Ca II emission from old red giants in the globular cluster M15

Abstract

MURPHYet al.¹ recently observed characteristic emission of Ca II from the globular cluster Ml5, and argued that this emission came from a population of primordial binary stars. Their conclusion appears theoretically attractive in relation to the study of the dynamics and evolution of globular clusters^2,3. We argue here, however, that the Ca II K-line emission in various types of hard binary systems differs in strength and width from the Ml5 spectra, and we demonstrate that the M15 emission closely resembles that from metal-deficient red giants of the halo population in our Galaxy. Chromospheric activity, indicated by emission in Ca II, Hα and Mg II, occurs in red giants in both globular clusters and halo-population field giants^4–6 A simple detection of Ca II emission is thus not a unique signature of binaries, and in the case of M15 we argue that red giants are the more likely cause.