135 research outputs found
The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters
We use archival Hubble Space Telescope observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M 51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of ~7.3 × 8.1 kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). We show the critical dependence of the age distribution on the sample selection, and confirm that using a constant mass cut-off, above which the sample is complete for the entire age range of interest, is essential. In particular, in this sample we are complete only for masses above 5× 104~M? for the last 1 Gyr. Using this dataset we find: i) that the cluster formation rate seems to have had a large increase ~50-70 Myr ago, which is coincident with the suggested second passage of its companion, NGC 5195; ii) a large number of extremely young (<10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next ~10 Myr; and iii) that the distribution of cluster sizes can be well approximated by a power-law with exponent, -? = -2.2 ± 0.2, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters
Hierarchical Star-Formation in M33: Fundamental properties of the star-forming regions
Star-formation within galaxies appears on multiple scales, from spiral
structure, to OB associations, to individual star clusters, and often
sub-structure within these clusters. This multitude of scales calls for
objective methods to find and classify star-forming regions, regardless of
spatial size. To this end, we present an analysis of star-forming groups in the
local group spiral galaxy M33, based on a new implementation of the Minimum
Spanning Tree (MST) method. Unlike previous studies which limited themselves to
a single spatial scale, we study star-forming structures from the effective
resolution limit (~20pc) to kpc scales. We find evidence for a continuum of
star-forming group sizes, from pc to kpc scales. We do not find a
characteristic scale for OB associations, unlike that found in previous
studies, and we suggest that the appearance of such a scale was caused by
spatial resolution and selection effects. The luminosity function of the groups
is found to be well represented by a power-law with an index, -2, similar to
that found for clusters and GMCs. Additionally, the groups follow a similar
mass-radius relation as GMCs. The size distribution of the groups is best
described by a log-normal distribution and we show that within a hierarchical
distribution, if a scale is selected to find structure, the resulting size
distribution will have a log-normal distribution. We find an abrupt drop of the
number of groups outside a galactic radius of ~4kpc, suggesting a change in the
structure of the star-forming ISM, possibly reflected in the lack of GMCs
beyond this radius. (abridged)Comment: 12 pages, 16 figures, accepted MNRA
Understanding the Dynamical State of Globular Clusters: Core-Collapsed vs Non Core-Collapsed
We study the dynamical evolution of globular clusters using our H\'enon-type
Monte Carlo code for stellar dynamics including all relevant physics such as
two-body relaxation, single and binary stellar evolution, Galactic tidal
stripping, and strong interactions such as physical collisions and binary
mediated scattering. We compute a large database of several hundred models
starting from broad ranges of initial conditions guided by observations of
young and massive star clusters. We show that these initial conditions very
naturally lead to present day clusters with properties including the central
density, core radius, half-light radius, half-mass relaxation time, and cluster
mass, that match well with those of the old Galactic globular clusters. In
particular, we can naturally reproduce the bimodal distribution in observed
core radii separating the "core-collapsed" vs the "non core-collapsed"
clusters. We see that the core-collapsed clusters are those that have reached
or are about to reach the equilibrium "binary burning" phase. The non
core-collapsed clusters are still undergoing gravo-thermal contraction.Comment: 42 pages, 12 figures, 1 table, submitted to MNRA
Monte Carlo Simulations of Globular Cluster Evolution. V. Binary Stellar Evolution
We study the dynamical evolution of globular clusters containing primordial
binaries, including full single and binary stellar evolution using our Monte
Carlo cluster evolution code updated with an adaptation of the single and
binary stellar evolution codes SSE/BSE from Hurley et. al (2000, 2002). We
describe the modifications we have made to the code. We present several test
calculations and comparisons with existing studies to illustrate the validity
of the code. We show that our code finds very good agreement with direct N-body
simulations including primordial binaries and stellar evolution. We find
significant differences in the evolution of the global properties of the
simulated clusters using stellar evolution compared to simulations without any
stellar evolution. In particular, we find that the mass loss from stellar
evolution acts as a significant energy production channel simply by reducing
the total gravitational binding energy and can significantly prolong the
initial core contraction phase before reaching the binary-burning quasi steady
state of the cluster evolution as noticed in Paper IV. We simulate a large grid
of clusters varying the initial cluster mass, binary fraction, and
concentration and compare properties of the simulated clusters with those of
the observed Galactic globular clusters (GGCs). We find that our simulated
cluster properties agree well with the observed GGC properties. We explore in
some detail qualitatively different clusters in different phases of their
evolution, and construct synthetic Hertzprung-Russell diagrams for these
clusters.Comment: 46 preprint pages, 18 figures, 3 tables, submitted to Ap
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