116 research outputs found
A new Extended Main Sequence Turnoff star cluster in the Large Magellanic Cloud
We present results on the age and metallicity estimates of the poorly studied
LMC cluster SL 529, from CCD SDSS gr photometry obtained at the Gemini South
telescope with the GMOS attached. The cluster MSTO region possesses an extended
structure, with an age spread (~ 0.5 Gyr) bigger than the mean age width of
known EMSTO LMC clusters. We report for the first time a mean cluster age of
2.25 Gyr and a mean cluster metallicity of Z=0.004, which place it as the most
metal-poor and oldest cluster in the EMSTO LMC cluster group. In addition, the
cluster RC appears to be formed by two concentrations of stars - although it is
not clear whether this feature can be caused, in part, by binary interactions
and mergers -, whereas the cluster core radius of 4.2 pc is in excellent
agreement with those determined for the previously 12 known EMSTO LMC clusters.Comment: 6 pages, 6 figures, MNRAS (accepted
Discovery of a loose star cluster in the Large Magellanic Cloud
We present results for an up-to-date uncatalogued star cluster projected
towards the Eastern side of the Large Magellanic Cloud (LMC) outer disc. The
new object was discovered from a search of loose star cluster in the Magellanic
Clouds' (MCs) outskirts using kernel density estimators on Washington CT1 deep
images. Contrarily to what would be commonly expected, the star cluster
resulted to be a young object (log(t /yr) = 8.45) with a slightly subsolar
metal content (Z = 0.013) and a total mass of 650Mo. Its core, half-mass and
tidal radii also are within the frequent values of LMC star clusters. However,
the new star cluster is placed at the Small Magellanic Cloud distance and at
11.3 kpc from the LMC centre. We speculate with the possibility that it was
born in the inner body of the LMC and soon after expeled into the intergalactic
space during the recent Milky Way/MCs interaction. Nevertheless, radial
velocity and chemical abundance measurements are needed to further understand
its origin, as well as extensive search for loose star clusters in order to
constrain the effectiveness of star cluster scattering during galaxy
interactions.Comment: MNRAS Letters, 5 pages, 2 figures (in press
Bruck 88 : a young star cluster with an old age resemblance in the outskirts of the Small Magellanic Cloud
We present spectroscopic and photometric results for the Small Magellanic
Cloud (SMC) cluster Bruck 88. From the comparison of the cluster integrated
spectrum with template cluster spectra we found that the Milky Way globular
cluster template spectra are the ones which best resemble it. However, the
extracted cluster colour magnitude diagram reveals that Bruck 88 is a young
cluster (log(t) = 8.1 +- 0.1). The derived cluster age is compatible with the
presence of a Bright Red Giant (BRG) star located ~ 2.6 arcsec in the sky from
the cluster centre. We serendipitously observed HW 33, a star cluster located ~
3 arcmin to the south-east from Bruck 88. We obtained for the cluster the same
age than Bruck 88 and surprisingly, a BRG star located within the cluster
radius also appears to be compatible with the cluster age. We estimated the MK
type of the BRG star in the Bruck 88 field to be in the range G9 II/Ib - K1
III. By combining the spectrum of a star within this MK type range with a
100-150 Myr template cluster integrated spectrum, we found that a proportion
85/15 in the sense BRG/template results in a spectrum which best resembles that
of Bruck 88. This result confirms that a BRG star dominates the cluster
integrated spectrum, so that it causes the globular cluster appearance of its
integrated light.Comment: MNRAS, accepte
The star cluster frequency throughout the Large Magellanic Cloud
We address the issue about the variation of the star cluster frequency (CF)
in the Large Magellanic Cloud (LMC) in terms of the cluster spatial
distribution. We adopted the LMC regions traced by Harris & Zaritsky (2009) and
used an updated version of the cluster database compiled by Baumgardt et al.
(2013). The CFs were produced by taking into account an appropriate selection
of age bins. Since the uncertainty in a cluster's age can be large compared to
the size of the age bins, we account for the fact that a cluster could actually
reside in one of a few adjacent age bins. We confirm that there exist some
variations of the LMC CFs in terms of their spatial distributions, although
some caveats should be pointed out. 30 Doradus resulted to be the region with
the highest relative frequency of youngest clusters, while the log() = 9-9.5
(1-3 Gyr) age range is featured by cluster formation at a higher rate in the
inner regions than in the outer ones. We compared the observed CFs to
theoretical CFs, which are based on the star formation histories of the field
stars in each region of the LMC, and found the former predicting more or fewer
clusters than observed depending on the field and age range considered.Comment: 27 pages, 14 figures, MNRAS, accepte
On the physical nature of globular cluster candidates in the Milky Way bulge
We present results from 2MASS JKs photometry on the physical reality of
recently reported globular cluster (GC) candidates in the Milky Way (MW) bulge.
We relied our analysis on photometric membership probabilities that allowed us
to distinguish real stellar aggregates from the composite field star
population. When building colour-magnitude diagrams and stellar density maps
for stars at different membership probability levels, the genuine GC candidate
populations are clearly highlighted. We then used the tip of the red giant
branch (RGB) as distance estimator, resulting heliocentric distances that place
many of the objects in regions near of the MW bulge where no GC had been
previously recognised. Some few GC candidates resulted to be MW halo/disc
objects.Metallicities estimated from the standard RGB method are in agreement
with the values expected according to the position of the GC candidates in the
Galaxy. We finally derived from the first time their structural parameters. We
found that the studied objects have core, half-light and tidal radii in the
ranges spanned by the population of known MW GCs. Their internal dynamical
evolutionary stages will be described properly when their masses are estimated.Comment: 11 pages, 7 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The age-metallicity relationship in the Small Magellanic Cloud periphery
We present results from Washington CT1 photometry for eleven star fields
located in the western outskirts of the Small Magellanic Cloud (SMC), which
cover angular distances to its centre from 2 up to 13 degrees (~ 2.2 - 13.8
kpc). The colour- magnitude diagrams, cleaned from the unavoidable Milky Way
(MW) and background galaxy signatures, reveal that the most distant dominant
main sequence (MS) stellar populations from the SMC centre are located at an
angular distance of ~ 5.7 deg (6.1 kpc); no sign of farther clear SMC MS is
visible other than the residuals from the MW/background field contamination.
The derived ages and metallicities for the dominant stellar populations of the
western SMC periphery show a constant metallicity level ([Fe/H] = -1.0 dex) and
an approximately constant age value (~ 7-8 Gyr). Their age-metallicity
relationship (AMR) do not clearly differ from the most comprehensive AMRs
derived for almost the entire SMC main body. Finally, the range of ages of the
dominant stellar populations in the western SMC periphery confirms that the
major stellar mass formation activity at the very early galaxy epoch peaked ~
7-8 Gyr ago.Comment: 20 pages, 7 figures, MNRAS (accepted
Extended Main Sequence Turn-Offs in Low Mass Intermediate Age Clusters
We present an imaging analysis of four low mass stellar clusters (< 5000 Mo)
in the outer regions of the LMC in order to shed light on the extended main
sequence turn-off (eMSTO) phenomenon observed in high mass clusters. The four
clusters have ages between 1-2 Gyr and two of them appear to host eMTSOs. The
discovery of eMSTOs in such low mass clusters - > 5 times less massive than the
eMSTO clusters previously studied - suggests that mass is not the controlling
factor in whether clusters host eMSTOs. Additionally, the narrow extent of the
eMSTO in the two older (~ 2 Gyr) clusters is in agreement with predictions of
the stellar rotation scenario, as lower mass stars are expected to be
magnetically braked, meaning that their CMDs should be better reproduced by
canonical simple stellar populations. We also performed a structural analysis
on all the clusters and found that a large core radius is not a requisite for a
cluster to exhibit an eMSTO.Comment: Astronomy & Astrophyscis, 12 pages, 7 figures, accepte
New age-metallicity diagnostic diagram for the Washington photometric system
The age calibration of the Washington deltaT1 index is mainly used to
estimate ages of star clusters older than 1 Gyr, no age-metallicity degeneracy
effect is considered. We have profusely exploited synthetic T1 versus C-T1
colour magnitude diagrams aiming at exploring the intrinsic behaviour of the
deltaT1 index. The analysis shows that deltaT1 varies with age and metal
content as well. In general, the dependence on age weakens for ages greater
than ~ 6 Gyr, and results even less sensitive to age as the metallicity
decreases. For ages younger than ~ 5 Gyr deltaT1 shows a strong correlation
with both age and metallicity. The deltaC index -defined as deltaT1 for the C
passband- is also a combined measurement of age and metallicity. We introduce a
new age-metallicity diagnostic diagram, deltaT1 versus deltaC - deltaT1, which
has shown the ability of unambiguously providing age and metallicity estimates,
simultaneously. The new procedure allows to derive ages from 1 up to 13 Gyr and
metallicities [Fe/H] from -2.0 up to +0.5 dex, and is independent of the
cluster reddening and distance modulus. It does solve the constraints found in
the deltaT1 index and surpasses the performance of the standard giant branch
metallicity method. All these features make the diagnostic diagram a powerful
tool for estimating accurate ages as well as metallicities.Comment: 14 pages, 7 figures, accepted for publication in MNRA
On the structure of Small Magellanic Cloud star clusters
It has been recently shown from observational data sets the variation of
structural parameters and internal dynamical evolution of star clusters in the
Milky Way and in the Large Magellanic Cloud (LMC), caused by the different
gravitational field strengths that they experience. We report here some hints
for such a differential tidal effects in structural parameters of star clusters
in the Small Magellanic Cloud (SMC), which is nearly 10 times less massive than
the LMC. A key contribution to this study is the consideration of the SMC as a
triaxial spheroid, from which we estimate the deprojected distances to the SMC
center of the statistically significant sample of star clusters analyzed. By
adopting a 3D geometry of the SMC, we avoid the spurious effects caused by
considering that a star cluster observed along the line-of-sight is close to
the galaxy center. When inspecting the relationships between the star cluster
sizes (represented by the 90% light radii), their eccentricities, masses and
ages with the deprojected distances, we find: (i) the star cluster sizes are
not visibly affected by tidal effects, because relatively small and large
objects are spread through the SMC body. (ii) Star clusters with large
eccentricities (> 0.4) are preferentially found located at deprojected
distances smaller than 7-8 kpc, although many star clusters with smaller
eccentricities are also found occupying a similar volume. (iii) Star clusters
more massive than log(M /Mo) 4.0 are among the oldest star clusters,
generally placed in the outermost SMC region and with a relative small level of
flattening. These findings contrast with the more elongated, generally younger,
less massive and innermost star clusters.Comment: 6 pages, 6 figures. Accepted por publication in Astronomy &
Astrophysic
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