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Astron. Astrophys. 361, 581-593 (2000)

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1. Introduction

The discovery of the TW Hydrae association as a nearby group of co-moving T Tauri Stars (TTS) far from molecular clouds has motivated the search for additional associations of pre-main sequence stars (PMS) close to Earth. In the course of a study of the Hipparcos catalog around IRAS 60[FORMULA] sources Zuckerman & Webb (2000) recently announced the identification of a co-moving group of stars with general youth indicators.

Youth can be deduced from the presence of the [FORMULA] Li absorption line, H[FORMULA] profile, rotation rate, IR excess indicative of circumstellar material, and X-ray emission. While any of these properties certainly are not sufficient to establish the youth of a star, the common presence of several of these characteristics is suggestive for PMS nature. However, no accurate age determination is possible by these means. In addition, for all but one probable Tucanae members, Hipparcos parallaxes are available (see Zuckerman & Webb 2000 and our Table 2 and Table 3), so that the stars can be placed accurately into the H-R diagram. They all fall near and/or above the zero-age main sequence, hence are really young.

Based on proper motion, distance, and the above mentioned youth indicators Zuckerman & Webb (2000) have divided their sample of potential Tucanae members in two groups: probable members with common distance, space motion, and general signs of youth, and improbable members. This latter group consists of stars which either could be members without signs of youth, or stars which by chance have distances and proper motion similar to the association. Zuckerman & Webb (2000) have identified 11 of the probable members with X-ray sources from the ROSAT All-Sky Survey Bright Source Catalog (RASS BSC; Voges et al. 1999), but only one of the non-members.

In this paper we present a more detailed study of the X-ray emission of Tucanae candidates based on data obtained from the ROSAT Public Data Archives. In particular we use X-ray luminosity distribution functions (XLDFs) to obtain further clues to the age of this association. Comparative studies of young stellar clusters and star-forming regions based on observations by the Einstein IPC (see e.g. Feigelson & Kriss 1989, Damiani et al. 1995) have shown that the X-ray luminosity decays with stellar age. This is manifest in a decrease of the median of [FORMULA] and a corresponding shift of the XLDFs with respect to each other. A comparison of the XLDF for the Tucanae stars with that for well studied star forming regions should therefore allow to put constraints on the age of this association.

In Sect. 2 we describe the observations and analysis of the raw data. We also provide tables which summarize the X-ray properties of detected and undetected potential Tucanae members. The XLDFs of Tucanae candidates and comparison samples are discussed in Sect. 3. In Sect. 4 we present the X-ray lightcurves of all detected stars and discuss their variability. Our results are summarized in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000