23 research outputs found
The Effect of Stellar Metallicity on the Sizes of Star Clusters
Observations indicate blue globular clusters have half-light radii
systematically larger than those of red globular clusters. In this paper, we
test whether the different metallicity-dependent stellar evolution timescales
and mass-loss rates within the clusters can impact their early dynamical
evolution. By means of N-body simulations including stellar evolution recipes
we simulate the early evolution of small centrally concentrated clusters with
and without primordial mass segregation. Our simulations include accurate
metallicity-dependent mass loss from massive stars. We find blue clusters to be
larger than red clusters regardless of whether the clusters have been
primordially mass segregated. In addition, the size difference is found to be
larger and consistent with observations for initial models with a low central
concentration. These results indicate that the systematic size difference found
between red and blue clusters can, at least in part, be attributed to the
dynamical effects of differing stellar evolution histories, driven by
metallicity.Comment: 8 pages, 5 figures, accepted by MNRA
The Origins of Blue Stragglers and Binarity in Globular Clusters
(abridged) We use newly available empirical binary fractions for globular
clusters to carry out a direct test of the binary evolution hypothesis, and of
collisional channels that involve binary stars. More specifically, using the
previously reported correlation between blue straggler numbers and core mass as
a benchmark, we test for correlations with the number of binary stars, as well
as with the rates of single-single, single-binary, and binary-binary
encounters. Surprisingly, we find that the simple correlation with core mass
remains by far the strongest predictor of blue straggler population size, even
in our joint models. This is despite the fact that the binary fractions
themselves strongly anti-correlate with core mass, just as expected in the
binary evolution model. At first sight, these results do not fit neatly with
either binary evolution or collisional models in their simplest forms. Arguably
the simplest and most intriguing possibility to explain this unexpected result
is that observational errors on the core binary fractions are larger than the
true intrinsic dispersion associated with their dependence on core mass. In the
context of the binary evolution model, this would explain why the combination
of binary fraction and core mass is a poorer predictor of blue straggler
numbers than core mass alone. It would also imply that core mass is a
remarkably clean predictor of core binary fractions. This would be of
considerable importance for the dynamical evolution of globular clusters, and
provides an important benchmark for models attempting to understand their
present-day properties.Comment: 10 pages, 9 figures, accepted for publication in MNRA
Understanding Compact Object Formation and Natal Kicks. IV. The case of IC 10 X-1
The extragalactic X-ray binary IC 10 X-1 has attracted attention as it is
possibly the host of the most massive stellar-mass black-hole (BH) known to
date. Here we consider all available observational constraints and construct
its evolutionary history up to the instant just before the formation of the BH.
Our analysis accounts for the simplest possible history that includes three
evolutionary phases: binary orbital dynamics at core collapse, common envelope
(CE) evolution, and evolution of the BH--helium star binary progenitor of the
observed system. We derive the complete set of constraints on the progenitor
system at various evolutionary stages. Specifically: right before the core
collapse event, we find the mass of the BH immediate progenitor to be > 31 Msun
(at 95% of confidence, same hereafter). The magnitude of the natal kick
imparted to the BH is constrained to be < 130 km/s. Furthermore, we find that
the "enthalpy" formalism recently suggested by Ivanova & Chaichenets is able to
explain the existence of IC 10 X-1 without the need of invoking unreasonably
high CE efficiencies. With this physically motivated formalism, we find that
the CE efficiency required to explain the system is in the range of 0.6--1.Comment: 15 pages, 9 figures, submitted to Ap
Multiple Populations in Globular Clusters: The Possible Contributions of Stellar Collisions
Globular clusters were thought to be simple stellar populations, but recent
photometric and spectroscopic evidence suggests that the clusters' early
formation history was more complicated. In particular, clusters show
star-to-star abundance variations, and multiple sequences in their
colour-magnitude diagrams. These effects seem to be restricted to globular
clusters, and are not found in open clusters or the field. In this paper, we
combine the two competing models for these multiple populations and include a
consideration of the effects of stellar collisions. Collisions are one of the
few phenomena which occur solely in dense stellar environments like
(proto-)globular clusters. We find that runaway collisions between massive
stars can produce material which has abundances comparable to the observed
second generations, but that very little total mass is produced by this
channel. We then add the contributions of rapidly-rotating massive stars (under
the assumption that massive stars are spun up by collisions and interactions),
and the contribution of asymptotic giant branch stars. We find that collisions
can help produce the extreme abundances which are seen in some clusters.
However, the total amount of material produced in these generations is still
too small (by at least a factor of 10) to match the observations. We conclude
with a discussion of the additional effects which probably need to be
considered to solve this particular problem.Comment: 9 pages, 3 figures. Accepted by MNRA
Multiple stellar populations and their influence on blue stragglers
It has become clear in recent years that globular clusters are not simple
stellar populations, but may host chemically distinct sub-populations,
typically with an enhanced helium abundance. These helium-rich populations can
make up a substantial fraction of all cluster stars.
One of the proposed formation channels for blue straggler stars is the
physical collision and merger of two stars. In the context of multiple
populations, collisions between stars with different helium abundances should
occur and contribute to the observed blue straggler population. This will
affect the predicted blue straggler colour and luminosity function.
We quantify this effect by calculating models of mergers resulting from
collisions between stars with different helium abundances and using these
models to model a merger population. We then compare these results to four
observed clusters, NGC 1851, NGC 2808, NGC 5634 and NGC 6093.
As in previous studies our models deviate from the observations, particularly
in the colour distributions. However, our results are consistent with
observations of multiple populations in these clusters. In NGC 2808, our best
fitting models include normal and helium enhanced populations, in agreement
with helium enhancement inferred in this cluster. The other three clusters show
better agreement with models that do not include helium enhancement. We discuss
future prospects to improve the modelling of blue straggler populations and the
role that the models we present here can play in such a study.Comment: 10 pages, 7 figures, 6 tables Accepted for publication in MNRAS, 15
June 201
A multiphysics and multiscale software environment for modeling astrophysical systems
We present MUSE, a software framework for combining existing computational
tools for different astrophysical domains into a single multiphysics,
multiscale application. MUSE facilitates the coupling of existing codes written
in different languages by providing inter-language tools and by specifying an
interface between each module and the framework that represents a balance
between generality and computational efficiency. This approach allows
scientists to use combinations of codes to solve highly-coupled problems
without the need to write new codes for other domains or significantly alter
their existing codes. MUSE currently incorporates the domains of stellar
dynamics, stellar evolution and stellar hydrodynamics for studying generalized
stellar systems. We have now reached a "Noah's Ark" milestone, with (at least)
two available numerical solvers for each domain. MUSE can treat multi-scale and
multi-physics systems in which the time- and size-scales are well separated,
like simulating the evolution of planetary systems, small stellar associations,
dense stellar clusters, galaxies and galactic nuclei.
In this paper we describe three examples calculated using MUSE: the merger of
two galaxies, the merger of two evolving stars, and a hybrid N-body simulation.
In addition, we demonstrate an implementation of MUSE on a distributed computer
which may also include special-purpose hardware, such as GRAPEs or GPUs, to
accelerate computations. The current MUSE code base is publicly available as
open source at http://muse.liComment: 24 pages, To appear in New Astronomy Source code available at
http://muse.l
Formation of the black-hole binary M33 X-7 via mass-exchange in a tight massive system
M33 X-7 is among the most massive X-Ray binary stellar systems known, hosting
a rapidly spinning 15.65 Msun black hole orbiting an underluminous 70 Msun Main
Sequence companion in a slightly eccentric 3.45 day orbit. Although
post-main-sequence mass transfer explains the masses and tight orbit, it leaves
unexplained the observed X-Ray luminosity, star's underluminosity, black hole's
spin, and eccentricity. A common envelope phase, or rotational mixing, could
explain the orbit, but the former would lead to a merger and the latter to an
overluminous companion. A merger would also ensue if mass transfer to the black
hole were invoked for its spin-up. Here we report that, if M33 X-7 started as a
primary of 85-99 Msun and a secondary of 28-32 Msun, in a 2.8-3.1 day orbit,
its observed properties can be consistently explained. In this model, the Main
Sequence primary transferred part of its envelope to the secondary and lost the
rest in a wind; it ended its life as a ~16 Msun He star with a Fe-Ni core which
collapsed to a black hole (with or without an accompanying supernova). The
release of binding energy and, possibly, collapse asymmetries "kicked" the
nascent black hole into an eccentric orbit. Wind accretion explains the X-Ray
luminosity, while the black hole spin can be natal.Comment: Manuscript: 18 pages, 2 tables, 2 figure. Supplementary Information:
34 pages, 6 figures. Advance Online Publication (AOP) on
http://www.nature.com/nature on October 20, 2010. To Appear in Nature on
November 4, 201