1 Introduction

One of the major results obtained with the imaging X-ray telescopes flown onboard the Einstein and ROSAT observatories is the ubiquity of X-ray emission from normal stars throughout the whole Hertzsprung-Russell (HR) diagram, the only exceptions being A-type main sequence stars and some of the late-type giants and supergiants. However, the more sensitive ROSAT observations have revealed X-ray emission even from those types of stars.

X-ray emission from late-type stars is generally attributed to magnetically heated stellar coronae. As suggested by the example of the Sun's corona, the X-ray emitting hot $(\gt\!\! 10^6\,{\rm K})$ plasma is believed to be confined by coronal magnetic fields, which ultimately originate from the interaction between rotation and outer convection zones. The solar X-ray luminosity varies approximately between $3 \ 10^{25}$ and $1\ 10^{27}$ergs^-1 (extrapolated to a 1-300 Å "bolometric X-ray band''; cf. Haisch & Schmitt 1996; Acton 1996) during the solar cycle and is known to be strongly correlated with other magnetic activity indicators (e.g., sunspot numbers, flare frequency, chromospheric CaII emission). Hence, X-ray luminosity can be regarded as a good activity indicator also for other late-type stars. Conversely, X-ray emission from O- and early B-type stars obeys (approximately) a $L_{\rm x}$/$L_{\rm bol} \approx 10^{-7}$ relation and is attributed to shocks generated and dissipated within the radiatively driven winds of those stars.

A thorough investigation of the properties of stellar X-ray emission requires large samples of stars in order to reduce the effects of individual scatter and to construct samples of stars with given properties such as mass, age, rotation rate etc. Especially important are complete, volume-limited samples of stars, for example, to derive X-ray luminosity distribution functions. In principle, there are two different approaches: i) to construct such a complete sample and to look for X-ray emission from those stars afterwards, and ii) to scan the whole sky for X-ray emission and then to extract the data for the sample stars (input positions). The first method has been applied by Schmitt et al. (1995) and Schmitt (1997) for the immediate solar environment, i.e., the K- and M-type stars within 7 pc and the A-, F-, and G-type stars within 13 pc from the Sun, respectively. These investigations revealed that virtually every late-type dwarf star with spectral type later than A7 can be detected as an X-ray source given data of sufficient sensitivity. These data are from both the ROSAT all-sky survey (RASS) and deep individual pointed ROSAT observations. A similar investigation on late-type giants within 25 pc has been performed by Hünsch et al. (1996).

The second method requires the existence of a sufficiently deep all-sky survey. Until now, the ROSAT observatory has undertaken the only sensitive X-ray survey. These data provide a flux-limited but otherwise unbiased sample of X-ray sources (Voges et al. 1996a). Of the $\approx\! 1.5 \ 10^5$ detected sources, about one third are considered to be coronal. We used these data to search for X-ray emission from stars contained in the Catalogue of Nearby stars (the so-called Gliese catalogue; Gliese & Jahreiß 1991). The Gliese catalogue is the most comprehensive list of stars in the solar environment and it is proposed to be as complete as possible within a space volume of 25 pc radius around the Sun. Our investigation is limited in completeness by both the completeness of the Gliese catalogue and the sensitivity of the RASS. However, also for those Gliese stars that are not detected, it is in principle possible to estimate upper limits for their X-ray luminosity (see Sect. 2.1).

A preliminary investigation of stellar activity of the nearby stars based on the data presented here has already been performed by Sterzik & Schmitt (1997). However, X-ray luminosities for those stars derived from ROSAT data combined with Hipparcos parallaxes are presented here for the first time.

We have also performed similar surveys of X-ray emission from the stars contained in the Bright Star catalogue (BSC; Hoffleit & Warren 1991). These results are presented as catalogues of X-ray data, which have already been published for OB stars (Berghöfer et al. 1996), late-type giants and supergiants (Hünsch et al. 1998a; hereafter HSV98), and late-type main-sequence stars and subgiant stars (Hünsch et al. 1998b).