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 giants from which soft X-ray emission was detected by the ROSAT satellite. Several 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-peaked and asymmetric, with the short-wavelength peak being stronger than the longward peak. This asymmetry is in the same sense as for the integrated disk of the Sun. The X-ray and Ca II K-line data indicate that giants of spectral types G and early K have coronae and chromospheres seemingly analogous 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 majority 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 examples of hybrid stars.