In principle, any late-type star should be able to sustain a corona, and henceforth, be an X-ray source. Indeed, already the Einstein Observatory detected X-ray emission from a large number of late-type stars (cf. Vaiana et al. 1981; Maggio et al. 1987). Detailed studies of the immediate solar environment (Schmitt et al. 1995; Schmitt 1997) 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; Schmitt (1997) argues that late-type stars emit X-rays at least at a level such that the apparent X-ray surface flux exceeds 104 ergcm-2 s-1. On the other hand, stellar X-ray luminosities appear to be bounded above by the so-called saturation limit /, which can be observed both for field stars and stars in open clusters, where one observes that the spectral type above which stars appear to be saturated moves toward later types with increasing age. This is probably related to the angular momentum evolution of young late-type main-sequence stars, which are spun down by magnetic breaking during their first yrs on the main-sequence. Enhanced activity can, however, last on much longer timescales, as indicated by example of the Hyades cluster.
Until now, the ROSAT observatory has undertaken the only sensitive X-ray survey (RASS) of the whole sky. These data provide a flux-limited but otherwise unbiased sample of X-ray sources (Voges et al. 1996a). Of the detected sources, about one third are considered to be coronal. Many of these coronal X-ray sources have optically faint counterparts which require optical follow-up observations. A smaller number of X-ray sources have bright optical counterparts, and we have systematically searched for X-ray emission from these optically bright stars. The results of our survey are presented as catalogs of X-ray data, which have already been published for OB stars (Berghöfer et al. 1996) and late-type giants and supergiants (Hünsch et al. 1998; hereafter HSV98).
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