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

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 <sup>1

1</sup> 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

Abstract

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  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 to  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

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