The core binary fractions of star clusters from realistic simulations
To appear in the Astrophysical Journal
The core binary fractions of star clusters from realistic simulations
Jarrod R. Hurley
Centre for Astrophysics and Supercomputing, Swinburne University of Technology,
P.O. Box 218, VIC 3122, Australia
jhurley@swin.edu.au
Sverre J. Aarseth
Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
sverre@ast.cam.ac.uk
Michael M. Shara
Department of Astrophysics, American Museum of Natural History,
Central Park West at 79th Street, New York, NY 10024
mshara@amnh.org
ABSTRACT
We investigate the evolution of binary fractions in star clusters using N -body
models of up to 100 000 stars. Primordial binary frequencies in these models
range from 5% to 50%. Simulations are performed with the NBODY4 code and
include a full mass spectrum of stars, stellar evolution, binary evolution and the
tidal field of the Galaxy.
We find that the overall binary fraction of a cluster almost always remains
close to the primordial value, except at late times when a cluster is near dis-
solution. A critical exception occurs in the central regions where we observe a
marked increase in binary fraction with time – a simulation starting with 100 000
stars and 5% binaries reached a core binary frequency as high as 40% at the end
of the core-collapse phase (occurring at 16Gyr with ∼ 20 000 stars remaining).
Binaries are destroyed in the core by a variety of processes as a cluster evolves,
but the combination of mass-segregation and creation of new binaries in exchange
interactions produces the observed increase in relative number. We also find that
http://arxiv.org/abs/0704.0290v1
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binaries are cycled into and out of cluster cores in a manner that is analogous to
convection in stars. For models of 100 000 stars we show that the evolution of
the core-radius up to the end of the initial phase of core-collapse is not affected
by the exact value of the primordial binary frequency (for frequencies of 10% or
less). We discuss the ramifications of our results for the likely primordial binary
content of globular clusters.
Subject headings: binaries: close, general — globular clusters: general— meth-
ods: N-body simulations — open clusters and associations: general — stellar
dynamics
1. Introduction
The binary content of a globular cluster is important in determining the frequency and
nature of cluster stellar exotica, as well as the dynamical evolution of the cluster. It has long
been recognized that binary formation is inevitable in a self-gravitating system1. Indeed,
the presence of binaries as a central energy source is vital to avoid complete core-collapse
(Goodman & Hut 1989). However, only more recently has it been realised that globular
clusters must also have formed with a sizeable binary population (see Hut et al. 1992 for
an early review). That globular clusters harbour a mixture of dynamically formed and
primordial binaries can be used to understand observations of their stellar content, such as
the diverse blue straggler population in 47 Tucanae (Mapelli et al. 2004).
Knowledge of the likely primordial binary fraction of globular clusters is essential as
input to models of globular cluster evolution. It also provides a constraint on the cluster
formation process. Considering that the presence of binaries in the cluster core has a pro-
nounced effect on the core properties and cluster evolution (Hut 1996), knowledge of the
central binary frequency is also important. Indications are that this is relatively small – of
the order of 20% (e.g. Bellazzini et al. 2002) or less (e.g. Cool & Bolton 2002) – when
compared to the frequencies of binaries observed in the solar neighbourhood (Duquennoy &
Mayor 1991) and open clusters such as M67 (Fan et al. 1996) which are of the order of 50%.
It would be particularly useful to take measurements of the current binary fraction in
globular clusters – whether that be in the core or outer regions – and extrapolate backwards to
gain a reliable determination of the primordial binary content. However, processes involved
1The ten-body gravitational calculations of von Hoerner (1960) are the earliest N -body calculations
published. They were continued until the first binary formed, at which point the calculations were halted.
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in the intervening cluster evolution make this difficult. For example, binaries can be formed
and destroyed in a variety of interactions between cluster members (Hurley & Shara 2002).
Binaries will on average be more massive than single stars and thus are affected differently
by mass segregation. Also, the escape rates of single stars and binaries will differ. Finally,
the internal evolution of the components of binaries can also lead to binaries’ destruction.
Current simulation techniques have been designed to model these (and other) processes
(Aarseth 2003) and have reached the level of sophistication required to produce realistic clus-
ter models. In this way the link between primordial and current cluster binary populations
can be investigated directly (e.g. Hurley et al. 2005; Ivanova et al. 2005). Aarseth (1996)
conducted an N -body simulation starting with 10 000 stars and a 5% binary frequency where
notably the stars were drawn from a realistic initial mass function (IMF), the cluster was
subject to the tidal field of the Galaxy, and both stellar and binary evolution were modelled.
This model cluster had a half-life of about 2Gyr at which point the core binary frequency
had risen to 20% primarily owing to mass-segregation. Thus binaries were not preferentially
depleted. In this case it was not necessary to include a large initial binary fraction in order
to halt core-collapse and yield a significant observed abundance in the central regions. The
earlier work of McMillan & Hut (1994) reported N -body simulations of 2 000 stars or less
and binary frequencies in the range of 5-20%. They included the Galactic tidal field but
only considered point-mass dynamics. McMillan & Hut (1994) showed that there is a criti-
cal primordial binary frequency of 10-15% below which the binaries are destroyed before the
cluster dissolves owing to the tidal field. Furthermore, they found that above this critical
value there exists a minimum possible binary mass fraction for the cluster – this result could
be used with observations of present-day binary frequency to place limits on the primordial
frequency. We note that the McMillan & Hut (1994) simulations were restricted to equal-
mass stars, and the binaries were a factor of two heavier then single stars – this could give
misleading results when applied to real clusters2.
These N -body simulations were definitely in the open cluster regime. Dynamical pro-
cesses which destroy (and equally may create) cluster binaries are density dependent. In
addition, the central stellar density of a cluster is a function of the number, N , of cluster
members. Thus, it is not clear that these prior results apply to globular cluster conditions.
More recently Ivanova et al. (2005) have conducted Monte Carlo simulations of clusters
2 Binaries would naturally be twice as massive as single stars on average if binaries form by random
pairings independent of the stellar IMF. In general correlated masses are assumed (e.g. Kroupa 1995)
although the exact situation is unclear – the recent survey of stars in the solar neighborhood and in young
open clusters compiled by Halbwachs et al. (2003) shows a distribution of mass-ratios, q, with a broad peak
between 0.2− 0.7 but also a sharp peak for q > 0.8.
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with up to 5 × 105 members and core number densities ranging from 103 − 106 stars pc−3.
They show that an initial binary frequency of 100% is required to produce a current core
binary frequency of 10% for a globular cluster such as 47 Tucanae. Depletion of binaries
in the cluster core is found to be the result of stellar evolution processes as well as three-
and four-body dynamical interactions. It is our intention in this paper to test these claims
by utilising direct N -body simulations of star clusters with up to N = 100 000 members
initially.
One aspect that will affect the evolution of the cluster binary population is the orbital
parameters of the primordial binaries – in particular the initial ratio of hard to soft binaries.
The boundary between these two regimes is determined by the mean kinetic energy of the
cluster stars (with binaries represented by their center-of-mass motion) where hard binaries
have a binding energy in excess of 2/3 of the mean kinetic energy (Hut et al. 1992). We
note that a useful estimate for the boundary in terms of the binary orbital separation is
given by twice the cluster half-mass radius divided by N . In three-body single–binary star
interactions hard binaries tend to harden and provide kinetic heating for the cluster (Heggie
1975; Hut 1983). Soft binaries are less strongly bound (and thus on average are wider) and
are efficiently destroyed in three- and four-body encounters. As noted by Hut et al. (1992) it
is for this reason that soft binaries are not generally included in cluster models. A common
misconception is that the omission of soft binaries is to aid the speed of simulation; however
it is binaries near the hard/soft boundary that provide the main threat to efficient simulation
(Aarseth 2003). The omission is more a realisation that soft binaries have little impact on
the cluster dynamics or exotic star formation and so the focus is on the more meaningful
binaries, so to speak. Neglecting soft binaries has the capacity to alter binary fractions in
the halo of a model cluster as binary encounters tend to occur in or near the cluster core.
For this reason we will attempt to account for any omitted soft binary populations when
making binary fraction comparisons.
Our simulation method and initial conditions are detailed in Sections 2 and 3. Results
are given in Section 4 followed by discussion in Section 5. We briefly summarize our results
in Section 6.
2. Models
All simulations utilized in this work were performed using the NBODY4 code (Aarseth
1999) on GRAPE-6 boards (Makino 2002) located at the American Museum of Natural
History. NBODY4 uses the 4th-order Hermite integration scheme and an individual timestep
algorithm to follow the orbits of cluster members and invokes regularization schemes to deal
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with the internal evolution of small-N subsystems (see Aarseth 2003 for details). Stellar and
binary evolution of the cluster stars are performed in concert with the dynamical integration
as described in Hurley et al. (2001).
The results of four extensive simulations (detailed below) form the dataset for this
paper. We will make use of data from two simulations that have previously been reported
in the literature – a simulation starting with 95 000 single stars and 5 000 binaries (Shara &
Hurley 2006) and a simulation starting with 12 000 single stars and 12 000 binaries (Hurley
et al. 2005). The former contained 100 000 members at birth, if we count each binary as
one object, and thus had a primordial binary frequency of 5%. We will refer to this as the
K100-5 simulation. After about nine Gyr of evolution the cluster membership was reduced
by half and at an age of 15 − 16Gyr the model cluster had reached the end of the main
core-collapse phase (associated with a minimum in core-radius, after which the size of the
core stabilizes, in relative terms). Figure 1a shows the behaviour of the core radius as the
K100-5 model evolves. Also shown is the 10% Lagrangian radius – the radius which encloses
the inner 10% of the cluster by mass. From Figure 1a we see that initially the inner regions of
the cluster expand owing to stellar evolution mass-loss before two-body effects take over and
drive a prolonged period of contraction. When the cluster is about 12 half-mass relaxation
times old (as denoted across the top of Fig. 1a) the core radius reaches a minimum of 0.17 pc
and the main core-collapse phase is halted. The 10% Lagrangian radius at this point is
0.94 pc. The core density of the model begins at 102 stars pc−3 and increases to a maximum
of 104 stars pc−3 just before termination of the model at 20Gyr.
The core radius in Figure 1 is actually the density radius commonly used in N -body
simulations (Casertano & Hut 1985). It is calculated from the density weighted average
of the distance of each star from the density centre (Aarseth 2003). This definition, in
combination with the effects of three-body interactions and the movement of binaries across
the core boundary, allows for the small-scale fluctuations in core radius observed in Figure 1.
Such fluctuations could be smoothed out (see Heggie, Hut & Trenti 2006, for example)
but we have chosen not to do this. This N -body core-radius is distinct from observational
determinations of core-radius calculated, for example, from the surface brightness profile
(SBP) of a cluster. As discussed by Wilkinson et al. (2003) there is no general relation
between the two quantities but usually the N -body value is the lesser of the two. This
is supported by an in-depth analysis of the core-radius evolution of the K100-5 simulation
that will be presented in an upcoming paper (Hurley, 2007, in preparation). Preliminary
results show that the the core-radius obtained from the two-dimensional projected SBP of
the K100-5 model agrees well with the N -body core radius for the first 7Gyr of evolution
but is about twice as large by the time the model reaches 16Gyr of age. Thus the binary
fraction within the 10% Lagrangian radius may often be a better number to compare with
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central binary fractions quoted for real clusters and we will give both this and the core binary
fraction in our results.
The second model had a primordial binary frequency of 50% and was tailored to in-
vestigate the evolution and stellar populations of the old open cluster M67. It had 24 000
members at birth and we will refer to this as the K24-50 simulation. It had a half-life of about
2Gyr and after 4Gyr of evolution only 2 000 stars and binaries remained. The core density
was about 102 stars pc−3 on average, reaching a maximum of 350 stars pc−3 at 3 480Myr with
a corresponding core radius of 0.3 pc. Figure 1b shows the evolution of the core and 10%
Lagrangian radii for the K24-50 simulation.
To investigate the evolution of binary fractions across a range of star cluster models we
will also make use of two simulations that have yet to be published. These are a simulation
that started with 90 000 single stars and 10 000 binaries (K100-10) and a simulation that
started with 40 000 single stars and 10 000 binaries (K50-20). In Table 1 we summarize the
properties of the four simulations.
For each model the initial setup is as follows. Masses for the single stars are drawn
from the IMF of Kroupa, Tout & Gilmore (1993) between the mass limits of 0.1 and 50M⊙.
Each binary mass is chosen from the IMF of Kroupa, Tout & Gilmore (1991), as this had
not been corrected for the effect of binaries, and the component masses are set by choosing
a mass-ratio from a uniform distribution. We assume that all stars are on the zero-age
main sequence (ZAMS) when the simulation begins and that any residual gas from the star
formation process has been removed. We use a Plummer density profile (Aarseth, Hénon
& Wielen 1974) and assume the stars and binaries are in virial equilibrium when assigning
the initial positions and velocities. There is no primordial segregation by mass, binary
properties, or any other discriminating factor in these models. Each cluster is subject to a
standard Galactic tidal field – a circular orbit in the Solar neighborhood. Stars are removed
from the simulation when their distance from the density centre exceeds twice that of the
tidal radius of the cluster. The metallicity of the stars in the two simulations starting with
100 000 stars (K100-5 and K100-10) was set to be Z = 0.001 while both the K24-50 and
K50-20 simulations were assigned solar metallicity (Z = 0.02).
3. Binary Period Distributions
The orbital separations of the 5 000 primordial binaries in the K100-5 simulation (Shara
& Hurley 2006) were drawn from the log-normal distribution suggested by Eggleton, Fitchett
& Tout (1989) with a peak at 30 au. This distribution is based on the properties of doubly-
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bright visual binaries in the Bright Star Catalogue (Hoffleit 1983) and is in agreement with
the survey data of Duquennoy & Mayor (1991) for binaries in the solar neighbourhood –
although the latter observations do not rule out a flat distribution. Orbital eccentricities of
the primordial binaries were assumed to follow a thermal distribution (Heggie 1975). In the
K100-5 model the initial separation distribution was capped at 100 au. With a half-mass
radius of 6.7 pc for the initial model the hard/soft binary boundary is at about 30 au. Thus
the maximum of 100 au excludes only the softest binaries from the distribution. Binaries
with an initial pericentre distance less than five times the radius of the primary star were
rejected in the setup of the model – for binaries closer than this it is assumed that interaction
during the formation process and on the Hayashi track would lead to collision. Rather
than enact such a collision we simply choose another set of binary parameters from the
distributions. In this way the intended primordial binary fraction is preserved. The resulting
period distribution of the K100-5 model is shown in Figure 2a. We see that the distribution
is peaked at 105 d and does not extend beyond 106 d. The K100-10 simulation had the
same binary setup as that of the K100-5 model. The K50-20 simulation also used the same
Eggleton, Fitchett & Tout (1989) distribution of orbital separations but with a cap at 50 au.
Primordial binaries in the M67 (or K24-50) simulation of Hurley et al. (2005) have
orbital separations drawn from a flat distribution of log a (Abt 1983). An upper cutoff
of 50 au was applied so that soft binaries were not included in the model – with a half-
mass radius of 3.9 pc the hard/soft binary limit for the starting model was about 40 au.
For this model very close primordial orbits were also rejected. The corresponding period
distribution for the primordial binaries in the K24-50 simulation is shown in Figure 2b. We
note that a goal of the K24-50 simulation was to reproduce the relatively large number of
blue stragglers observed in M67. For this purpose an Eggleton, Fitchett & Tout (1989)
separation distribution was ruled out as it did not lead to enough blue straggler production
from Case A mass transfer in close binaries. Uncorrelated masses of the component stars in
binaries were also ruled out for the same reason (see Hurley et al. 2005 for details).
During this work we will be making comparisons to the Monte Carlo models presented by
Ivanova et al. (2005). In their study binary periods were chosen from a uniform distribution
in logP between the limits of 0.1 and 107 d. Thus they assumed a wider distribution of
primordial binaries. If, for example, the Eggleton, Fitchett & Tout (1989) distribution used
in the K100-5 simulation was extended to include all periods up to 107 d, rather than being
curtailed at 100 au, the 5 000 binaries that make up the distribution shown in Figure 2a
would represent about 5/6 of the full population. So effectively there would be 1 000 soft
binaries that have been neglected and the true primordial frequency would be 6%. One could
then assume that these soft binaries were broken-up at the very start of the simulation –
although this may not be true for soft binaries residing in the less-dense outer regions of
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the cluster. However, we note that there is no evidence that binary periods in star clusters
should extend as far as 107 d (Meylan & Heggie 1997).
In terms of hard binaries one could argue, for the sake of semantics, that in comparison
to a population drawn from a uniform distribution of periods extending from 0.1 d (without
restriction) our initial distributions are under-sampling the contribution of hard binaries. A
key point here is that short-period binaries were not excluded from the primordial popula-
tions of our simulations by some ad-hoc process. Instead the distribution of orbital periods is
dictated by using distributions borne from observations in combination with accounting for
pre-main sequence (MS) evolution – before contracting along the Hayashi track the stellar
radius of a pre-MS star can be a factor of five or more greater than on the ZAMS (Siess,
Dufour & Forestini 2000) and birth periods must allow for this (Kroupa 1995). Pre-MS evo-
lution was not considered by Ivanova et al. (2005) although they did reject systems where
one or both stars would initially fill their Roche-lobes at pericentre – this was also assumed
in our models.
4. Results
In Figure 3 we show the evolution of the core binary fraction for the four N -body
simulations introduced above. Also shown is the binary fraction within the 10% Lagrangian
radius and the overall binary fraction of the model clusters.
Except at late times in the K24-50 model, when the cluster has lost more than 90%
of its original mass and is nearing dissolution, we see that in each case the cluster binary
fraction remains close to the primordial value. Focussing on the K100-5 simulation, Figure 4
shows the fractions of single stars and binaries (compared to their respective initial number)
in the cluster. Following on from Figure 3a the fractions are similar at all times as expected.
However, Figure 4 also shows the fractions of single stars and binaries that have escaped the
cluster and we see that from about 2Gyr onwards the fractional escape rate of single stars
is greater than that of the binaries. At the end of the simulation (20Gyr) the difference is
34%. This is offset somewhat by evolution processes (stellar and binary) that destroy binaries
(see the dotted line in Figure 4). These processes include binaries becoming unbound due
to supernova mass-loss and/or kicks (only relevant for the first 100Myr of evolution) and
mass transfer-induced mergers in close binaries. The remaining difference is balanced by
the destruction of binaries in dynamical encounters and this becomes more important as the
cluster evolves. We note that even though the cluster binary fraction is relatively static as
the cluster evolves the characteristics of the binary population change markedly over time
with hard binaries favoured at late times.
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Evident from Figure 3 is an overall trend for the core binary fraction to increase with
time, irrespective of simulation type. For the core binary population of the K100-5 model we
see that this rises from an initial 5% to as high as 40% around the time that the core-collapse
phase is halted. After this time the core binary fraction becomes quite noisy owing to the
small size of the core (see Figure 1) and the small numbers of binaries and stars in the core.
However, the value always remains greater than the initial value. We see also from Figure 3a
that the binary frequency within the inner 10% Lagrangian radius rises to a maximum of
16% just prior to the end of the core-collapse phase.
It is important to note at this point that we are working with radii derived from spherical
data whereas observational determinations of binary fractions are based on two-dimensional
projected data. With our models it is possible to test the effect of this discrepancy on our
findings. If we calculate the 10% Lagrangian radius for model K100-5 from a two-dimensional
projection we find that the radius is reduced by about 20-40% across the evolution (the
choice of projection axis does not affect this result). This is consistent with the expectation
suggested by Fleck et al. (2006). A similar relationship is reported by Baumgardt, Makino &
Hut (2005) in that the half-light radius (calculated from projected data) is approximately half
the size of the half-mass radius (calculated from spherical data). However, the binary fraction
within the projected 10% Lagrangian radius of our K100-5 model is almost indistinguishable
from that of the result shown in Figure 3a (the dotted curve).
We now aim to understand the processes underlying the evolution of the core binary
fraction of star clusters, focussing again on the K100-5 simulation. Figure 5 shows the
number of single stars and binaries in the core, relative to their total number in the cluster,
as the cluster evolves. For the first 10Gyr of evolution the ratio of binaries in the core to
binaries in the cluster is fairly static – roughly 1 in 10 binaries is in the core. However,
the ratio of single stars found in the core is decreasing sharply over the same timeframe
and thus single stars are being lost from the core at a greater rate than from the cluster in
general (comparing Figs. 4 and 5). From 10Gyr onwards the ratio of binaries in the core
also decreases. This corresponds to a period of increasing core density: prior to 10Gyr the
core density of stars hovers around the 102 stars pc−3 mark but from 10− 15Gyr it increases
by an order of magnitude. The binary fraction continues to rise in the core over this period
indicating that single stars continue to be lost from the core at a greater rate than binaries.
We note that mass loss from stellar evolution is reduced considerably at this stage compared
to earlier in the cluster lifetime when more massive stars were present.
Figure 6 confirms that the number of binaries in the core is decreasing with time even
though the binary fraction, fb,c, is increasing. We also see from this figure that at least half of
the binaries in the core at any time were not present in the core the last time the population
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was sampled (this is done at intervals of 80Myr). So the core binary population is by no
means static as many binaries are being created/destroyed, or moving in and out of the core,
on the 80Myr timescale. It is important to note for comparison that the relaxation time
in the core is approximately 200Myr initially and decreases to about 50Myr at late times.
Individual binaries in cluster cores are both promiscuous and mobile – transient residents.
In Figure 7 we examine the fraction of core binaries that were created in exchange
interactions. These are short-lived 3- and 4-body gravitational encounters where a star is
exchanged into an existing binary displacing one of the members of that binary (Heggie
1975). Thus it is a process by which primordial binaries can be destroyed and replaced by
new dynamical, or exchange, binaries. We see from Figure 7a that these non-primordial
binaries come to dominate the core population towards the end of the core-collapse phase
in the K100-5 simulation. Figure 7a also shows that the double-degenerate binary content
increases steadily in the core with time and comprises about 30% of the core binaries subse-
quent to the completion of the core-collapse phase. In Figure 7b we see that the exchange
binary content in the core of the K100-10 model does not reach the heights of the K100-5
model. Presumably this is a consequence of the lower core-density of the K100-10 model.
The fraction of double-degenerate binaries is similar – any decrease in double-degenerate pro-
duction via dynamical means in the K100-10 model is compensated by the increased number
of primordial binaries. The fraction of exchange binaries in the core of the K24-50 simulation
(Figure 7d is comparatively low whereas the K50-20 simulation (Figure 7c exhibits a much
larger fraction. Clearly there is a positive correlation between core density and the fraction
of exchange binaries in the core.
Figure 8a shows the number of binaries created and destroyed in exchange interactions
occurring in the core in intervals of 80Myr. Also shown is the number of core binaries
destroyed by all processes (exchanges, orbital perturbations, supernovae, mergers) in each
interval. The key point to note here is that on average exchange interactions are creating
as many binaries as they are destroying. For the entire cluster there were 1 024 binaries
destroyed in exchange interactions during the simulation and 933 binaries created.
Figure 8b looks at the movement of binaries in and out of the core as the cluster evolves.
Across each 80Myr interval it shows the fraction of core binaries that move out of the core
during the interval and the fraction of binaries that have moved into the core during the
interval. We see that the inwards and outwards fluxes are equal. Also shown is the fraction
of binaries entering the core that have previously been in the core – most binaries that leave
the core eventually revisit it. We see a pattern where binaries move outwards across the
core boundary owing to recoil velocities from gravitational encounters, or as a result of the
shrinking core. The core binary population is then replenished by binaries sinking inwards
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owing to mass-segregation effects. In the discussion below we will refer to this pattern as
binary convection. We note that binaries on radial orbits with a moderate to high eccentricity
will also make an apparent contribution to this process.
An analysis of binary disruption for the K100-5 simulation is given in Figure 9 in terms
of cumulative events. Exchange interactions and orbital perturbations from nearby stars are
by far the dominant causes of binary disruption and these are shown in the top panel. We see
that perturbation events are more likely at early times in the evolution but, as soft binaries
disappear and the binary population becomes skewed towards hard binaries, exchange events
eventually overtake perturbations as the major cause of disruption. However there is an
important distinction to make between these two types of event. Exchange interactions are
counted as a disruption event in Figure 9a even if the event also leads to the creation of a
new binary and as we have seen in Figure 8a this is more than likely. On the other hand if
a binary is broken up owing to an orbital perturbation (also known as a fly-by) there is no
possibility of a replacement binary being created in the event.
The lower panel of Figure 9 shows the number of binaries that were ejected from the core
and escaped the cluster. There is a sharp correlation between the incidence of escape and
the increase in core density after 10Gyr. Even so, the total number of binaries lost owing
to this process remains an order of magnitude less than either perturbation or exchange
disruption. There is an initial burst of stellar/binary evolution induced mergers in short-
period primordial binaries followed by a gradual depletion of binaries owing to this process
and collisions in highly eccentric binaries. The cluster had a total of 287 binaries that
experienced either a merger or an internal collision and 67 of these events occurred in the core.
We also see from Figure 9b that supernova events do not make a meaningful contribution to
depletion of the core binary population.
Figure 10 repeats Figure 9 for the K24-50 simulation. In this simulation mergers and
collisions are the most likely cause of core binary loss. This is linked to the increased
primordial binary fraction and decreased core density, compared to the K100-5 simulation.
For similar reasons exchange disruption is more likely than perturbed disruption over the
course of the evolution. In fact in this simulation even the loss of binaries from the core as
a result of escape is greater than that from perturbed break-up. A key distinction between
the K24-50 and K100-5 simulations is that in the K24-50 case the ratio of binary destruction
to creation in exchanges is 3:1 whereas it was close to 1:1 for the K100-5 simulation.
The effect of a substantial primordial binary population on the evolution of open clusters
has been documented in the past (McMillan, Hut & Makino 1990, for example and see
Meylan & Heggie 1997 for a review). The main results are that in comparison to simulations
without primordial binaries the core-collapse phase of evolution is less dramatic and the
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cluster lifetime is reduced. Little has been done on this subject for globular clusters to
date primarily because direct simulations have not been possible. However, our simulations
starting with 100 000 stars can start to shed some light on the expected behaviour. We see
from Table 1 that increasing the primordial binary frequency from 5% (K100-5 simulation)
to 10% (K100-10) does not reduce the cluster half-life significantly. In contrast the K24-50
simulation with 50% binaries has a half-life of 2 060Myr while a comparable simulation of
30 000 single stars with no primordial binaries has a half-life of 3 600Myr. As noted in Hurley
& Shara (2003) the presence of a large number of primordial binaries in an open cluster leads
to an enhanced rate of escaping stars via recoil velocity kicks obtained in 3-body interactions.
In comparison, the K100-5 and K100-10 clusters have deeper potential wells and also the
change in binary fraction between the two models is much less than for the open cluster
example. So a sharp change in the escape rate is not to be expected.
Figure 11 shows that the core radius evolution of the K100-5 and K100-10 simulations
is similar up to 15Gyr (when the K100-10 simulation was stopped). We note however that
the core density of the K100-10 model at this time is only half that of the K100-5 model. So
the presence of additional primordial binaries has reduced the number density of stars in the
core. Also in Figure 11 we compare the core radius evolution of a 100 000 star simulation
with no primordial binaries (a K100-0 model). Here we see that the core radius evolution
is slightly more irregular but overall the evolution is once again similar up to 15Gyr. After
core-collapse has been halted the situation is different as the single star model experiences a
fluctuating, and generally increasing, core-radius while the core-radius of the K100-5 model
remains approximately constant (see Figure 1a). The K100-0 model has a greater core
density than the K100-5 model at the end of the main core-collapse phase. The fluctuating
core-radius of the K100-0 model in the post-core-collapse phase is indicative of the core
bounce and subsequent oscillations expected for such a model – these phenomena are more
pronounced for models without primordial binaries (see the related discussion in Heggie &
Hut 2003 and Heggie, Trenti & Hut 2006).
In Figure 12 we investigate the radial distribution of the K100-5 binary population at
times of 6, 12 and 18Gyr, i.e. before, during and after the deep core-collapse phase. We see
that outside of the half-mass radius the binary fraction is effectively constant with radius
and changes little with time. The binary population in this region is also dominated by
primordial binaries – exchange binaries are unlikely to be found outside of the half-mass
radius. Within the half-mass radius the binary fraction rises sharply towards the centre of
the cluster and binaries become more centrally concentrated as the cluster evolves. Note
that the inner radial bin corresponds to the inner 5% Lagrangian radius so the core is not
resolved in Figure 12.
– 13 –
5. Discussion
Our N -body results clearly show that the core binary fraction of an evolved star cluster
is expected to be greater than the primordial binary fraction. We see this behaviour in each
of the models presented and at all times in the evolution. The most striking case is our main
model (K100-5) which started with 95 000 single stars and 5 000 binaries and experienced a
factor of eight increase in the core binary fraction after 16Gyr of evolution (coinciding with
the end of the main core-collapse phase).
At face value our results appear to be in clear contradiction to the Monte-Carlo results
recently presented by Ivanova et al. (2005). Their main reference model has a primordial
binary fraction fb,0 = 1.0 and a stellar density of nc = 10
5 stars pc−3. Processes such
as exchange interactions, orbital perturbations, binary evolution and mass-segregation are
included and the model is reduced to fb,c = 0.095 at 14Gyr. Ivanova et al. (2005) then repeat
the simulation with fb,0 = 0.5 and end up with fb,c = 0.07 so, as they note, the relationship
between primordial binary fraction and final core binary fraction is not linear. Coming at
this from the other direction our models show a possible saturation effect as the primordial
binary fraction increases. Looking back at Figures 3a and 3b we see that the core binary
fraction of the K100-5 model at the 14Gyr mark is 0.2 (up from fb,0 = 0.05) while it is 0.3
(up from fb,0 = 0.1) for the K100-10 model. So the simulation with the lower primordial
binary fraction has experienced the greater relative increase in core binary content. Our
K24-50 model, which started with fb,0 = 0.5, has fb,c = 0.8 at a similar dynamical age so
the relative increase is less again. These results raise the possibility that decreasing fb,0
below 0.5 in the Monte Carlo models may lead to conditions where fb,c can increase. It is
interesting to note that the idealized models presented recently by Heggie, Trenti & Hut
(2006) showed saturation effects in the core for initial binary frequencies greater than 10%
and also recorded an increase in the core binary fraction with time.
Ivanova et al. (2005) also performed a model to compare with 47 Tucanae. This was
similar to their main reference model although slightly more dense and with an increased
velocity dispersion. The result for fb,0 = 1.0 was fb,c = 0.07 – this lead to the conclusion
that the primordial binary frequencies of globular clusters such as 47 Tucanae must have
been close to 100% to explain current observations. However, Ivanova et al. (2005) also
ran the same simulation with fb,0 = 0.75, 0.5 and 0.25 and reported little or no variation in
the final core binary fraction. It would seem safe to assume that repeating the simulation
with fb,0 = 0.1 may give the same result or even an increase in binary fraction. This would
act to remove any obvious discrepancies between the N -body and Monte Carlo results. We
would certainly be interested in seeing the results of a Monte Carlo simulation conducted
with fb,0 = 0.1 and a similar setup of the primordial binary population as used in this work
– 14 –
– much easier than repeating a large N -body simulation with 100% binaries.
A major distinction between our N -body models and the Monte Carlo simulations
mentioned above is that the stellar density is at least an order of magnitude greater in the
latter. Fortunately, Ivanova et al. (2005) performed a simulation with nc = 10
3 stars pc−3
which facilitates a more direct comparison with our K50-20 model which had a similar core
density throughout the evolution. The K50-20 model experienced an almost factor of two
increase in core binary fraction as it evolved from 0 − 8Gyr. The comparable Monte Carlo
model showed a reduction in core binary fraction of more than a factor of two over the same
period. So there is an obvious deviation in behaviour. Of course there is a large difference
in the primordial binary fractions (0.2 compared to 1.0). The effect of this will be discussed
further below. However, we note at this stage that the initial hard binary fraction in the
Monte Carlo model was ∼ 30% (Ivanova, private communication) and this rose to 37% – so
the hard binary fraction increases and subsequently the models do show agreement at some
level. Another consideration is the velocity dispersion which is generally around 3−4 km s−1
for our models and was set to 10 km s−1 for most of the Monte Carlo models. However,
Ivanova et al. (2005) did perform two models (D4 and M12) similar in all respects except
that σ = 10 km s−1 in one and 4.5 km s−1 in the other. There was no significant difference in
the final core binary fractions of these models.
In Section 3 we discussed that in the setup of our models we might be neglecting a
fraction of soft binaries from the true primordial population. This results from imposing
a maximum initial orbital separation and at most would cause the binary fraction to be
underestimated by a few per cent. Thus we are confident that our choice of initial parameters
for the binary populations in our models is not affecting the result that the core binary
fraction increases as a cluster evolves. We also note that differences in the setup of primordial
binaries between our simulations and those of Ivanova et al. (2005) make it difficult to
directly compare quoted binary frequencies. For example, by not accounting for pre-MS
stellar radii as we do, Ivanova et al. (2005) have a greater relative number of close binaries
in their primordial populations. Such an excess would result in a greater number of evolution-
induced binary mergers. If we were to adopt the period distribution and methods used by
Ivanova et al. (2005) we would need to choose ∼ 11 000 binaries in order to recover the 5 000
in our K100-5 model at birth. This gives an effective primordial binary frequency of 11%, for
the sake of comparison. The effective primordial binary frequency for the K24-50 simulation
would be 80%. Adopting these values, in the worst case scenario, would still not lead us to
conclude that the core binary fraction of an evolved cluster is decreased from the primordial
value.
The comparable rates of binary disruption and creation owing to exchanges in our
– 15 –
K100-5 simulation indicates that 3-body interactions dominated over 4-body interactions.
This is because the most likely outcome of a binary-binary encounter is a binary and two
single stars. So a binary is lost from the overall count. This is not the case for binary-
single encounters where the most likely outcome is a binary and a single star, although the
pairing of stars in the binary and/or the orbital parameters may have changed. By contrast,
exchange interactions in the K24-50 simulation produced a binary disruption rate much
higher than the binary creation rate. Here we had a much higher proportion of primordial
binaries and thus binary-binary encounters were more likely. Thus, in terms of exchange
interactions, increasing the primordial binary fraction can lead to a greater rate of binary
destruction. This would certainly be expected to be true of models with comparable stellar
densities. However, a competing effect comes from the fact that the central density is less for
simulations with higher primordial binary fractions. We certainly see this when comparing
our K100-5 and K100-10 models. The setup of these models was identical in all respects
except for the change in primordial binary frequency from 5% to 10%. The models have
similar half-lives and we showed that the core radius evolution is also similar. So at any
particular time in the evolution they are at a comparable dynamical age. But there is one
clear difference – the model with twice as many primordial binaries has a central stellar
density that is a factor of two less. This translates to a lower incidence of close stellar
encounters and as we saw from Figure 7 a greatly reduced fraction of exchange binaries
in the core. Previous simulations, albeit with small-N , have indicated that the effects of
primordial binaries saturate at some level (Wilkinson et al. 2003) so this is not necessarily
a trend that we expect will continue as the primordial binary fraction is increased towards
unity. However it is certainly significant for clusters with frequencies of 10% or less.
Another point to note is that in a 3-body exchange, not only is a binary not lost, but also
a more massive single star is swapped for a less massive one, increasing the likelihood that
the single star will be lost from the core via mass-segregation. So the exchange process has
indirectly increased the core binary fraction. The process of binary convection that became
evident from Figure 8b also is related to mass-segregation and acts to keep the core binary
fraction healthy. Both single stars and binaries in the core are subject to velocity kicks from
gravitational encounters. These kicks can remove an object from the core and even from the
cluster entirely. For binaries this is less likely to occur primarily because they are on average
more massive than single stars. Also, the average stellar mass decreases radially outwards
in an evolved cluster. So if a core binary suddenly finds itself outside of the core it can be
expected to be one of the more massive objects in its new local environment and thus to
quickly sink back towards the core. We note that we found the movement of binaries inwards
and outwards across the core boundary, as exhibited by Figures 6 and 8, to be quite striking.
Our K100-5 N -body simulation creates a realistic model of a moderate-size globular
– 16 –
cluster. It combines stellar and binary evolution with a self-consistent treatment of the
cluster dynamics. It includes primordial binaries and accounts for the tidal field of the
Galaxy. Thus it provides us with a solid picture of how such a cluster evolves. Single stars
escape from the cluster at a greater rate than binaries do – single stars are less massive on
average so they are more likely to be tidally stripped after segregating to the outer regions
of the cluster and also more likely to be ejected from the cluster in gravitational encounters.
However, binaries are also lost from the cluster population owing to supernova disruption,
evolution-induced mergers and dynamical encounters. These effects balance and the ratio
of single stars to binaries is similar at all times in the evolution. As the cluster evolves
binaries sink towards the centre and the binary fraction increases in the central regions.
The core radius decreases as core-collapse proceeds and dynamical encounters become more
prevalent. These encounters not only break-up binaries but also create new binaries. The
cluster evolves to a state where primordial binaries dominate the binary population in the
outer regions and non-primordial binaries dominate towards the centre.
In the centre of the cluster soft binaries are broken-up as a result of orbital perturbations
from gravitational encounters. Binaries become involved in exchange interactions, primarily
three-body, but these tend to create as many binaries as they destroy. Hard binaries are lost
when the components merge as a result of close binary evolution or a collision at periastron.
These are ongoing processes as the cluster evolves. At an age of 10Gyr the rate of exchange
interactions is greater than that of perturbed break-ups and mergers. However, perturbed
break-ups are the dominant cause of binary loss. This is compared to the Monte Carlo
model of Ivanova et al. (2005) which found that evolutionary mergers were the dominant
event at the same age. We also find that after 10Gyr, as the core density increases, that
binaries can be kicked out of the cluster directly from the core. Partly as a result of the
combination of these processes the number of binaries in the core decreases as the cluster
evolves. Also to blame is the movement of binaries outwards across the core boundary owing
to the decreasing size of the core and recoil velocities invoked in gravitational encounters.
However, the movement of single stars outwards across the core boundary is greater and the
net effect is an increase in the core binary fraction. This is also helped by binary convection
where binaries that were previously resident in the core are cycled back in.
Noting that the typical membership of Galactic globular clusters exceeds 300 000 stars
(Gnedin & Ostriker 1997, for example) we must ask the question – to what extent can
we expect this behaviour to extend to globular clusters in general? We can start with
the ejection rate, tej, of stars from an isolated cluster calculated by Hénon (1969) which
gives tej ∝ ln (0.4N) trh (Binney & Tremaine 1987). Here trh is the half-mass relaxation
timescale and we can relate this to behaviour near the core of a cluster if we assume that
core-mass scales with total mass and that radii do not vary appreciably with cluster mass.
– 17 –
This indicates that the relative rate of outward binary ejection and inward mass-segregation
(which occurs on a relaxation timescale) is only weakly dependent on the cluster mass. If
we look in detail at the local relaxation timescale this scales as
ρ ln (0.4N)
(Davies, Piotto & De Angeli 2004, as derived from Binney & Tremaine 1987) where σ is the
velocity dispersion of the cluster stars and ρ is the mass density. We can take σ ∝
M/rh ∝
c and ρ ∝ Mc, using the above assumptions, to show that tr ∝ M
c / ln (0.4N). Here
Mc is the cluster core-mass, M is the total cluster mass and rh is the half-mass radius. The
timescale for a typical binary in the core of a globular cluster to have a close encounter with
another star scales as
tenc ∝
(Davies, Piotto & De Angeli 2004) where n is the number density and n ∼ ρ if the average
stellar mass is of the order of M⊙, as it is in an evolved cluster core. This gives us tenc ∝
c . To escape the core a binary must acquire a boost in energy of the order of GMc/2 rc
(where G is the Gravitational constant). So, assuming that the average energy imparted in
an encounter does not vary strongly with mass, we have tej ∝ M
c . This rather simplified
analysis returns Hénon’s result and shows that asM (or N) increases there will be relatively
less binary convection as both the ejection and relaxation timescales increase. However, the
effect on the observed core binary fraction can be expected to be minimal.
We cannot definitively use our results to make predictions regarding globular clusters
such as 47 Tucanae because the central density in these clusters is at least an order of
magnitude higher than that reached by our models. However, we note that our model with
the highest core density showed the greatest increase in core binary fraction. Furthermore,
we have considered a range of cluster types. It does not appear, from our simulations, that
an initial binary fraction anywhere near as high as 100% is required to give a core population
of 20% or less at later times. We also note that proper-motion cleaned colour-magnitude
diagrams recently presented for NGC6397 (Richer et al. 2006) and M4 (Richer et al. 2004)
show a distinct lack of binaries in regions outside of the cluster centre – this cannot be
reconciled with a large primordial binary population.
6. Summary
We have presented a range of simulations typical of rich open clusters and moderate-size
globular clusters. In each case we find that the fraction of binaries in the core of a cluster
– 18 –
does not decrease as the cluster evolves. In fact the overriding trend is for an increase in core
binary fraction from the primordial value. Thus we do not agree with Ivanova et al. (2005)
that the binary fraction in the core will be depleted in time. We also do not agree that
models of globular cluster evolution need necessarily include large populations of primordial
binaries.
Our simulations have shown that the binary population in the core of a cluster is con-
tinually being replenished by stars from outside the core, many of which were previously in
the core. This is a process we have termed binary convection. We also find that the binary
content of an evolved star cluster is dominated by exchange binaries provided that the stellar
density is relatively high. This is true of our moderate-size globular cluster models and we
expect it to be true in more massive clusters. We also show that increasing the primordial
binary fraction does not necessarily lead to an increase in the final binary fraction – in fact
it gives more scope for binary depletion. A key and paradoxical result is that a final binary
fraction that can be achieved by choosing a higher primordial binary fraction may also be
replicated by choosing an initially lower binary fraction.
We find that the overall binary fraction of a cluster does not vary appreciably from the
primordial value as a cluster evolves. This is a result of binary destruction being balanced
by a greater rate of escape of single stars compared to binaries. We also find that the
primordial binary frequency of a cluster is well preserved outside of the cluster half-mass
radius. Therefore, observations of the current binary fraction in these regions is a good
indicator of the primordial binary fraction while determination of the core binary fraction
provides an upper limit.
We acknowledge the generous support of the Cordelia Corporation and that of Edward
Norton which has enabled AMNH to purchase GRAPE-6 boards and supporting hardware.
We thank the anonymous referee for extremely helpful comments and especially for alerting
us to the scaling considerations.
– 19 –
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This preprint was prepared with the AAS LATEX macros v5.2.
– 22 –
Fig. 1.— Evolution of the core radius (solid line) and the radius containing the inner 10% of
the cluster mass (dotted line) for: a) the K100-5 simulation; and b) the K24-50 simulation.
The numbers across the top show the number of half-mass relaxation times that have elapsed.
Note that Ns,0 and Nb,0 refer to the number of single stars and binaries, respectively, in the
starting model.
– 23 –
Fig. 2.— Period distribution of the primordial binary populations in: a) the K100-5 sim-
ulation (starting with 5 000 binaries); and b) the K24-50 simulation (starting with 12 000
binaries).
– 24 –
Fig. 3.— Evolution of the binary fraction in the core (solid line), within the 10% Lagrangian
radius (dotted line), and for the entire cluster (dashed line). Results are shown for the: a)
K100-5; b) K100-10; c) K50-20; and d) K24-50 simulations (see Table 1 for details).
– 25 –
Fig. 4.— Fraction of single stars (solid line) and binaries (dashed line) remaining in the
cluster as a function of time (lines decreasing from top-left). Each population is scaled by the
initial number of that population. Also shown are the fractions of single stars and binaries
that have escaped from the cluster (lines increasing from bottom-left). The dotted line is the
combined fraction of binaries lost to escape and binary/stellar evolution processes. Results
are for the K100-5 simulation.
– 26 –
Fig. 5.— Number of single stars in the core as a fraction of the number of single stars in the
cluster (solid line) and number of binaries in the core as a fraction of the number of binaries
in the cluster (dashed line). Results are for the K100-5 simulation.
– 27 –
Fig. 6.— Number of core binaries as a function of time (solid line). Also shown at each
time is the number of binaries that have remained in the core from the previous sampling
(dashed line). Results are for the K100-5 simulation and the data are sampled every 80Myr.
– 28 –
Fig. 7.— Fraction of binaries in the core that were created in an exchange interaction (solid
line) and fraction of core binaries that contain two degenerate stars (dotted line). Results
are shown for the: a) K100-5; b) K100-10; c) K50-20; and d K24-50 simulations (as described
in Table 1).
– 29 –
Fig. 8.— Statistics regarding core binaries across intervals of 80Myr as the K100-5 model
cluster evolves. Shown are: a) number of binaries destroyed in an exchange interaction
occurring in the core (solid line), number of binaries created in exchange interactions in the
core (dashed line) and the number of binaries destroyed by any means (dotted line); and b)
fraction of binaries that have moved out of the core but remained in the cluster (solid line:
as a fraction of the number of binaries in the core at the start of the interval), number of
binaries that have moved into the core (dashed line: as a fraction of the number of binaries
in the core at the end of the interval) and the fraction of binaries entering the core that
have previously resided in the core (dotted line). Note that the data have been moderately
smoothed – over a width of three bins (or 240Myr). Further smoothing would hide the
naturally irregular behaviour of the binary destruction/creation processes.
– 30 –
Fig. 9.— Cumulative numbers of events that lead to the destruction of binaries in the core.
Shown are: a) binaries broken-up in exchange encounters (solid line) and binaries broken-up
owing to orbital perturbations (dotted line); b) binaries that were ejected from the core
and escaped from the cluster (dashed line), binaries broken-up as a result of supernovae
explosions (dotted line) and binaries in which the stars merged (solid line) – this includes
stellar evolution induced mergers and collisions at periastron in highly eccentric binaries.
Results are for the K100-5 simulation.
– 31 –
Fig. 10.— As for Figure 9 but for the K24-50 simulation.
– 32 –
Fig. 11.— Comparison of core-radius evolution for models starting with 100 000 stars. The
K100-5 simulation is taken as a reference model and shown are differences between the core
radius of this model and models starting with 0% (dotted line) and 10% primordial binaries
(K100-10: dashed line). The difference is scaled by the core radius of the K100-5 model.
Note that for each simulation the core radius used is the average core radius in a 250Myr
interval.
– 33 –
Fig. 12.— Binary data as a function of radial position for the K100-5 model. Shown at
times of 6Gyr (solid line), 12Gyr (dashed line) and 18Gyr (dotted line) are: a) distribu-
tion of binary fraction; b) fraction of binaries that are primordial. At each time there are
twenty radial bins each containing the same mass, i.e. corresponding to Lagrangian radii
incremented by 5%. Thus the core is not resolved.
– 34 –
Table 1. Details of the four N -body simulations utilised in this work. Columns 1 and 2
show the number of single stars and binaries in the starting model. The distribution used
to select the orbital separations of the primordial binaries is given in column 3 and this is
followed by the maximum applied to the distribution (in au). Column 5 lists the primordial
binary fraction and in column 6 we show the typical stellar density in the core for the
simulation (stars/pc3). The half-life of the simulation (time in Myr for Ns +Nb to drop to
half the initial value) is given in column 7 and finally an identifying label is supplied for
each simulation.
Ns,0 Nb,0 ψ(a) amax fb,0 nc t1/2 label
95000 5000 EFT30 100 0.05 102 − 104 8920 K100-5
90000 10000 EFT30 100 0.10 100− 500 8850 K100-10
40000 10000 EFT30 50 0.20 103 5560 K50-20
12000 12000 log a 50 0.50 100− 350 2060 K24-50
	Introduction
	Models
	Binary Period Distributions
	Results
	Discussion
	Summary
