Chromospheric Ca II H and K emission among subdwarfs

Abstract

Echelle spectra have been obtained of the Ca ii H and K lines for a sample of metal-poor subdwarf stars as well as for a number of nearby Population I dwarfs selected from among those included in the Mount Wilson HK survey. The main conclusion of this paper is that Ca ii H- and K-line emission does occur among subdwarfs. It is particularly notable among those subdwarfs with colours of B−V≥0.75; all such stars observed exhibit chromospheric emission, although emission is observed among some subdwarfs bluer than this colour. The Ca ii K emission profile in most subdwarfs exhibits an asymmetry of V/R>1, similar to that seen in the integrated light of the solar disc. Two quantitative indicators of the contrast between the peaks in the emission profile and the neighbouring photospheric line profile are introduced. Measurements of these indicators show that the level of Ca ii emission among the subdwarfs is similar to that among low-activity Population I dwarfs.