Late-type stars are among the most frequent classes of X-ray
sources. As suggested by the study of the Sun, their X-ray emission is
believed to be thermal emission from hot (
) coronal plasma.
Hundreds of late-type stars were already detected by the
Einstein and EXOSAT Observatories (cf. Vaiana et al. 1981)
with X-ray luminosities mostly ranging from about solar level
(i.e., 
) up to 
.
Recent ROSAT observations of a complete sample of the nearest
stars (Schmitt et al. 1995; Schmitt 1997) have demonstrated that
virtually all late-type main-sequence stars are X-ray sources.
It is generally accepted that the existence of outer convection
zones (starting at late A-types) as well as rotation are essential
conditions for magnetically heated stellar coronae. A relation
between X-ray luminosity and rotation velocity was first
established by Pallavicini et al. (1981), and gave an incentive
for numerous further studies in the context of the rotation-activity
relationship. In connection with
angular momentum evolution stellar X-ray emission can thus be
regarded as a rough age-indicator.
Contrary to main-sequence stars, not all late-type giants and supergiants were found as X-ray sources. In particular, IUE and Einstein observations suggested that in certain regions of the Hertzsprung-Russell (HR) diagram late-type giants possess hot transitions regions and coronae while in other regions such hot material is absent. The boundary between these different regions in the HR diagram is indicated by several dividing lines based on different observational criteria (cf. Linsky & Haisch 1979; Ayres et al. 1981; Haisch et al. 1991, 1992). Sensitive new ROSAT observations have substantially revised this picture. While Hünsch et al. (1996a) have shown from a complete, volume-limited sample that probably all but the very latest giants are X-ray sources of mostly about solar level, Reimers et al. (1996) detected X-ray emission from several bright giants and supergiants (the so-called hybrid stars) obviously violating the strict dividing line concept. Recently, Hünsch & Schröder (1996) suggested a scenario in which the observed coronal properties of late-type giants and supergiants can be qualitatively understood in the context of stellar and angular momentum evolution.
It is obvious that all such investigations are severely hampered by the scarcity of X-ray detected evolved stars. A compilation of the Einstein detections is given by Maggio et al. (1990) and includes 69 detected giants and supergiants of spectral types F to M. Additionally, for 310 such stars upper limits of X-ray emission were derived. However, we have to keep in mind that the Einstein observations only covered about 10% of the sky with rather inhomogeneous exposure times. The Maggio et al. data consist of both pointed observations on individual stars and serendipitious observations in the field-of-view of other targets.
Contrary to the Einstein observations, the ROSAT observatory scanned the whole sky during the first half year of its mission. The ROSAT all-sky survey (RASS) resulted in about 150000 detected X-ray sources, which form a flux-limited but otherwise unbiased sample (Voges et al. 1996a).
We have searched for X-ray detections of optically bright late-type giants and supergiants in the RASS data. The aim of this paper is to provide the X-ray data of those detections in the form of a homogeneous catalogue. We describe the way the catalogue was created and how the X-ray properties of the stars were derived. For a detailed astrophysical discussion we refer to an accompanying paper in the main journal (Hünsch & Schmitt 1998).
We further note that a similar catalogue of all RASS-detected bright OB-type stars (irrespective of luminosity class) has already been published by Berghöfer et al. (1996).