Abstract
M15 is the prototypical collapsed-core globular cluster. Having undergone collapse, its core is believed now to be expanding, with energy for the re-expansion provided by binary stars, which turn gravitational potential energy into kinetic energy1. Because these binary stars are generally more massive than single stars, they will have settled to the centre of the cluster2. We report here that several of the stars at the core of M15 show Ca II H- and K-line emission, characteristic of young, rapidly rotating stars and close binaries3. We argue that the emission from M15 comes from primordial binaries, in which a period of spin-up has led to magnetic field generation by enhanced dynamo action, which in turn causes heating of the stellar chromospheres. If this interpretation is correct, the Ca H and K emission may provide an important diagnostic tool of the binary population in cluster cores, and thus of the cluster dynamics.
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