Astron. Astrophys. 324, 185-195 (1997)

Model atmospheres of cool, low-metallicity stars: the importance of collision-induced absorption

Aleksandra Borysow ^{1, 2}, Uffe Gråe Jorgensen ¹ and Chunguang Zheng <sup>2

1</sup> Niels Bohr Institute, Copenhagen University Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
² Physics Department, Michigan Technological University, Houghton, MI 49931, USA

Received 29 July 1996 / Accepted 12 December 1996

Abstract

We have extended our data base of collision induced absorption (CIA) in the high-temperature regime applicable to stellar atmospheres. Improvements of existing data include computation of series of hot-bands and extension of the spectral wings much further away from the maxima than hitherto. Computation of new bands includes the first and the second overtone bands of H₂ -H₂. We apply these data to an extensive grid of oxygen-rich model atmospheres with scaled solar metallicities in order to investigate for which range of fundamental stellar parameters (i.e., effective temperature, gravity, and chemical composition) CIA has a significant impact on the atmospheric structure. Besides the CIA due to H₂ -H₂ and H₂ -He pairs, our models include complete molecular line data for TiO, H₂ O, CN, CH, and SiO.

For stellar models with low effective temperatures, high gravity, and low metallicity, the atmospheric structure and the emergent spectrum are completely dominated by the effects of CIA. For our test-models of lowest effective temperature and lowest metallicity ( = 2800 K and Z = 10^-3 ) the effect of CIA is pronounced even for sub-giant stars with log(g) = 2.0. For dwarf models with Z = 10^-3 and log(g) = 5.0 the effects are visible in the overall flux distribution for effective temperatures as high as 4000 K, and for models with = 2800 K and log(g) = 5.0 CIA has effects on the spectrum of stars with metallicities as high as 0.1 .

Key words: molecular data molecular processes stars: atmospheres stars: late-type stars: low-mass infrared: stars

Send offprint requests to: Aleksandra Borysow

Contents

• 1. Introduction
• 2. CIA opacities from existing quantum mechanical sources
  • 2.1. H₂ -H₂ roto-translational band
  • 2.2. H₂ -H₂ fundamental band
  • 2.3. H₂ -He roto-translational band
  • 2.4. H₂ -He fundamental band
  • 2.5. H₂ -He higher overtones
• 3. New H₂ -H CIA opacities
  • 3.1. The first overtone band
  • 3.2. The second overtone band
  • 3.3. The combined quantum mechanical CIA data
• 4. Comparison with previous data
• 5. Impact of CIA on stellar model atmospheres and synthetic spectra
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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