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Astron. Astrophys. 348, 579-583 (1999)

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Water vapor absorption in early M-type stars

M. Matsuura 1,2, I. Yamamura 3,2, H. Murakami 1, M.M. Freund 1,4 and M. Tanaka 1

1 Institute of Space and Astronautical Science (ISAS), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
2 Department of Astronomy, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
3 Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
4 NASA Ames Research Center, MS 239-4, Moffett Field, CA 94035-1000, USA

Received 27 October 1998 / Accepted 26 May 1999


The spectrometers onboard the Infrared Telescope in Space (IRTS) reveal water vapor absorption in early M-type stars, as early as M2. Previous observations detected H2O vapor absorption only in stars later than M6, with the exception of the recent detection of H2O in [FORMULA] Peg (M2.5 II-III). In our sample of 108 stars, 67 stars have spectral types earlier than M6. The spectral types are established by means of their near-infrared colors on a statistical basis. Among the 67 stars of spectral types earlier than M6, we find water vapor absorption in six stars. The observed absorption features are interpreted using a local thermodynamic equilibrium model. The features are reasonably fitted by model spectra with excitation temperatures of 1000-1500 K and water column densities of [FORMULA] to [FORMULA] cm-2. These numbers imply that the H2O molecules are present in a region of the atmosphere, located above the photosphere. Furthermore, our analysis shows a good correlation between the H2O absorption band strength, and the mid-infrared excess due to the circumstellar dust. We discuss the relation between the outer atmosphere and the mass loss.

Key words: infrared: stars – stars: late-type – stars: atmospheres – stars: circumstellar matter – stars: variables: general – surveys

Send offprint requests to: M. Matsuura (mikako@astro.isas.ac.jp)

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999