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Astron. Astrophys. 363, 1065-1080 (2000)

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H2 in the 2 micron infrared spectra of long period variables

I. Observations

K.H. Hinkle 1, B. Aringer 2, T. Lebzelter 2, C.L. Martin 1,3 and S.T. Ridgway 1

1 Kitt Peak National Observatory, National Optical Astronomy Observatories *, 950 N. Cherry Avenue, P.O. Box 26732, Tucson, AZ 85726, USA
2 Institut für Astronomie, Türkenschanzstrasse 17, 1180 Vienna, Austria
3 Present Address: STScI, 3700 San Martin Drive, Baltimore, Maryland 21218, USA

Received 11 July 2000 / Accepted 20 July 2000

Abstract

Up to seven lines in the S branch of molecular hydrogen have been detected in the 2 µm infrared absorption line spectra of long period variables. The H2 lines show very strong phase dependent changes in velocity, equivalent width, and line profile, with the strongest lines near minimum light. The strength of the H2 lines also depends on variable type with Mira variables having deep lines (at minimum light), SRa variables generally having detectable lines, and the SRb and non-variables having weak to non-detectable lines. In addition H2 line strengths depend on period and on C/O abundance, with long period (i.e. cool), pure S-type Miras having H2 lines with central depths of up to 80%. The appearance of H2 lines in the spectra of cool giants is linked with geometrical extension of the atmosphere. The greatly increased strength of the H2 lines in Mira variables is observational evidence of the role of stellar pulsation in greatly extending these stellar atmospheres. Weak emission is seen in H2 near maximum light in Miras. We suggest this is resonant scattering in the spherically extended stellar atmosphere. The significant role of H2 opacity in the blue is noted.

Key words: line: profiles – stars: AGB and post-AGB – stars: atmospheres – stars: variables: general – infrared: stars

* Operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation.

Send offprint requests to: K.H. Hinkle (hinkle@noao.edu)

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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