Line profile variations in M giants: Clues to mass-loss and chromospheric heating mechanisms

We analyze time-series, high dispersion spectra of the Mg II h and k and Ca II H and K lines of the semiregular giants ρ Per (M4 H-III, periodicity P∼50 days), R Lyr (M5 III, P∼46 days), and g Her (M6 III, P∼90 days). The targets were strategically selected to study the relative importance of convective motions and global stellar pulsations in determining the structure of the outer atmospheres. These first or second ascent stars lie in a crucial region of the HR diagram where circumstellar dust shells begin to form. We relate line profile variations to photospheric variations using the Fine Error Sensor on the IUE satellite and ground-based UBV photometry. We have detected small amplitude (typically ±10%) but real changes in the profiles of Mg II and Ca II lines in all three stars which are not obviously related to underlying changes in the photospheres. We argue that the observed variability is due to changes in chromospheric conditions and not variations within the circumstellar shell. The observations support the picture of a steady-state "chromosphere" which is modulated on long time scales (∼weeks). In g Her, we find evidence for a localized heating event which we tentatively assign to the overshooting of a large supergranule cell. Unfortunately, some important conclusions of related work by Eaton et al. [ApJ, 364, 259 (1990)] warrant a re-examination in the light of our analysis. We conclude that the geometric scales over which most of the energy is deposited in the outer atmospheres must be much less than a stellar radius, in contrast to large amplitude variables which are only slightly more evolved.