Stellar chromospheres : H alpha and CA II K profiles.

A set of medium to high-resolution observations of H-alpha and Ca II K lines in a sample of Population I stars is presented in order to examine the systematics of H-alpha absorption profiles and to determine empirically the extent to which velocity fields observed therein are reflected in the chromospheric component of the Ca II line. For main-sequence stars, bright Ca II K emission profiles accompany shallow H-alpha lines with sharp central cores, unlike the apparently U-shaped H-alpha cores of stars displaying relatively weak Ca II K emission. For giants and supergiants, the H-alpha line is generally wider than a computed LTE photospheric profile, with significant K(3) absorption present in the Ca II K(2) reversal profile. The excess widths appear to correlate with the strength of the K(3) absorption. Estimates of the strength of Ca II K(3) indicate severe modifications of Ca II K(2) widths and intensities, strongly affecting the cooling role of Ca II K in the upper chromospheres.