Ca II K emission-line asymmetry among red giants detected by the ROSAT satellite

Spectra of the Ca II H and K lines are reported for a number of field giant s from which soft X-ray emission was detected by the ROSAT satellite. Sever al of these stars are RS CVn systems and exhibit very strong Ca II emission . The majority of the noninteracting giants in the sample have as M-V > -2. 0, determined from Hipparcos parallaxes, and spectral types earlier than K3 . The Ca II K emission-line profile for these stars is most often double-pe aked and asymmetric, with the short-wavelength peak being stronger than the longward peak. This asymmetry is in the same sense as for the integrated d isk of the Sun. The X-ray and Ca II K-line data indicate that giants of spe ctral types G and early K have coronae and chromospheres seemingly analogou s to those of the Sun. Four M giants that were detected by ROSAT were also observed. Their Ca II emission spectra show asymmetries in which the violet wing is weaker than the red wing, a phenomenon that is typical of M giants in general and indicative of mass outflows in their chromospheres. The maj ority of these M giants, but not all, are known to be in binary systems, so it is possible that the X-ray emission for at least some of them may come from a companion. Alternatively, some or all of these M giants may be examp les of hybrid stars.