> Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, A-1180 Wien, Austria aringer@astro.ast.univie.ac.at
² University Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark uffegj@nbivms.nbi.dk
³ NASA Ames Research Center, Moffett Field, USA langhoff@pegasus.arc.nasa.gov

Received 10 September 1996 / Accepted 28 November 1996

Abstract

In order to study the behavior of the SiO bands of cool oxygen-rich stars we have computed a grid of 138 hydrostatic model atmospheres using an improved version of the MARCS code and new SiO line data. We found that the corresponding opacity never produces large changes of the atmospheric structure, especially if one compares it with the enormous effects from TiO or water. Based on the atmospheres, synthetic SiO spectra have been calculated with a high resolution for the photometric L-band and with a low resolution (opacity sampling) for the large wavelength range between 2.0 and 12.5 µm, which includes the fundamental as well as the first and second overtone transitions. Thus, we could study the behavior of the SiO bands as a function of different stellar parameters. It turned out that the intensity of the absorption features monotonously increases with lower temperature and gravitational acceleration as well as with higher metallicity. We expect the strongest bands in the coolest and most extended objects. When we compared our results to observations of K and M giants, we found a good agreement showing the correctness of the line data and atmospheres. On the other hand the calculations can not reproduce the relatively weak and variable bands of AGB stars indicating that our classical model atmospheres may not always work for this kind of objects. This will be discussed in a second paper, where we will focus on our observations of AGB stars.

Key words: stars: atmospheres stars: fundamental parameters stars: late-type stars: AGB and post-AGB infrared: stars

Send offprint requests to: B. Aringer

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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