112 research outputs found
Ratios of star cluster core and half-mass radii: a cautionary note on intermediate-mass black holes in star clusters
There is currently much interest in the possible presence of
intermediate-mass black holes in the cores of globular clusters. Based on
theoretical arguments and simulation results it has previously been suggested
that a large core radius -- or particularly a large ratio of the core radius to
half-mass radius -- is a promising indicator for finding such a black hole in a
star cluster. In this study N-body models of 100000 stars with and without
primordial binaries are used to investigate the long-term structural evolution
of star clusters. Importantly, the simulation data is analysed using the same
processes by which structural parameters are extracted from observed star
clusters. This gives a ratio of the core and half-mass (or half-light) radii
that is directly comparable to the Galactic globular cluster sample. As a
result, it is shown that the ratios observed for the bulk of this sample can be
explained without the need for an intermediate-mass black hole. Furthermore, it
is possible that clusters with large core to half-light radius ratios harbour a
black-hole binary (comprised of stellar mass black holes) rather than a single
massive black hole. This work does not rule out the existence of
intermediate-mass black holes in the cores of at least some star clusters.Comment: 14 pages, 7 figures, accepted for publication in MNRA
Populating the Galaxy with low-mass X-ray binaries
We perform binary population synthesis calculations to investigate the
incidence of low-mass X-ray binaries and their birth rate in the Galaxy. We use
a binary evolution algorithm that models all the relevant processes including
tidal circularization and synchronization. Parameters in the evolution
algorithm that are uncertain and may affect X-ray binary formation are allowed
to vary during the investigation. We agree with previous studies that under
standard assumptions of binary evolution the formation rate and number of
black-hole low-mass X-ray binaries predicted by the model are more than an
order of magnitude less than what is indicated by observations. We find that
the common-envelope process cannot be manipulated to produce significant
numbers of black-hole low-mass X-ray binaries. However, by simply reducing the
mass-loss rate from helium stars adopted in the standard model, to a rate that
agrees with the latest data, we produce a good match to the observations.
Including low-mass X-ray binaries that evolve from intermediate-mass systems
also leads to favourable results. We stress that constraints on the X-ray
binary population provided by observations are used here merely as a guide as
surveys suffer from incompleteness and much uncertainty is involved in the
interpretation of results.Comment: 17 pages and 9 figures; accepted by MNRA
A direct N-body model of core-collapse and core oscillations
We report on the results of a direct N-body simulation of a star cluster that
started with N = 200 000, comprising 195 000 single stars and 5 000 primordial
binaries. The code used for the simulation includes stellar evolution, binary
evolution, an external tidal field and the effects of two-body relaxation. The
model cluster is evolved to 12 Gyr, losing more than 80% of its stars in the
process. It reaches the end of the main core-collapse phase at 10.5 Gyr and
experiences core oscillations from that point onwards -- direct numerical
confirmation of this phenomenon. However, we find that after a further 1 Gyr
the core oscillations are halted by the ejection of a massive binary comprised
of two black holes from the core, producing a core that shows no signature of
the prior core-collapse. We also show that the results of previous studies with
N ranging from 500 to 100 000 scale well to this new model with larger N. In
particular, the timescale to core-collapse (in units of the relaxation
timescale), mass segregation, velocity dispersion, and the energies of the
binary population all show similar behaviour at different N.Comment: 9 pages, 8 figures, accepted for publication in MNRA
Evolution of binary stars and the effect of tides on binary populations
We present a rapid binary evolution algorithm that enables modelling of even
the most complex binary systems. In addition to all aspects of single star
evolution, features such as mass transfer, mass accretion, common-envelope
evolution, collisions, supernova kicks and angular momentum loss mechanisms are
included. In particular, circularization and synchronization of the orbit by
tidal interactions are calculated for convective, radiative and degenerate
damping mechanisms. We use this algorithm to study the formation and evolution
of various binary systems. We also investigate the effect that tidal friction
has on the outcome of binary evolution. Using the rapid binary code, we
generate a series of large binary populations and evaluate the formation rate
of interesting individual species and events. By comparing the results for
populations with and without tidal friction we quantify the hitherto ignored
systematic effect of tides and show that modelling of tidal evolution in binary
systems is necessary in order to draw accurate conclusions from population
synthesis work. Tidal synchronism is important but because orbits generally
circularize before Roche-lobe overflow the outcome of the interactions of
systems with the same semi-latus rectum is almost independent of eccentricity.
It is not necessary to include a distribution of eccentricities in population
synthesis of interacting binaries, however, the initial separations should be
distributed according to the observed distribution of semi-latera recta rather
than periods or semi-major axes.Comment: 36 pages, 12 figures, to be published in the Monthly Notices of the
Royal Astronomical Societ
Dynamical Interactions Make Hot Jupiters in Open Star Clusters
Explaining the origin and evolution of exoplanetary "hot Jupiters" remains a
significant challenge. One possible mechanism for their production is
planet-planet interactions, which produces hot Jupiters from planets born far
from their host stars but near their dynamical stability limits. In the much
more likely case of planets born far from their dynamical stability limits, can
hot Jupiters can be formed in star clusters? Our N-body simulations of
planetary systems inside star clusters answer this question in the affirmative,
and show that hot Jupiter formation is not a rare event. We detail three case
studies of the dynamics-induced births of hot Jupiters on highly eccentric
orbits that can only occur inside star clusters. The hot Jupiters' orbits bear
remarkable similarities to those of some of the most extreme exoplanets known:
HAT-P-32 b, HAT-P-2 b, HD 80606 b and GJ 876 d. If stellar perturbations formed
these hot Jupiters then our simulations predict that these very hot, inner
planets are often accompanied by much more distant gas giants in highly
eccentric orbits.Comment: 18 pages, 4 figure
Direct N-body Modelling of Stellar Populations: Blue Stragglers in M67
We present a state-of-the-art N-body code which includes a detailed treatment
of stellar and binary evolution as well as the cluster dynamics. This code is
ideal for investigating all aspects relating to the evolution of star clusters
and their stellar populations. It is applicable to open and globular clusters
of any age. We use the N-body code to model the blue straggler population of
the old open cluster M67. Preliminary calculations with our binary population
synthesis code show that binary evolution alone cannot explain the observed
numbers or properties of the blue stragglers. On the other hand, our N-body
model of M67 generates the required number of blue stragglers and provides
formation paths for all the various types found in M67. This demonstrates the
effectiveness of the cluster environment in modifying the nature of the stars
it contains and highlights the importance of combining dynamics with stellar
evolution. We also perform a series of N = 10000 simulations in order to
quantify the rate of escape of stars from a cluster subject to the Galactic
tidal field.Comment: 26 pages, 18 figures, accepted for publication in MNRA
A Complete N-body Model of the Old Open Cluster M67
The old open cluster M67 is an ideal testbed for current cluster evolution
models because of its dynamically evolved structure and rich stellar
populations that show clear signs of interaction between stellar, binary and
cluster evolution. Here we present the first truly direct N-body model for M67,
evolved from zero age to 4 Gyr taking full account of cluster dynamics as well
as stellar and binary evolution. Our preferred model starts with 12000 single
stars and 12000 binaries placed in a Galactic tidal field at 8.0 kpc from the
Galactic Centre. Our choices for the initial conditions and for the primordial
binary population are explained in detail. At 4 Gyr, the age of M67, the total
mass has reduced by 90% as a result of mass loss and stellar escapes. The mass
and half-mass radius of luminous stars in the cluster are a good match to
observations although the model is more centrally concentrated than
observations indicate. The stellar mass and luminosity functions are
significantly flattened by preferential escape of low-mass stars. We find that
M67 is dynamically old enough that information about the initial mass function
is lost, both from the current luminosity function and from the current mass
fraction in white dwarfs. The model contains 20 blue stragglers at 4 Gyr which
is slightly less than the 28 observed in M67. Nine are in binaries. The blue
stragglers were formed by a variety of means and we find formation paths for
the whole variety observed in M67. Both the primordial binary population and
the dynamical cluster environment play an essential role in shaping the
population. A substantial population of short-period primordial binaries (with
periods less than a few days) is needed to explain the observed number of blue
stragglers in M67.Comment: 32 pages, 17 figures, submitted to MNRA
Tracking Cluster Debris (TraCD) – I. Dissolution of clusters and searching for the solar cradle
The capability to reconstruct dissolved stellar systems in dynamical and chemical space is a key factor in improving our understanding of the evolution of the Milky Way. Here we concentrate on the dynamical aspect and given that a significant portion of the stars in the Milky Way have been born in stellar associations or clusters that have lived a few Myr up to several Gyr, we further restrict our attention to the evolution of star clusters. We have carried out our simulations in two steps: (1) we create a simulation of dissolution and mixing processes which yields a close fit to the present-day Milky Way dynamics and (2) we have evolved three sets of stellar clusters with masses of 400, 1000 and 15 000 M⊙ to dissolution. The birth location of these sets was 4, 6, 8 and 10 kpc for the 400 and 1000 M⊙ clusters and 4, 6, 8, 10 and 12 kpc for the 15 000 M⊙. We have focused our efforts on studying the state of the escapers from these clusters after 4.5 Gyr of evolution with particular attention to stars that reach the solar annulus, i.e. 7.5 ≤ Rgc ≤ 8.5 kpc. We give results for solar twins and siblings over a wide range of radii and cluster masses for two dissolution mechanisms. From kinematics alone, we conclude that the Sun was ∼50 per cent more likely to have been born near its current Galactocentric radius, rather than have migrated (radially) ∼2 kpc since birth. We conclude our analysis by calculating magnitudes and colours of our single stars for comparison with the samples that the Gaia, Gaia-ESO and GALAH-AAO surveys will obtain. In terms of reconstructing dissolved star clusters, we find that on short time-scales we cannot rely on kinematic evolution alone and thus it will be necessary to extend our study to include information on chemical space
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