Binaries in Galactic globular clusters
Roles, abundances & models
Outline
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Stability of GCs
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Blue stragglers
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LMXBs
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Abundance of binaries vs cluster age
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Binary creation & disruption processes
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Models
Stability of GCs
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First successful models of GCs in the sixties
(e. g. King, Michie, Henon)
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Some models (Henon, and later in 1980s e.g.
Lynden-Bell, Cohn, Goodman) – gravothermal
catastrophe (core collapse)
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Binary stars as the energy source which
stabilize GC cores against collapse
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Gravothermal oscillation cycle
Blue stragglers
CMD for M3 (Sandage 1953)
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First discovered in GC M3
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Produced by stellar collisions – lone stars or
binaries in the process or after merging
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Mass transfer in tight binaries
LMXBs
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Hard to form due to the difference in masses of the components – potential
disruption due to SN explosion
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Overabundant in GCs (~20 times) + runaway velocity problem -> different
formation channels?
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Tidal capture & Three-Body scattering
Abundance of binaries in GCs
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ξ neigh ~ 50%
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ξ GC-spec, phot ~ 20% ± huge
uncertainities (e.g. Yan &
Mateo (1994), Yan & Reid
(1996), Albrow (2001), ...)
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Use of simulated CMDs - not
universal, many free
parameters -> need to
properly choose the right
sample – low extinction, low
central density, high galactic
lattitude – but quite precise
(Sollima et al. (2007))
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Is there a correlation of the
binary population with the
age of GCs?
Sollima et al. (2007)
Models
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N-body simulations – ~100 000 stars, high demands for computational power
(Hurley et al. (2007))
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Monte-Carlo simulations– faster, but sparse & giving conflicting results (Ivanova et
al. (2005)
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Statistical approach – simpler but consistent with N-body simulations, number of
simplifications, less demanding to compute -> more simulations can be made
(Sollima (2008))
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Statistical treatment of interactions – tidal capture, ionization, exchange, stellar
evolution, evaporation, mass segregation, cluster dynamical evolution
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Different initial conditions – central density, velocity dispersion, initial binary fraction,
evaporation efficiency
Sollima (2008)
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Sudden drop followed by a
sharp increase
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More binaries in the GC
core
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Binary ionization dominant
in the first 2 Gyr
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Steady increase of the
binary fraction by
evaporation of the single
stars in the later stages
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Other processes negligible
Sollima (2008)
Summary
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Binaries are crucial for stability of the globular
clusters
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Globular clusters provide a suitable enviroment
for formation for a variety of exotic astrophysical
objects (blue stragglers, LMXBs, cataclysmic
variables, etc.)
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Binary fraction in the globular clusters evolve in
time – disruption during the early stages and
then a steady increase