149 research outputs found
Mass loss of stars in star clusters: an energy source for dynamical evolution
Dense star clusters expand until their sizes are limited by the tidal field
of their host galaxy. During this expansion phase the member stars evolve and
lose mass. We show that for clusters with short initial relaxation time scales
(<~100 Myr) the dynamical expansion is largely powered by mass loss from stars
in the core, but happens on a relaxation time scale. That is, the energy
release following stellar mass loss is in balance with the amount of energy
that is transported outward by two-body relaxation.Comment: 4 pages, to appear in the proceedings of the conference "370 years of
Astronomy in Utrecht", eds. G. Pugliese, A. de Koter and M. Wijbur
The mass and radius evolution of globular clusters in tidal fields
We present a simple theory for the evolution of initially compact clusters in
a tidal field. The fundamental ingredient of the model is that a cluster
conducts a constant fraction of its own energy through the half-mass radius by
two-body interactions every half-mass relaxation time. This energy is produced
in a self-regulative way in the core by an (unspecified) energy source. We find
that the half-mass radius increases during the first part (roughly half) of the
evolution and decreases in the second half, while the escape rate is constant
and set by the tidal field. We present evolutionary tracks and isochrones for
clusters in terms of cluster half-mass density, cluster mass and
galacto-centric radius. We find substantial agreement between model isochrones
and Milky Way globular cluster parameters, which suggests that there is a
balance between the flow of energy and the central energy production for almost
all globular clusters. We also find that the majority of the globular clusters
are still expanding towards their tidal radius. Finally, a fast code for
cluster evolution is presented.Comment: 5 pages, 1 figure. To appear in the proceedings of the conference
`Reading the book of globular clusters with the lens of stellar evolution',
Rome, Nov. 26-28, 2012, to be published in Memorie della Societa Astronomica
Italiana, Eds. Paolo Ventura and Corinne Charbonne
Dynamical evolution of stellar clusters
The evolution of star clusters is determined by several internal and external
processes. Here we focus on two dominant internal effects, namely energy
exchange between stars through close encounters (two-body relaxation) and
mass-loss of the member stars through stellar winds and supernovae explosions.
Despite the fact that the former operates on the relaxation timescale of the
cluster and the latter on a stellar evolution timescale, these processes work
together in driving a nearly self-similar expansion, without forming (hard)
binaries. Low-mass clusters expand more, such that after some time the radii of
clusters depend very little on their masses, even if all clusters have the same
(surface) density initially. Throughout it is assumed that star clusters are in
virial equilibrium and well within their tidal boundary shortly after
formation, motivated by observations of young (few Myrs) clusters. We start
with a discussion on how star clusters can be distinguished from (unbound)
associations at these young ages.Comment: 6 pages, 4 figures, proceedings of "Stellar Clusters and Associations
- A RIA workshop on GAIA", 23-27 May 2011, Granada, Spai
A family of lowered isothermal models
We present a family of self-consistent, spherical, lowered isothermal models,
consisting of one or more mass components, with parameterised prescriptions for
the energy truncation and for the amount of radially biased pressure
anisotropy. The models are particularly suited to describe the phase-space
density of stars in tidally limited, mass-segregated star clusters in all
stages of their life-cycle. The models extend a family of isotropic,
single-mass models by Gomez-Leyton and Velazquez, of which the well-known
Woolley, King and Wilson (in the non-rotating and isotropic limit) models are
members. We derive analytic expressions for the density and velocity dispersion
components in terms of potential and radius, and introduce a fast model solver
in PYTHON (LIMEPY), that can be used for data fitting or for generating
discrete samples.Comment: 17 pages, 10 figures, 4 appendices, MNRAS, updated to match final
journal styl
A flexible method to evolve collisional systems and their tidal debris in external potentials
We introduce a numerical method to integrate tidal effects on collisional
systems, using any definition of the external potential as a function of space
and time. Rather than using a linearisation of the tidal field, this new method
follows a differential technique to numerically evaluate the tidal acceleration
and its time derivative. Theses are then used to integrate the motions of the
components of the collisional systems, like stars in star clusters, using a
predictor-corrector scheme. The versatility of this approach allows the study
of star clusters, including their tidal tails, in complex, multi-components,
time-evolving external potentials. The method is implemented in the code nbody6
(Aarseth 2003).Comment: MNRAS accepted. Code available here:
http://personal.ph.surrey.ac.uk/~fr0005/nbody6tt.ph
The distinction between star clusters and associations
In Galactic studies a distinction is made between (open) star clusters and
associations. For barely resolved objects at a distance of several Mpc this
distinction is not trivial to make. Here we provide an objective definition by
comparing the age of the stars to the crossing time of nearby stellar
agglomerates. We find that a satisfactory separation can be made where this
ratio equals unity. Stellar agglomerates for which the age of the stars exceeds
the crossing time are bound, and are referred to as star clusters.
Alternatively, those for which the crossing time exceeds the stellar age are
unbound and are referred to as associations. This definition is useful whenever
reliable measurements for the mass, radius and age are available.Comment: 2 pages, 2 figures, accepted for MNRAS Letter
The origin of the Milky Way globular clusters
We present a cosmological zoom-in simulation of a Milky Way-like galaxy used
to explore the formation and evolution of star clusters. We investigate in
particular the origin of the bimodality observed in the colour and metallicity
of globular clusters, and the environmental evolution through cosmic times in
the form of tidal tensors. Our results self-consistently confirm previous
findings that the blue, metal-poor clusters form in satellite galaxies which
are accreted onto the Milky Way, while the red, metal-rich clusters form mostly
in situ or, to a lower extent in massive, self-enriched galaxies merging with
the Milky Way. By monitoring the tidal fields these populations experience, we
find that clusters formed in situ (generally centrally concentrated) feel
significantly stronger tides than the accreted ones, both in the present-day,
and when averaged over their entire life. Furthermore, we note that the tidal
field experienced by Milky Way clusters is significantly weaker in the past
than at present-day, confirming that it is unlikely that a power-law cluster
initial mass function like that of young massive clusters, is transformed into
the observed peaked distribution in the Milky Way with relaxation-driven
evaporation in a tidal field.Comment: MNRAS accepte
Evolution of star clusters in arbitrary tidal fields
We present a novel and flexible tensor approach to computing the effect of a
time-dependent tidal field acting on a stellar system. The tidal forces are
recovered from the tensor by polynomial interpolation in time. The method has
been implemented in a direct-summation stellar dynamics integrator (NBODY6) and
test-proved through a set of reference calculations: heating, dissolution time
and structural evolution of model star clusters are all recovered accurately.
The tensor method is applicable to arbitrary configurations, including the
important situation where the background potential is a strong function of
time. This opens up new perspectives in stellar population studies reaching to
the formation epoch of the host galaxy or galaxy cluster, as well as for
star-burst events taking place during the merger of large galaxies. A pilot
application to a star cluster in the merging galaxies NGC 4038/39 (the
Antennae) is presented.Comment: 12 pages, 8 figures. Accepted for publication in MNRA
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