The Impact of Triple Stars on the Formation of the NGC 188 Blue Stragglers

Type Conference Paper
Names Aaron M. Geller, J. R. Hurley, R. D. Mathieu
Proceedings Title Bulletin of the American Astronomical Society
Conference Name American Astronomical Society, AAS Meeting #217, #327.02
Volume 43
Pages 32702
Date January 1, 2011
Library Catalog NASA ADS
Abstract We recently completed a detailed radial-velocity study of the old (7 Gyr) open cluster NGC 188 in which we identify all solar-type cluster members, and study the binaries and blue stragglers (BSs) in great detail. Using these data for guidance, we attempt to recreate the observed properties of NGC 188 within a sophisticated N-body model. Specifically, we study how a population of primordial triples impacts the dynamical evolution of the solar-type main-sequence binary population as well as the BS production rate and mechanisms. We employ the observed main-sequence binary population of M35 (180 Myr) to set our initial binary frequency and distributions of orbital parameters. Thus we create the first N-body model whose initial binary population is defined by such detailed observations of a binary population in a young open cluster. Without any triples, the 7 Gyr main-sequence solar-type hard-binary population in the simulation matches that of NGC 188 in both frequency and distributions of orbital parameters. This simulation is also consistent with the observed spatial density profile and mass of NGC 188. However, our simulations with binaries alone cannot reproduce the large number of BSs observed in NGC 188 or their distinctive eccentricity - period distribution. Adding tertiary companions to the initial binaries produces an additional population of BSs, primarily formed through dynamical encounters that lead to direct collisions of the stars in the inner binary. Importantly, these added BSs have a high binary frequency similar to what we observe in NGC 188. Comparisons between our simulations and observations of the NGC 188 BSs place important constraints on the initial triple population, which in turn results in detailed predictions for future observations of triples in young star clusters. We suggest that dynamical encounters with triples likely played a significant role in creating the observed NGC 188 BSs.
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