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

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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 1

1 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 [FORMULA] between [FORMULA] and [FORMULA] in an area [FORMULA] 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 [FORMULA]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 [FORMULA]150 mÅ and [FORMULA]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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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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