H<i>α</i>and the Ca II H and K lines as activity proxies for late-type stars

Abstract

Context.The main chromospheric activity indicator is the S index, which is the ratio of the flux in the core of the Ca II H and K lines to the continuum nearby, and is well studied for stars from F to K. Another chromospheric proxy is the Hα line, which is believed to be tightly correlated with the Ca II index.