Astron. Astrophys. 326, 647-654

On the widespread Weak-Line T-Tauri population detected
in the ROSAT All-Sky Survey^*

F. Favata¹, G. Micela², and S. Sciortino²

¹Astrophysics Division - ESA/ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
²Istituto e Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy

Received 14 January 1997 / Accepted 14 May 1997

Abstract

We discuss the apparent widespread presence of Weak-Line T-Tauri stars (WTTS) among stellar coronal sources detected in the ROSAT All-Sky Survey (RASS), and their relative number with respect to young main-sequence stars in the same samples. The approach taken in most of the current literature for identifying and classifying WTT stars among RASS X-ray sources is based on the usage of low-resolution optical spectra only and on simple, mass-independent thresholds on the equivalent width of the Li I 6707.8 Å doublet. We show that this approach is likely to lead to putative WTTS samples which contain a large number of normal, young main-sequence stars masquerading as WTTS sources. Young main-sequence stars are known to be the dominant contributor in stellar X-ray selected samples at the limiting flux levels of the RASS, yet they appear to be very rare in the RASS samples discussed here. We argue that many of the putative WTTS sources are actually mis-classified young main-sequence stars, and that thus there is likely not a true ''WTTS question'' in the RASS samples.

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

^*Based on observations collected at the ESO La Silla observatory

Send offprint requests to: F. Favata (ffavata@astro.estec.esa.nl)

© European Southern Observatory (ESO) 1997

Online publication: September 18, 1997
Last change: October 15, 1997
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