arXiv subjects

Andrés E. Piatti

Publications and source records attributed to Andrés E. Piatti.

At least 73 records · Page 4Linked to original sources

arXiv2018

Two kinematically distinct old globular cluster populations in the Large Magellanic Cloud

We report results of proper motions of 15 known Large Magellanic Cloud (LMC) old globular clusters (GCs) derived from the Gaia DR2 data sets. When these mean proper motions are gathered with existent radial velocity measurements to compose the GCs' velocity vectors, we found that the projection of the velocity vectors onto the LMC plane and those perpendicular to it tell us about two distinct kinematic GC populations. Such a distinction becomes clear if the GCs are split at a perpendicular velocity of 10 km/s (absolute value). The two different kinematic groups also exhibit different spatial distributions. Those with smaller vertical velocities are part of the LMC disc, while those with larger values are closely distributed like a spherical component. Since GCs in both kinematic-structural components share similar ages and metallicities, we speculate with the possibility that their origins could have occurred through a fast collapse that formed halo and disc concurrently.

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arXiv2018

Search for a metallicity spread in the multiple population Large Magellanic Cloud cluster NGC 1978

We report on the spread of [Fe/H] values in the massive Large Magellanic Cloud cluster NGC1978, recently confirmed to harbor multiple populations of nearly the same age. We used accurate Stromgren photometry of carefully selected cluster red giant branch stars along with a high-dispersion spectroscopy-based calibration of the metallicity-sensitive index m1. Once accounting for the photometry quality, assessed from extensive artificial stars tests to trace the photometric uncertainties as a function of the position to the cluster's center as well as the stellar brightness, and those from the metallicity calibration, we found that NGC1978 exhibits a small metallicity spread of 0.035 dex (+- 0.019-0.023), depending on whether stars with individual sigma[Fe/H] < 0.15 dex or those located in the cluster's outer areas are considered. Such a spread in [Fe/H] is consistent with a cluster formation model with self-enrichment, if mass loss higher than 90 per cent due to stellar evolutionary and galactic tidal effects is assumed. Nevertheless, scenarios where the apparent [Fe/H] variation reflects CN abundance anomalies or less extreme mass loss models with environmentally-dependent self enrichment should not be ruled out.

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arXiv2018

A likely runaway star cluster in the outer disc of the Large Magellanic Cloud

We present results from photometric and spectroscopic data obtained with SOAR and Gemini observatory facilities in the field of a recently discovered star cluster. The cluster, projected towards the Eastern side of the outer disc of the Large Magellanic Cloud (LMC), was originally placed nearly 10 kpc behind the LMC with an age and metallicity typical of the innermost LMC star cluster population. We assigned radial velocity (RV) memberships to stars observed spectroscopically, and derived the cluster age and distance from theoretical isochrone fitting to the cluster colour-magnitude diagram. The new object turned out to be a 0.9 Gyr old outer LMC disc cluster, which possibly reached the present position after being scattered from the innermost LMC regions where it might have been born. We arrived at this conclusion by examining the spatial distribution of LMC star clusters of similar age, by comparing the derived spectroscopic metallicity with that expected for an outside-in galaxy formation scenario, by considering the cluster internal dynamical stage as inferred from its derived structural parameters and by estimating the circular velocity of a disc that rotates with the corresponding star cluster radial velocity at the cluster's deprojected distance, which resulted to be nearly 60 per cent higher than that of most of the outer LMC disc clusters.

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arXiv2018

Hints for multiple populations in intermediate-age clusters of the Small Magellanic Cloud

We report on the magnitude of the intrinsic [Fe/H] spread in the Small Magellanic Cloud (SMC) intermediate-age massive clusters NGC 339, 361, Lindsay 1 and 113, respectively. In order to measure the cluster metallicity dispersions, we used accurate Stromgren photometry of carefully selected cluster red giant branch (RGB) stars. We determined the Fe-abundance spreads by employing a maximum likelihood approach. The spreads obtained using the more accurate photometry of the brighter RGB stars resulted to be marginal (~ 0.05+-0.03 dex) for NGC 339 and NGC 361, while for Lindsay 1 and Lindsay 113 we obtained metallicity spreads of 0.00+-0.04 dex. From these results, we speculated with the possibility that NGC 361 is added to the group of four SMC clusters with observational evidence of multiple populations (MPs). Furthermore, in the context of the present debate about the existence of Fe-abundance inhomogeneities among old clusters with MPs, these outcomes put new constrains to recent theoretical speculations for making this phenomenon visible.

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arXiv2018

Search for an intrinsic metallicity spread in old globular clusters of the Large Magellanic Cloud

We report for the first time on the magnitude of the intrinsic [Fe/H] spread among ten old globular clusters (GCs) of the Large Magellanic Cloud (LMC). Such spreads are merely observed in approximately five per cent of the Milky Way GCs and recently gained more attention in theoretical models of GC evolution. We derived metallicities with a typical precision of 0.05 dex < sigma[Fe/H] < 0.20 dex for an average of 14 red giant branch stars per GC from Strömgren photometry. The respective, metallicity-sensitive indices have been calibrated to precise and accurate high-dispersion spectroscopy. For all clusters we found null [Fe/H] spreads with a typical uncertainty of 0.04 dex, with the possible exception of NGC1786 that shows an intrinsic dispersion of 0.07+-0.04 dex. The mean, observed standard deviation of the derived metallicities for nearly 40 per cent of our GC sample amounted to smaller than 0.05 dex. At present, we cannot exclude that the remaining GCs also have intrinsic Fe-abundance variations in excess of 0.05 dex, but in order to significantly detect those, the measurement errors on individual [Fe/H]-values would need to be lowered to the 0.03--0.07 dex level. These findings suggest, along with those from ages and light-element abundances, that the LMC GCs studied here are alike to the majority of Galactic GCs.

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arXiv2018

Towards a comprehensive knowledge of the star cluster population in the Small Magellanic Cloud

The Small Magellanic Cloud (SMC) has recently been found to harbour more than two hundred per cent increase of its known cluster population. We provide here with solid evidence that such an unprecedented number of clusters could be largely overestimated. On the one hand, the fully-automatic procedure used to identify such an enormous cluster candidate sample did not recover ~ 50 per cent, in average, of the known relatively bright clusters located in the SMC main body. On the other hand, the number of new cluster candidates per time unit as a function of time results noticeably different to the intrinsic SMC cluster frequency (CF), which should not be the case if these new detections were genuine physical systems. We additionally found that the SMC CF varies spatially, in such a way that it resembles an outside-in process coupled with the effects of a relatively recent interaction with the Large Magellanic Cloud. By assuming that clusters and field stars share the same formation history, we showed for the first time that the cluster dissolution rate also depends on the position in the galaxy. The cluster dissolution results higher as the concentration of galaxy mass increases or external tidal forces are present.

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arXiv2018

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.

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arXiv2017

Sizing the star cluster population of the Large Magellanic Cloud

The number of star clusters that populate the Large Magellanic Cloud (LMC) at deprojected distances < 4 deg has been recently found to be nearly double the known size of the system. Because of the unprecedented consequences of this outcome in our knowledge of the LMC cluster formation and dissolution histories, we closely revisited such a compilation of objects and found that only ~ 35 per cent of the previously known catalogued clusters has been included. The remaining entries are likely related to stellar overdensities of the LMC composite star field, because there is a remarkable enhancement of objects with assigned ages older than log(t yr-1) ~ 9.4, which contrasts with the existence of the LMC cluster age gap; the assumption of a cluster formation rate similar to that of the LMC star field does not help to conciliate so large amount of clusters either; and nearly 50 per cent of them come from cluster search procedures known to produce more than 90 per cent of false detections. The lack of further analyses to confirm the physical reality as genuine star clusters of the identified overdensities also glooms those results. We support that the actual size of the LMC main body cluster population is close to that previously known.

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arXiv2017

Stellar density distribution along the minor axis of the Large Magellanic Cloud

We studied the spatial distribution of young and old stellar populations along the western half part of the minor axis of the Large Magellanic Cloud (LMC) using Washington MT1 photometry of selected fields, which span a deprojected distance range from the LMC bar centre out to ~ 31.6 kpc. We found that both stellar populations share a mean LMC limiting radius of 8.9+-0.4 kpc; old populations are three times more dense that young populations at that LMC limit. When comparing this result with recent values for the LMC extension due to north, the old populations resulted significantly more elongated than the young ones. Bearing in mind previous claims that the elongation of the outermost LMC regions may be due to the tidal effects of the Milky Way (MW), our findings suggest that such a tidal interaction should not have taken place recently. The existence of young populations in the outermost western regions also supports previous results about ram pressure stripping effects of the LMC gaseous disc due to the motion of the LMC in the MW halo.

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arXiv2017

The VMC Survey XXVII. Young Stellar Structures in the LMC$'$s Bar Star-Forming Complex

Star formation is a hierarchical process, forming young stellar structures of star clusters, associations, and complexes over a wide scale range. The star-forming complex in the bar region of the Large Magellanic Cloud is investigated with upper main-sequence stars observed by the VISTA Survey of the Magellanic Clouds. The upper main-sequence stars exhibit highly non-uniform distributions. Young stellar structures inside the complex are identified from the stellar density map as density enhancements of different significance levels. We find that these structures are hierarchically organized such that larger, lower-density structures contain one or several smaller, higher-density ones. They follow power-law size and mass distributions as well as a lognormal surface density distribution. All these results support a scenario of hierarchical star formation regulated by turbulence. The temporal evolution of young stellar structures is explored by using subsamples of upper main-sequence stars with different magnitude and age ranges. While the youngest subsample, with a median age of log($τ$/yr)~=~7.2, contains most substructure, progressively older ones are less and less substructured. The oldest subsample, with a median age of log($τ$/yr)~=~8.0, is almost indistinguishable from a uniform distribution on spatial scales of 30--300~pc, suggesting that the young stellar structures are completely dispersed on a timescale of $\sim$100~Myr. These results are consistent with the characteristics of the 30~Doradus complex and the entire Large Magellanic Cloud, suggesting no significant environmental effects. We further point out that the fractal dimension may be method-dependent for stellar samples with significant age spreads.

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arXiv2017

On the extended stellar structure around NGC 288

We report on observational evidence of an extra-tidal clumpy structure around NGC 288 from an homogeneous coverage of a large area with the Pan-STARRS PS1 database. The extra-tidal star population has been disentangled from that of the Milky Way field by using a cleaning technique that successfully reproduced the stellar density, luminosity function and colour distributions of MW field stars. We have produced the cluster stellar density radial profile and a stellar density map from independent approaches, from which we found results in excellent agreement : the feature extends up to 3.5 times the cluster tidal radius. Previous works based on shallower photometric data sets have speculated on the existence of several long tidal tails, similar to that found in Pal 5. The present outcome shows that NGC 288 could hardly have such tails, but favours the notion that interactions with the MW tidal field has been a relatively inefficient process for stripping stars off the cluster. These results point to the need of a renewed overall study of the external regions of Galactic globular clusters (GGCs) in order to reliably characterise them. Hence, it will be possible to investigate whether there is any connection between detected tidal tails, extra-tidal stellar populations, extent diffuse halo-like structures with the GGCs' dynamical histories in the Galaxy.

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arXiv2017

Star cluster formation history along the minor axis of the Large Magellanic Cloud

We analysed Washington $CMT_1$ photometry of star clusters located along the minor axis of the LMC, from the LMC optical centre up to $\sim$ 39 degrees outwards to the North-West. The data base was exploited in order to search for new star cluster candidates, to produce cluster CMDs cleaned from field star contamination and to derive age estimates for a statistically complete cluster sample. We confirmed that 146 star cluster candidates are genuine physical systems, and concluded that an overall $\sim$ 30 per cent of catalogued clusters in the surveyed regions are unlikely to be true physical systems. We did not find any new cluster candidates in the outskirts of the LMC (deprojected distance $\ge$ 8 degrees). The derived ages of the studied clusters are in the range 7.2 < log($t$ yr$^{-1}$) $\le$ 9.4, with the sole exception of the globular cluster NGC\,1786 (log($t$ yr$^{-1}$) = 10.10). We also calculated the cluster frequency for each region, from which we confirmed previously proposed outside-in formation scenarios. In addition, we found that the outer LMC fields show a sudden episode of cluster formation (log($t$ yr$^{-1}$) $\sim$ 7.8-7.9) that continued until log($t$ yr$^{-1}$) $\sim$ 7.3 only in the outermost LMC region. We link these features to the first pericentre passage of the LMC to the MW, which could have triggered cluster formation due to ram pressure interaction between the LMC and MW halo.

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arXiv2017

Detection of a diffuse extended halo-like structure around 47 Tuc

We constructed for the first time a stellar density profile of 47 Tucanae (47 Tuc) out of $\sim$ 5.5 times its tidal radius ($r_t$) using high-quality deep $BV$ photometry. After carefully considering the influence of photometric errors, and Milky Way and Small Magellanic Cloud composite stellar population contamination, we found that the cluster stellar density profile reaches a nearly constant value from $\sim$ 1.7$r_t$ outwards, which does not depend on the direction from the cluster's center considered. These results visibly contrast with recent distinct theoretical predictions on the existence of tidal tails or on a density profile that falls as $r^{-4}$ at large distances, and with observational outcomes of a clumpy structure as well. Our results suggest that the envelope of 47 Tuc is a halo- like nearly constant low density structure.

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arXiv2017

The real population of star clusters in the bar of the Large Magellanic Cloud

We report results on star clusters located in the South-Eastern half of the Large Magellanic (LMC) bar from Washington $CT_1$ photometry. Using appropriate kernel density estimators we detected 73 star cluster candidates, three of which do not show any detectable trace of star cluster sequences in their colour-magnitude diagrams (CMDs). We did not detect other 38 previously catalogued clusters, which could not be recognized when visually inspecting the $C$ and $T_1$ images either; the distribution of stars in their respective fields do not resemble that of an stellar aggregate. They represent $\sim$ 33 per cent of all catalogued objects located within the analysed LMC bar field. From matching theoretical isochrones to the cluster CMDs cleaned from field star contamination, we derived ages in the range 7.2 < log($t$ yr$^{-1}$) < 10.1. As far as we are aware, this is the first time homogeneous age estimates based on resolved stellar photometry are obtained for most of the studied clusters. We built the cluster frequency (CF) for the surveyed area, and found that the major star cluster formation activity has taken place during the period log($t$ yr$^{-1}$) $\sim$ 8.0 -- 9.0. Since $\sim$ 100 Myr ago, clusters have been formed during few bursting formation episodes. When comparing the observed CF to that recovered from the star formation rate we found noticeable differences, which suggests that field star and star cluster formation histories could have been significantly different.

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arXiv2017

Observational hints of a real age spread in the young LMC star cluster NGC 1971

We report the serendipitous young Large Magellanic Cloud cluster, NGC 1971, exhibits an extended main-sequence turnoff (eMSTO) possibly originated by mostly a real age spread. We used CT1 Washington photometry to produce a colour-magnitude diagram (CMD) with the fiducial cluster features. From its eMSTO, we estimated an age spread of ~ 170 Myr (observed age range 100-280 Myr), once observational errors, stellar binarity, overall metalicity variations and stellar rotation effects were subtracted in quadrature from the observed age width.

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arXiv2017

Representative galaxy age-metallicity relationships

The ongoing surveys of galaxies and those for the next generation of telescopes will demand the execution of high-CPU consuming machine codes for recovering detailed star formation histories (SFHs) and hence age-metallicity relationships (AMRs). We present here an expeditive method which provides quick-look AMRs on the basis of representative ages and metallicities obtained from colour-magnitude diagram (CMD) analyses. We have tested its perfomance by generating synthetic CMDs for a wide variety of galaxy SFHs. The representative AMRs turn out to be reliable down to a magnitude limit with a photometric completeness factor higher than $\sim$ 85 per cent, and trace the chemical evolution history for any stellar population (represented by a mean age and an intrinsic age spread) with a total mass within ~ 40 per cent of the more massive stellar population in the galaxy.

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arXiv2017

Candidate star clusters toward the inner Milky Way discovered on deep-stacked Ks-band images from the VVV Survey

The census of star clusters in the inner Milky Way is incomplete because of extinction and crowding. We embarked on a program to expand the star cluster list in the direction of the inner Milky Way using deep stacks of Ks-band images from the VISTA Variables in Via Lactea (VVV) Survey. We applied an automated two-step procedure to the point-source catalog derived from the deep Ks images: first, we identified overdensities of stars, and then we selected only candidate clusters with probable member stars that match an isochrone with a certain age, distance, and extinction on the color-magnitude diagram. This pilot project only investigates the cluster population in part of one VVV tile, that is, b201. We identified nine cluster candidates and estimated their parameters. The new candidates are compact with a typical radius on the sky of ~0.2--0.4 arcmin (~0.4-1.6 pc at their estimated distances). They are located at distances of ~5-14 kpc from the Sun and are subject to moderate extinction of E(B-V)=0.4-1.0 mag. They are sparse, probably evolved, with typical ages log(t/1 yr)~9. Based on the locations of the objects inside the Milky Way, we conclude that one of these objects is probably associated with the disk or halo and the remaining objects are associated with the bulge or the halo. The cluster candidates reported here push the VVV Survey cluster detection to the limit. These new objects demonstrate that the VVV survey has the potential to identify thousands of additional cluster candidates. The sub-arcsec angular resolution nd the near-infrared wavelength regimen give it a critical advantage over other surveys.

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arXiv2017

Astrophysical properties of star clusters in the Magellanic Clouds homogeneously estimated by $\texttt{ASteCA}$

Aims. To produce an homogeneous catalog of astrophysical parameters of 239 resolved star clusters located in the Small and Large Magellanic Clouds, observed in the Washington photometric system. Methods. The cluster sample was processed with the recently introduced Automated Stellar Cluster Analysis ($\texttt{ASteCA}$) package, which ensures both an automatized and a fully reproducible treatment, together with a statistically based analysis of their fundamental parameters and associated uncertainties. The fundamental parameters determined with this tool for each cluster, via a color-magnitude diagram (CMD) analysis, are: metallicity, age, reddening, distance modulus, and total mass. Results. We generated an homogeneous catalog of structural and fundamental parameters for the studied cluster sample, and performed a detailed internal error analysis along with a thorough comparison with values taken from twenty-six published articles. We studied the distribution of cluster fundamental parameters in both Clouds, and obtained their age-metallicity relationships. Conclusions. The $\texttt{ASteCA}$ package can be applied to an unsupervised determination of fundamental cluster parameters; a task of increasing relevance as more data becomes available through upcoming surveys.

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