EDP Sciences
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Volume 384, Number 3, March IV 2002
Page(s) 965 - 981
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020063

A&A 384, 965-981 (2002)
DOI: 10.1051/0004-6361:20020063

CO 1st overtone spectra of cool evolved stars: Diagnostics for hydrodynamic atmosphere models

J. H. Bieging, M. J. Rieke and G. H. Rieke

Steward Observatory, The University of Arizona, Tucson AZ 85721, USA

(Received 4 October 2001 / Accepted 11 January 2002 )

We present spectra covering the wavelength range 2.28 to 2.36  $\mu$m at a resolution of $\Delta \lambda$ = 0.0007  $\mu$m (or R = 3500) for a sample of 24 cool evolved stars. The sample comprises 8 M supergiants, 5 M giants, 3 S stars, 6 carbon stars, and 2 RV Tauri variables. The wavelengths covered include the main parts of the 12C 16O v = 2-0 and 3-1 overtone bands, as well as the v = 4-2 and 13CO v = 2-0 bandhead regions. CO lines dominate the spectrum for all the stars observed, and at this resolution most of the observed features can be identified with individual CO R- or P-branch lines or blends. The observed transitions arise from a wide range of energy levels extending from the ground state to $E/k~>~20\,000$ K. We looked for correlations between the intensities of various CO absorption line features and other stellar properties, including IR colors and mass loss rates. Two useful CO line features are the v = 2-0 R14 line, and the CO v = 2-0 bandhead. The intensity of the 2-0 bandhead shows a trend with K-[12] color such that the reddest stars (K-[12] > 3 mag) exhibit a wide range in 2-0 bandhead depth, while the least reddened have the deepest 2-0 bandheads, with a small range of variation from star to star. Gas mass loss rates for both the AGB stars and the red supergiants in our sample correlate with the K-[12] color, consistent with other studies. The data imply that stars with $\dot M_{\rm gas} < 5\times 10^{-7}~M_{\odot}$ y -1 exhibit a much narrower range in the relative strengths of CO 2-0 band features than stars with higher mass loss rates. The range in observed spectral properties implies that there are significant differences in atmospheric structure among the stars in this sample.

Key words: stars: AGB and post-AGB -- stars: atmospheres -- stars: supergiants -- infrared: stars

Offprint request: J. Bieging, jbieging@as.arizona.edu

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