Chromospheric activity in Galactic open clusters

Abstract

Ca II observations of post-main-sequence stars in the Galactic open clusters NGC 2477 and IC 4756 and the Hyades Supercluster are described. After calibrating the spectra in absolute surface flux units an index of chromospheric activity (ΔFCaII) is derived for each star. Using the ΔFCaII indices we examine the decline of chromospheric activity for ∼2 M⊙ stars evolving off the main sequence, comparing our indices with the theoretical predictions of Rutten & Pylyser. Our observations indicate that their predictions overestimate the levels of activity (and therefore the rotation velocities) of 2 M⊙ stars. We conclude that the assumptions of rigid-body rotation and/or conservation of total angular momentum should be relaxed to explain the decline in observed rotational velocities and chromospheric activity for post-main-sequence stars. Show more