Astron. Astrophys. 341, 751-767 (1999)

1. Introduction

Observations of open clusters allow us to study the evolution of physical quantities during the main sequence life of stars. In the last few years, ROSAT observations have explored coronal properties in a large number of open clusters: Pleiades (Stauffer et al. 1994, hereafter Paper I, Schmitt et al. 1993, Gagne et al. 1995, Micela et al. 1996, hereafter Paper II), Hyades (Stern et al. 1992, 1994; Pye et al. 1994), IC 2391 (Patten & Simon 1993; 1996; Simon & Patten 1998), NGC 6475 (James & Jeffries 1997; Prosser et al. 1995a), IC 2602 (Randich et al. 1995), NGC 2516 (Dachs & Hummel 1996, Jeffries et al. 1997), Praesepe (Randich & Schmitt 1995), Per (Randich et al. 1996a, Prosser et al. 1996, Prosser & Randich 1997, Prosser et al. 1997), Coma (Randich et al. 1996b), NGC 2422 (Barbera et al. in preparation), IC 4665 (Giampapa et al. 1998), NGC 752 (Belloni & Verbunt 1996), NGC 6940 (Belloni & Tagliaferri 1997) and IC 4651 (Belloni & Tagliaferri 1998). These observations have substantially increased our knowledge of the evolution of X-ray activity levels of late-type stars.

Notwithstanding this flourishing of new data, a number of issues remain unresolved. For example, the origin of the spread of the X-ray luminosity function (XLF) for open cluster members of a given spectral type is not understood: Thus, while it is known that this spread is smaller than the corresponding spread for field stars of the same spectral type, that the mean (and median) decrease with age, and that fast rotators have activity levels higher than those of slow rotators, it is nevertheless unclear if the observed spread can be attributed simply to intrinsic variability of coronal emission. Furthermore, we do not understand how the evolution of X-ray luminosity depends on stellar mass, nor do we understand if and how this evolution is different for single and binary stars.

The Pleiades cluster represents a key sample of stars in any study of these issues: It is one of the best-studied open clusters, with a fairly complete membership list down to the mid-M dwarfs in the central region where our HRI fields are located, and has been the subject of an extensive campaign to measure rotational velocity and photometric periods (cf. Prosser et al. 1993a, 1993b, 1995b; Jones et al. 1996, Krishnamurthi et al. 1998, Queloz et al. 1998). Furthermore, its richness and proximity make it one of the best X-ray targets because it is possible to detect a significant fraction of the cluster stars with relatively small numbers of distinct pointings (in contrast to the Hyades, for example, in which the loose spatial distribution makes it quite time-consuming to obtain good coverage of its members). As a consequence, the Pleiades is the most deeply studied open cluster at X-ray wavelengths; since the launch of Einstein , X-ray survey observations have been reported by Caillault & Helfand (1985), Gagné et al. (1995), Micela et al. (1985, 1990, 1996), Schmitt et al. (1993), and Stauffer et al. (1994).

In order to explore some of the unresolved issues mentioned above, it is important to gain as complete a knowledge as possible of the X-ray emission properties of the Pleiades stars. As a further step in toward that goal, this paper reports and summarizes the full set of ROSAT HRI observations devoted to the investigation of this cluster.

Our paper is organized as follow: in Sect. 2 we describe the observations and data analysis and present data for the Pleiades stars; we discuss the relationship between the X-ray luminosity and rotation in Sect. 3 and summarize our results in Sect. 4. In the appendix A we present X-ray data for field stars, in appendix B, for sources not identified with cataloged objects together with new photometric observations of their optical counterparts. In the appendix C we present CCD photometry of cataloged stars around unidentified sources.

© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998
helpdesk.link@springer.de