Astron. Astrophys. 339, 457-466 (1998)

Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources

VI. K to M-type stars south of Taurus-Auriga

F.-J. Zickgraf ¹, J.M. Alcalá ^{4, 6}, J. Krautter ², M.F. Sterzik ^{5, 3}, I. Appenzeller ², C. Motch ¹ and M.W. Pakull <sup>1

1</sup> Observatoire Astronomique de Strasbourg, 11, rue de l'Université, F-67000 Strasbourg, France
² Landessternwarte Königstuhl, D-69117 Heidelberg, Germany
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, Postfach 1603, D-85740 Garching, Germany
⁴ Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy
⁵ European Southern Observatory, Alonso de Cordova 3107, Santiago 19, Chile
⁶ Instituto Nacional de Astrofísica, Optica y Electrónica, A.P. 51 y 216 C.P. 72000, Puebla, México

Received 14 April 1998 / Accepted 27 July 1998

Abstract

We present first results of a spectroscopic study of lithium absorption lines in late-type stars at high galactic latitude. The investigated stars were identified previously as optical counterparts of ROSAT All-Sky Survey sources. They are contained in a complete X-ray count-rate limited representive subsample of the RASS which was completely optically identified by our group. No selection criteria other than requiring RASS count rates larger than 0.03 cts s^-1 were applied. The stars are located at galactic latitudes between and in an area south of the Taurus-Auriga star forming region. In the sample of 35 stars with spectral types between K0 and M5 we found 9 stars with significant Li I 6708 absorption lines, 8 K stars and 1 M0 star. No lithium was found in stars with spectral types later than M0. The Li I equivalent widths (corrected for the contribution of Fe I ) are between 150 mÅ and 400 mÅ . The lithium strength of these stars is consistent with ages on the order of 10-30 Myr. These stars are still in a pre-main sequence evolutionary stage, and belong to the weak-line Tauri stars.

Key words: surveys stars: formation stars: late-type stars: pre-main sequence X-rays: stars

Send offprint requests to: F.-J. Zickgraf

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Contents

• 1. Introduction
• 2. The observed sample
• 3. Observations
• 4. Results
• 5. Discussion
  • 5.1. Age of the lithium-rich stars
  • 5.2. Comparison with galactic distribution models
  • 5.3. The lithium-rich stars and the Gould Belt
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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