1996A&A...310..801H

We have investigated a complete sample of the nearest 39 late type giants (d≤25pc) for which we have probed the X-ray luminosity function with unprecedented sensitivity by deep (3...18ksec) ROSAT PSPC-observations in the pointed mode, together with ROSAT All-Sky survey (RASS) data. We confirm the X-ray dividing line for luminosity class III giants as proposed by Haisch et al. (1991, 1992) and we find evidence, that essentially all luminosity class III giants with B-V<1.2 or spectral type <K3, that is to the hot side of the X-ray dividing line in the HR diagram, show X-ray emission with typical luminosities of about 10²⁷erg/s. A few stars like β Cet have considerably higher X-ray luminosities. On the right, cool side of the X-ray dividing line, we find only one X-ray emitting star in our sample, while the other stars have upper limits for their ratios of X-ray to bolometric luminosities as low as logL_x/L_bol≤-10. We find observational evidence for a minimum X-ray surface flux to the left of the dividing line of the order of ≃100erg/cm²/s, which seems to be independent of B-V colour. The data and their implications for our understanding of coronae of late-type giants, in particular the questions concerning the dividing lines and possible acousting heating of the coronae are discussed in some detail.