On the extreme blue stragglers and the fate of subgiants in the core of the globular cluster M15

Details On the extreme blue stragglers and the fate of subgiants in the core of the globular cluster M15 On the extreme blue stragglers and the fate of subgiants in the core of the globular cluster M15

Feb 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.272..730d&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 272, Issue 4, pp. 730-736.

Astronomy and Astrophysics

Astronomy

9

Binaries: General, Blue Stragglers, Stars: Evolution, Stars: Interiors, White Dwarfs, Globular Clusters: Individual: M15

Scientific paper

We consider the nature of the 13 extreme blue stragglers discovered in the core of the globular cluster M15 by De Marchi & Paresce. These stars probably have 3>10^7 yr, from which we deduce that they must be core helium-burning stars. We then show that the only such star satisfying the observational requirements, lifetime constraint and core confinement is one produced by the merger of two small helium white dwarfs, each below the helium-ignition mass, but above such a limit after merger, with tau_l~=2x10^8 yr. A scenario for the formation of such objects (which we call MDDs for merged degenerate dwarfs) in the core of M15 is then discussed, which depends critically on the fraction of stars in binaries; this is discussed in the light of several observational arguments. When helium-degenerate dwarfs, formed and still contained in a binary that confines them to the core, collide with a red giant, the extensive hydrogen envelope of the red giant is stripped and the two helium-degenerate cores merge. Only such collisions are likely to lead to a transformation of the helium dwarfs, all others leading to mass accretion on the primary, and/or mass loss from the system. We then deduce a rate of formation for these objects, ~=10^-7 yr^-1, which agrees with the abundance of MDDs in M15. We predict (1) that there is a correlation between the depletion of giants in dense cores, the abundance of blue stragglers and the number of MDDs, and (2) that MDDs are slow rotators.

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