Aaron M. Geller, Robert D. Mathieu
We present an in-depth study of the hard-binary population of the old (7 Gyr)
open cluster NGC 188. The main-sequence solar-type hard binaries in NGC 188 are
nearly indistinguishable from similar binaries in the Galactic field. We find a
global solar-type main-sequence hard-binary frequency in NGC 188 of 29 +/- 3 %
for binaries with periods less than 10^4 days. For main-sequence hard binaries
in the cluster we observe a log-period distribution that rises towards our
detection limit, a roughly Gaussian eccentricity distribution centered on e =
0.35 (for binaries with periods longer than the circularization period), and a
secondary-mass distribution that rises towards lower-mass companions.
Importantly, the NGC 188 blue straggler binaries show significantly different
characteristics than the solar-type main sequence binaries in NGC 188. We
observe a blue straggler hard-binary frequency of 76 +/- 19 %, three times that
of the main sequence. The blue straggler binary eccentricity - log period
distribution is distinct from that of the main sequence at the 99% confidence
level, with the majority of the blue straggler binaries having periods of order
1000 days and lower eccentricities. The secondary-mass distribution for these
long-period blue straggler binaries is narrow and peaked with a mean value of
about 0.5 Msun. Predictions for mass-transfer products are most closely
consistent with the binary properties of these NGC 188 blue stragglers, which
comprise two-thirds of the blue straggler population. Additionally we compare
the NGC 188 binaries to those evolved within the sophisticated Hurley et al.
(2005) N-body open cluster simulation. We find that additional simulations with
initial conditions that are better motivated by observations are necessary to
properly investigate the dynamical evolution of a rich binary population in
open clusters like NGC 188. (abridged)
View original:
http://arxiv.org/abs/1111.3950
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