Ca II K Emission Line Asymmetries Among Red Giants

Abstract

In the spectra of red giants the chromospheric emission feature found in the core of the Ca II K line often exhibits an asymmetric profile. This asymmetry can be documented by a parameter V/R which is classified as > 1, 1, or < 1 if the violet wing of the emission profile is of greater, equal, or lower intensity than the redward wing. A literature search has been conducted to compile a V/R dataset which builds on the large survey of bright field giants made by Wilson (1976). Among stars of luminosity classes II–III–IV the majority of those with V/R > 1 are found to be bluer than B-V =1.3, while those with V/R < 1 are mostly redder than this colour. Stars with nearly symmetric profiles, V/R≈ 1, are found throughout the colour range 0.8 < B-V < 1.5. There is no sharp transition line separating stars of V/R > 1 and < 1 in the colour-magnitude diagram, but rather a ‘transition zone’ centered at B-V ≈ 1.3. The center of this zone coincides closely with a ‘coronal dividing line’ identified by Haish, Schmitt and Rosso (1991) as the red envelope in the H–R diagram of giants detected in soft x-ray emission by ROSAT. It is suggested that both the transition to a Ca II K emission asymmetry of V/R < 1 and the drop in soft x-ray activity across the coronal dividing line are related to changes in the dynamical state of the chromospheres of red giants. By contrast, the onset of photometric variability due to pulsation occurs among stars of early-M spectral type, that are redward of the mid-point of the Ca II V/R ‘transition zone’, suggesting that the chromospheric motions which produce an asymmetry of V/R < 1 are established prior to the onset of pulsation.