STELLAR COLLISIONS AND BLUE STRAGGLER STARS IN DENSE GLOBULAR CLUSTERS
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Blue straggler stars (BSS) are abundantly observed in all Galactic globular
clusters (GGC) where data exist. However, observations alone cannot reveal the
relative importance of various formation channels or the typical formation
times for this well studied population of anomalous stars. Using a
state-of-the-art H\'enon-type Monte Carlo code that includes all relevant
physical processes, we create 128 models with properties typical of the
observed GGCs. These models include realistic numbers of single and binary
stars, use observationally motivated initial conditions, and span large ranges
in central density, concentration, binary fraction, and mass. Their properties
can be directly compared with those of observed GGCs. We can easily identify
the BSSs in our models and determine their formation channels and birth times.
We find that for central densities above ~10^3 Msun/pc^3 the dominant formation
channel is stellar collisions while for lower density clusters, mass transfer
in binaries provides a significant contribution (up to ~ 60% in our models).
The majority of these collisions are binary-mediated, occurring during 3-body
and 4-body interactions. As a result a strong correlation between the specific
frequency of BSSs and the binary fraction in a cluster can be seen in our
models. We find that the number of BSSs in the core shows only a weak
correlation with the collision rate estimator \Gamma traditionally used by
observers, in agreement with the latest Hubble Space Telescope (ACS) data.
Using an idealized "full mixing" prescription for collision products, our
models indicate that the BSSs observed today may have formed several Gyrs ago.
However, denser clusters tend to have younger (~1 Gyr) BSSs.
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