205 research outputs found
On Collision Course: The Nature of the Binary Star Cluster NGC 2006 / SL 538
The LMC hosts a rich variety of star clusters seen in close projected
proximity. Ages have been derived for few of them showing differences up to few
million years, hinting at being binary star clusters. However, final
confirmation needs to be done through spectroscopic analysis. Here we focus on
the LMC cluster pair NGC2006-SL538 and aim to determine whether the star
cluster pair is a bound entity (binary star cluster) or a chance alignment.
Using the MIKE echelle spectrograph at LCO we have acquired integrated-light
spectra for each cluster. We have measured radial velocities by two methods: a)
direct line profile measurement yields v km/s for NGC2006 and
km/s for SL538. b) By comparing observed spectra with
synthetic bootstrapped spectra yielding km/s for NGC2006 and
km/s for SL538. Finally when spectra are directly compared,
we find a km/s. Full-spectrum SED fits reveal that the
stellar population ages lie in the range 13-21 Myr with a metallicity of
Z=0.008. We find indications for differences in the chemical abundance patterns
as revealed by the helium absorption lines between the two clusters. The
dynamical analysis shows that the two clusters are likely to merge within the
next 150 Myr. The NGC2006-SL538 cluster pair shows radial velocities,
stellar population and dynamical parameters consistent with a gravitational
bound entity. We conclude that this is a genuine binary cluster pair, and we
propose that their differences in ages and stellar population chemistry is most
likely due to variances in their chemical enrichment history within their
environment. We suggest that their formation may have taken place in a loosely
bound star-formation complex which saw initial fragmentation but then had its
clusters become a gravitationally bound pair by tidal capture.Comment: Accepted for publication in Astronomy & Astrophysics. 15 pages, 10
figures in low resolutio
Extremely alpha-Enriched Globular Clusters in Early-Type Galaxies: A Step towards the Dawn of Stellar Populations?
We compare [alpha/Fe], metallicity, and age distributions of globular
clusters in elliptical, lenticular, and spiral galaxies, which we derive from
Lick line index measurements. We find a large number of globular clusters in
elliptical galaxies that reach significantly higher [alpha/Fe] values (>0.5)
than any clusters in lenticular and spiral galaxies. Most of these extremely
alpha-enriched globular clusters are old (t > 8 Gyr) and cover the metallicity
range -1 <~ [Z/H] <~ 0. A comparison with supernova yield models suggests that
the progenitor gas clouds of these globular clusters must have been
predominantly enriched by massive stars (>~20 M_sol) with little contribution
from lower-mass stars. The measured [alpha/Fe] ratios are also consistent with
yields of very massive pair-instability supernovae (~130-190 M_sol). Both
scenarios imply that the chemical enrichment of the progenitor gas was
completed on extremely short timescales of the order of a few Myr. Given the
lower [alpha/Fe] average ratios of the diffuse stellar population in early-type
galaxies, our results suggest that these extremely alpha-enhanced globular
clusters could be members of the very first generation of star clusters formed,
and that their formation epochs would predate the formation of the majority of
stars in giant early-type galaxies.Comment: 6 pages, 3 figures, accepted for publication in the Astrophysical
Journa
Wide-Field Hubble Space Telescope Observations of the Globular Cluster System in NGC1399
We present a comprehensive high spatial-resolution imaging study of globular
clusters (GCs) in NGC1399, the central giant elliptical cD galaxy in the Fornax
galaxy cluster, conducted with HST/ACS. Using a novel technique to construct
drizzled PSF libraries for HST/ACS data, we accurately determine the fidelity
of GC structural parameter measurements from detailed artificial star cluster
experiments. The measurement of rh for the major fraction of the NGC1399 GC
system reveals a trend of increasing rh versus galactocentric distance, Rgal,
out to about 10 kpc and a flat relation beyond. This trend is very similar for
blue and red GCs which are found to have a mean size ratio of
rh(red)/rh(blue)=0.82+/-0.11 at all galactocentric radii from the core regions
of the galaxy out to ~40 kpc. This suggests that the size difference between
blue and red GCs is due to internal mechanisms related to the evolution of
their constituent stellar populations. Modeling the mass density profile of
NGC1399 shows that additional external dynamical mechanisms are required to
limit the GC size in the galaxy halo regions to rh~2 pc. We suggest that this
may be realized by an exotic GC orbit distribution function, an extended dark
matter halo, and/or tidal stress induced by the increased stochasticity in the
dwarf halo substructure at larger Rgal. We match our GC rh measurements with
radial velocity data from the literature and find that compact GCs show a
significantly smaller line-of-sight velocity dispersion, =225+/-25
km/s, than their extended counterparts, =317+/-21 km/s. Considering
the weaker statistical correlation in the GC rh-color and the GC rh-Rgal
relations, the more significant GC size-dynamics relation appears to be
astrophysically more relevant and hints at the dominant influence of the GC
orbit distribution function on the evolution of GC structural parameters.Comment: 22 pages, 17 figures, accepted for publication in The Astrophysical
Journal, a high-quality PDF version is available at
http://www.astro.puc.cl/~tpuzia/PUC/Home.htm
New Constraints on the Star Formation History of the Star Cluster NGC 1856
We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain
deep, high-resolution photometry of the young (age ~ 300 Myr) star cluster
NGC1856 in the Large Magellanic Cloud. We compare the observed colour-magnitude
diagram (CMD), after having applied a correction for differential reddening,
with Monte Carlo simulations of simple stellar populations (SSPs) of various
ages. We find that the main sequence turn-off (MSTO) region is wider than that
derived from the simulation of a single SSP. Using constraints based on the
distribution of stars in the MSTO region and the red clump, we find that the
CMD is best reproduced using a combination of two different SSPs with ages
separated by 80 Myr (0.30 and 0.38 Gyr, respectively). However, we can not
formally exclude that the width of the MSTO could be due to a range of stellar
rotation velocities if the efficiency of rotational mixing is higher than
typically assumed. Using a King-model fit to the surface number density profile
in conjunction with dynamical evolution models, we determine the evolution of
cluster mass and escape velocity from an age of 10 Myr to the present age,
taking into account the possible effects of primordial mass segregation. We
find that the cluster has an escape velocity Vesc ~ 17 km/s at an age of 10
Myr, and it remains high enough during a period of ~ 100 Myr to retain material
ejected by slow winds of first-generation stars. Our results are consistent
with the presence of an age spread in NGC1856, in contradiction to the results
of Bastian & Silva-Villa (2013).Comment: 17 pages, 14 figures. Re-submitted to MNRAS after addressing all the
comments by the refere
Some Constraints On the Effects of Age and Metallicity on the Low Mass X-ray Binary Formation Rate
We have studied the low mass X-ray binary (LMXB) populations within and
outside globular clusters (GC) in NGC 4365 and NGC 3115. Using published age
and metallicity constraints from optical and IR observations of their GCs, we
do not find any evidence for an increase in the LMXB formation rate in the
intermediate age cluster population of NGC 4365, as has been proposed in some
scenarios of dynamical LMXB formation in GCs. The old, metal-rich, red
population of GCs in NGC 3115 on the other hand is {\it at least} three times
as efficient at creating LMXBs as the old, metal-poor, blue clusters. These
data suggest that the higher formation efficiency of LMXBs in the red GC
subsystems of many galaxies is largely a consequence of their higher
metallicity. A comparison of the densities of field LMXBs in different galaxies
does not reveal an obvious correlation with the age of the field stars as
predicted by models in which the LMXB formation rate in the field drops
monotonically with time after an initial burst. This suggests that either a
significant fraction of the field LMXBs are created in GCs and subsequently
injected into the field, or the LMXB formation rate has a more complex time
evolution pattern.Comment: Accepted for publication in ApJ Letters. 7 pages including 5 figure
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