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

Disentangling the physical reality of star cluster candidates projected towards the inner disc of the Large Magellanic Cloud

We have used Washington photometry for 90 star cluster candidates of small angular size -typically ~ 11" in radius- distributed within nine selected regions in the inner disc of the Large Magellanic Cloud (LMC) to disentangle whether they are genuine physical system, and to estimate the ages for the confirmed clusters. In order to avoid a misleading interpretation of the cluster colour-magnitude diagrams (CMDs), we applied a subtraction procedure to statistically clean them from field star contamination. Out of the 90 candidate clusters studied, 61 of them resulted to be genuine physical systems, whereas the remaining ones were classified as possible non- clusters since either their CMDs and/or the distribution of stars in the respective fields do not resemble those of stellar aggregates. We statistically show that ~ (13 +- 6)% of the catalogued clusters in the inner disc could be possible non-clusters, independently of their deprojected distances. We derived the ages for the confirmed clusters from the fit of theoretical isochrones to the cleaned cluster CMDs. The derived ages resulted to be in the age range 7.8 < log(t) < 9.2. Finally, we built cluster frequencies for the different studied regions and found that there exists some spatial variation of the LMC CF throughout the inner disc. Particularly, the innermost field contains a handful of clusters older than ~ 2 Gyr, while the wider spread between different CFs has taken place during the most recent 50 Myr of the galaxy lifetime.Comment: MNRAS, accepte

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

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($t$) = 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

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

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

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