H α Emission among Evolved FGKM Stars: A Compendium from the Literature

Emission in the wings of the H α line among late-type giant stars occurs as a consequence of conditions in their chromospheres. Data from the literature have been used to document the incidence of H α emission among Population I giant stars as a function of position in various color–magnitude diagrams (CMDs), including infrared versions that plot M K s versus ( J − K s ) and ( V − K s ). Population I and metal-poor Population II red giants are compared with regard to certain characteristics of H α emission. In the case of stars with emission, both observed photometry and photometry corrected for interstellar reddening are considered. Population I and II giants having H α emission are found to share a number of phenomena: (a) they occupy similar regions of the visual and infrared CMDs; (b) the emission in the blue and red wings is often of unequal strength, suggestive of mass motions in the chromosphere; (c) there are indications of time variability for a few stars that have been observed on multiple occasions; (d) red giants of both Population I and II having H α emission are often long-period or semiregular variables; and (e) a subset of them, but by no means all, evince circumstellar dust. While some giants with H α emission are pulsating red variables that have formed dusty circumstellar envelopes, others may be in prior phases of evolution where pulsations have commenced, which is restructuring the chromosphere and producing outflows, but has not yet generated the conditions needed for dust formation at higher circumstellar altitudes.