2,724 research outputs found
Evidence for the Strong Effect of Gas Removal on the Internal Dynamics of Young Stellar Clusters
We present detailed luminosity profiles of the young massive clusters M82-F,
NGC 1569-A, and NGC 1705-1 which show significant departures from equilibrium
(King and EFF) profiles. We compare these profiles with those from N-body
simulations of clusters which have undergone the rapid removal of a significant
fraction of their mass due to gas expulsion. We show that the observations and
simulations agree very well with each other suggesting that these young
clusters are undergoing violent relaxation and are also losing a significant
fraction of their stellar mass. That these clusters are not in equilibrium can
explain the discrepant mass-to-light ratios observed in many young clusters
with respect to simple stellar population models without resorting to
non-standard initial stellar mass functions as claimed for M82-F and NGC
1705-1. We also discuss the effect of rapid gas removal on the complete
disruption of a large fraction of young massive clusters (``infant
mortality''). Finally we note that even bound clusters may lose >50% of their
initial stellar mass due to rapid gas loss (``infant weight-loss'').Comment: 6 pages, 3 figures, MNRAS letters, accepte
The effects of spatially distributed ionisation sources on the temperature structure of HII region
Spatially resolved studies of star forming regions show that the assumption
of spherical geometry is not realistic in most cases, with a major complication
posed by the gas being ionised by multiple non-centrally located stars or star
clusters. We try to isolate the effects of multiple non-centrally located stars
on the temperature and ionisation structure of HII regions, via the
construction of 3D photoionisation models using the 3D Monte Carlo
photoionisation code MOCASSIN. We find that the true temperature fluctuations
due to the stellar distribution (as opposed to the large-scale temperature
gradients due to other gas properties) are small in all cases and not a
significant cause of error in metallicity studies. Strong emission lines from
HII regions are often used to study the metallicity of star-forming regions. We
compare integrated emission line spectra from our models and quantify any
systematic errors caused by the simplifying assumption of a single, central
location for all ionising sources. We find that the dependence of the
metallicity indicators on the ionisation parameter causes a clear bias, due to
the fact that models with a fully distributed configuration of stars always
display lower ionisation parameters than their fully concentrated counterparts.
The errors found imply that the geometrical distribution of ionisation sources
may partly account for the large scatter in metallicities derived using
model-calibrated empirical methods.Comment: 13 pages, 6 figures, Accepted by MNRA
Dynamical Masses of Young Star Clusters: Constraints on the Stellar IMF and Star-Formation Efficiency
Many recent works have attempted to constrain the stellar initial mass
function (IMF) inside massive clusters by comparing their dynamical mass
estimates to the measured light. These studies have come to different
conclusions, with some claiming standard Kroupa-type IMFs, while others have
claimed extreme non-standard IMFs. However, the results appear to be correlated
with the age of the clusters, as older clusters (>80 Myr) all appear to be well
fit by a Kroupa-type IMF whereas younger clusters display significant scatter
in their best fitting IMF. Here we show that this is likely due to the fact
that young clusters are out of virial equilibrium and therefore cannot be used
for such studies. Hence only the older clusters are suitable for IMF studies.
Using only these clusters we find that the IMF does not vary significantly. The
youngest clusters can be used instead to constrain the star-formation
efficiency (SFE) within clusters. We find that the SFE varies between 20 and
60% and we conclude that approximately 60% of young clusters are unbound and
will not survive for more than a few 10's of Myr (i.e. "infant mortality").Comment: 4 pages, contribution to "Globular Clusters: Guides to Galaxies",
March 6th-10th, 200
The Star Cluster Population in the Tidal Tails of NGC 6872
We present a photometric analysis of the rich star cluster population in the
tidal tails of NGC 6872. We find star clusters with ages between 1 - 100 Myr
distributed in the tidal tails, while the tails themselves have an age of less
than 150 Myr. Most of the young massive ()
clusters are found in the outer regions of the galactic disk or the tidal
tails. The mass distribution of the cluster population can be well described by
power-law of the form , where , in very good agreement with other young cluster populations found in a
variety of different environments. We estimate the star formation rate for
three separate regions of the galaxy, and find that the eastern tail is forming
stars at times the rate of the western tail and times the
rate of the main body of the galaxy. By comparing our observations with
published N-body models of the fate of material in tidal tails in a galaxy
cluster potential, we see that many of these young clusters will be lost into
the intergalactic medium. We speculate that this mechanism may also be at work
in larger galaxy clusters such as Fornax, and suggest that the so-called
ultra-compact dwarf galaxies could be the most massive star clusters that have
formed in the tidal tails of an ancient galactic merger.Comment: 12 pages, 10 figures, accepted A&
The Maximum Mass of Star Clusters
When an universal untruncated star cluster initial mass function (CIMF)
described by a power-law distribution is assumed, the mass of the most massive
star cluster in a galaxy (M_max) is the result of the size-of-sample (SoS)
effect. This implies a dependence of M_max on the total number of star clusters
(N). The SoS effect also implies that M_max within a cluster population
increases with equal logarithmic intervals of age. This is because the number
of clusters formed in logarithmic age intervals increases (assuming a constant
cluster formation rate). This effect has been observed in the SMC and LMC.
Based on the maximum pressure (P_int) inside molecular clouds, it has been
suggested that a physical maximum mass (M_max[phys]) should exist. The theory
predicts that M_max[phys] should be observable, i.e. lower than M_max that
follows from statistical arguments, in big galaxies with a high star formation
rate. We compare the SoS relations in the SMC and LMC with the ones in M51 and
model the integrated cluster luminosity function (CLF) for two cases: 1) M_max
is determined by the SoS effect and 2) M_max=M_max[phys]=constant. The observed
CLF of M51 and the comparison of the SoS relations with the SMC and LMC both
suggest that there exists a M_max[phys] of 5*10^5 M_sun in M51. The CLF of M51
looks very similar to the one observed in the ``Antennae'' galaxies. A direct
comparison with our model suggests that there M_max[phys]=2*10^6 M_sun.Comment: 4 pages, contribution to "Globular Clusters: Guides to Galaxies",
March 6th-10th, 200
Stellar Clusters in NGC 1313: Evidence for Infant Mortality
We present evidence that infant mortality of stellar clusters is likely to be
a major and very efficient process for the dissolution of young clusters in the
spiral galaxy NGC 1313. Performing stellar PSF photometry on archival HST/ACS
images of the galaxy, we find that a large fraction of early B-type stars are
seen outside of star clusters and well spread within the galactic disk,
consistent with the scenario of infant mortality. We also calculate the UV flux
produced by the stars in and out the clusters and find that 75 to 90% of the UV
flux in NGC 1313 is produced by stars outside the clusters. These results
suggest that the infant mortality of star clusters is probably the underlying
cause of the diffuse UV emission in starburst galaxies. Infant mortality would
also explain the numerous B-type stars observed in the background field of our
Galaxy as well. We exclude the possibility that unresolved low-mass star
clusters and scaled OB associations might be the main source for the diffuse UV
emission.Comment: 5 pages, 3 figures, accepted for publication in the Astrophysical
Journal Letter
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