Nature 354, 284 - 286 (28 November 1991); doi:10.1038/354284a0

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

A. K. Dupree & B. A. Whitney

Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

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, H 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.

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