On the self-consistent physical parameters of LMC intermediate-age clusters

The LMC clusters are unique templates of simple stellar population (SSP), being crucial to calibrate models describing the integral light as well as to test the stellar evolution theory. With this in mind we analyzed HST/WFPC2 (V, B--V) colour-magnitude diagrams (CMDs) of 15 populous LMC clusters with ages between ~0.3 Gyr and ~4 Gyr using different stellar evolutionary models (Padova, PEL or Pisa, BaSTI or Teramo). Following the approach described by Kerber, Santiago & Brocato (2007), we determined accurate and self-consistent physical parameters (age, metallicity, distance modulus and reddening) for each cluster by comparing the observed CMDs with synthetic ones. We found significant trends in the physical parameters due to the choice of stellar evolutionary model and treatment of convective core overshooting. In general, models that incorporate overshooting presented more reliable results than those that do not. Comparisons with the results found in the literature demonstrated that our derived metallicities are in good agreement with the ones from the spectroscopy of red giants. We also confirmed that, independent of the adopted stellar evolutionary library, the recovered 3D distribution for these clusters is consistent with a thick disk roughly aligned with the LMC disk as defined by field stars. Finally, we also provide new estimates of distance modulus to the LMC center, that are marginally consistent with the canonical value of 18.50.Comment: 6 pages, 4 figures, conference contribution to IAU Symposium 256, van Loon J.T. & Oliviera J.M., ed

SMC west halo: a slice of the galaxy that is being tidally stripped? Star clusters trace age and metallicity gradients

(ABRIDGED) The evolution and structure of the Magellanic Clouds is presently under debate. The classical scenario where both the Large and Small Magellanic Clouds (LMC, SMC) are orbiting the Milky Way has been challenged by an alternative where the LMC and SMC are in their first close passage to our Galaxy. Detailed studies of stellar populations in the galaxies should constrain the proposed scenarios. In particular, the west halo of the SMC was recently characterized with radial trends in age and metallicity which indicates tidal disruption. We increase the sample of star clusters in the west halo of the SMC with homogeneous age, metallicity, and distance derivations, to determine better age and metallicity gradients. Comparisons of observed and synthetic V,(B-V) colour-magnitude diagrams are used to derive parameters for west halo star clusters. We derived age and metallicity for the reference cluster NGC 152 compatible with literature parameters. Age and metallicity gradients are confirmed in the west halo: 2.6 +/- 0.6 Gyr/deg and -0.19 +/- 0.09 dex/deg, respectively. Age-metallicity relation for the west halo has low dispersion in metallicity and it is compatible with a burst model of chemical enrichment. All WH clusters seem to follow the same predicted stellar distribution, with exception of AM-3 that should belong to the counter-bridge. Bruck 6 is only 130 +/- 40 Myr old and it could have been formed during a recent tidal interaction of the SMC-LMC. We suggest that it is crucial to split the SMC cluster population in groups: main body, wing/bridge, counter-bridge and west halo. This is the way to analyse the complex star formation and dynamical history of our neighbour. In particular we show that west halo has clear age and metallicity gradients and age-metallicity relation, also compatible with the dynamical model of tidal influence of the LMC over the SMC.Comment: 17 pages, 16 figures, 6 tables, in press in A&

Colour-Magnitude Diagrams of candidate age-gap filling LMC clusters

The LMC has a rich star cluster system spanning a wide range of ages and masses. One striking feature of the LMC cluster system is the existence of an age gap between 3-10 Gyrs. Four LMC clusters whose integrated colours are consistent with those of intermediate age simple stellar populations have been imaged with the Optical Imager (SOI) at the Southern Telescope for Astrophysical Research (SOAR). Their colour-magnitude diagrams (CMDs) reach V ~ 24. Isochrone fits, based on Padova evolutionary models, were carried out to these CMDs, after subtraction of field contamination. The preliminary results are as follows: KMK88-38 has an age of ~ 1.3 Gyr, assuming typical LMC metallicity and distance modulus, and a very low redenning. For OGLE-LMC0531, the best eye fits to isochrones yield an age ~ 1.6 Gyr and E(B-V)=0.03. BSDL917 is younger, ~ 150 yrs, and subjected to larger extinction (E(B-V)=0.08). The remaining cluster is currently under analysis. Therefore, we conclude that these clusers are unlikely to fill in the LMC cluster age gap, even when fitting uncertainties in the parameters are considered.Comment: 4 pages, 5 figures, compact form of the poster published online in the proceedings of IAUS256: The Magellanic System: Stars, Gas, and Galaxie

Star clusters with dual red clumps

A few star clusters in the Magellanic Clouds exhibit composite structures in the red-clump region of their colour-magnitude diagrams. The most striking case is NGC419 in the SMC, where the red clump is composed of a main blob as well as a distinct secondary feature. This structure is demonstrated to be real and corresponds to the simultaneous presence of stars which passed through electron degeneracy after central-hydrogen exhaustion and those that did not. This rare occurrence in a single cluster allows us to set stringent constraints on its age and on the efficiency of convective-core overshooting during main-sequence evolution. We present a more detailed analysis of NGC419, together with a first look at other populous LMC clusters which are apparently in the same phase: NGC1751, NGC1783, NGC1806, NGC1846, NGC1852 and NGC1917. We also compare these Magellanic Cloud cases with their Galactic counterparts, NGC752 and NGC7789. We emphasise the extraordinary potential of these clusters as absolute calibration marks on the age scale of stellar populations.Comment: contributed talk at IAUS 266 'Star clusters: basic galactic building blocks', eds R. de Grijs and J. Lepine. A high resolution version of Fig. 1 is available in http://stev.oapd.inaf.it/~lgirardi/NGC419_hr.pd

Self-consistent physical parameters for MC clusters from CMD modelling: application to SMC clusters observed with the SOAR telescope

The Magellanic Clouds (MCs) present a rich system of stellar clusters that can be used to probe the dynamical and chemical evolution of these neighboring and interacting irregular galaxies. In particular, these stellar clusters (SCs) present combinations of age and metallicity that are not found for this class of objects in the Milky Way, being therefore very useful templates to test and to calibrate integrated light simple stellar population (SSP) models applied to unresolved distance galaxies. On its turn, the age and metallicity for a cluster can be determined spatially resolving its stars, by means of analysis of its colour-magnitude diagrams (CMDs). In this work we present our method to determine self-consistent physical parameters (age, metallicity, distance modulus and reddening) for a stellar cluster, from CMDs modelling of relatively unstudied SCs in the Small Magellanic Cloud (SMC) imaged in the BVI filters with the 4.1 m SOAR telescope. Our preliminary results confirm our expectations that come from a previous integrated spectra and colour analysis: at least one of them (Lindsay 2) is an intermediate-age stellar cluster with ~ 2.6 Gyr and [Fe/H] ~ -1.3, being therefore a new interesting witness regarding the reactivation of the star formation in the MCs in the last 4 Gyr.Comment: 4 pages, 2 figures. To be published in the proceedings of IAUS256: The Magellanic System: Stars, Gas, and Galaxie

The star formation history of the SMC star cluster NGC419

The rich SMC star cluster NGC419 has recently been found to present both a broad main sequence turn-off and a dual red clump of giants, in the sharp colour-magnitude diagrams (CMD) derived from the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope. In this work, we apply to the NGC419 data the classical method of star formation history (SFH) recovery via CMD reconstruction, deriving for the first time this function for a star cluster with multiple turn-offs. The values for the cluster metallicity, reddening, distance and binary fraction, were varied within the limits allowed by present observations. The global best-fitting solution is an excellent fit to the data, reproducing all the CMD features with striking accuracy. The corresponding star formation rate is provided together with estimates of its random and systematic errors. Star formation is found to last for at least 700 Myr, and to have a marked peak at the middle of this interval, for an age of 1.5 Gyr. Our findings argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1 Gyr. It remains to be tested whether alternative hypotheses, such as a main sequence spread caused by rotation, could produce similarly good fits to the data.Comment: 10 pages, MNRAS in pres

The star formation history of the Large Magellanic Cloud star cluster NGC1751

The HST/ACS colour-magnitude diagrams (CMD) of the populous LMC star cluster NGC1751 present both a broad main sequence turn-off and a dual clump of red giants. We show that the latter feature is real and associate it to the first appearance of electron-degeneracy in the H-exhausted cores of the cluster stars. We then apply to the NGC1751 data the classical method of star formation history (SFH) recovery via CMD reconstruction, for different radii corresponding to the cluster centre, the cluster outskirts, and the underlying LMC field. The mean SFH derived from the LMC field is taken into account during the stage of SFH-recovery in the cluster regions, in a novel approach which is shown to significantly improve the quality of the SFH results. For the cluster centre, we find a best-fitting solution corresponding to prolonged star formation for a for a timespan of 460 Myr, instead of the two peaks separated by 200 Myr favoured by a previous work based on isochrone fitting. Remarkably, our global best-fitting solution provides an excellent fit to the data - with chi^2 and residuals close to the theoretical minimum - reproducing all the CMD features including the dual red clump. The results for a larger ring region around the centre indicate even longer star formation, but in this case the results are of lower quality, probably because of the differential extinction detected in the area. Therefore, the presence of age gradients in NGC1751 could not be probed. Together with our previous findings for the SMC cluster NGC419, the present results for the NGC1751 centre argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1.5 Gyr.Comment: To appear soon in MNRAS. 12 pages, better printed in colou

Discovery of two distinct red clumps in NGC419: a rare snapshot of a cluster at the onset of degeneracy

Colour-magnitude diagrams (CMD) of the SMC star cluster NGC419, derived from HST/ACS data, reveal a well-delineated secondary clump located below the classical compact red clump typical of intermediate-age populations. We demonstrate that this feature belongs to the cluster itself, rather than to the underlying SMC field. Then, we use synthetic CMDs to show that it corresponds very well to the secondary clump predicted to appear as a result of He-ignition in stars just massive enough to avoid electron-degeneracy settling in their H-exhausted cores. The main red clump instead is made of the slightly less massive stars which passed through electron-degeneracy and ignited He at the tip of the RGB. In other words, NGC419 is the rare snapshot of a cluster while undergoing the fast transition from classical to degenerate H-exhausted cores. At this particular moment of a cluster's life, the colour distance between the main sequence turn-off and the red clump(s) depends sensitively on the amount of convective core overshooting, Lambda_c. By coupling measurements of this colour separation with fits to the red clump morphology, we are able to estimate simultaneously the cluster mean age (1.35(-0.04,+0.11) Gyr) and overshooting efficiency (Lambda_c=0.47(-0.04,+0.14)). Therefore, clusters like NGC419 may constitute important marks in the age scale of intermediate-age populations. After eye inspection of other CMDs derived from HST/ACS data, we suggest that the same secondary clump may also be present in the LMC clusters NGC1751, 1783, 1806, 1846, 1852, and 1917.Comment: To appear in MNRAS Letters (www.blackwell-synergy.com). Better printed in colou

New record of Prozostrodon brasiliensis (Eucynodontia: Prozostrodontia) from its type-locality (Upper Triassic, Southern Brazil): comments on the endocranial morphology

Here we described a new specimen (UFRGS-PV-0543-T) of the non-mamaliaform cynodont Prozostrodon brasiliensis, collected in the Faixa Nova locality (Santa Maria City, State of Rio Grande do Sul, Brazil), which is referred to the Upper Triassic, Hyperodapedon Assemblage Zone. The new specimen includes a portion of the cranium (fragmented nasals, lacrimals, prefrontals, frontals, palatines, vomer, and fragments of the right premaxilla, left maxilla, and pterygoid), preserving the natural endocast of the nasal cavity, and a fragmented right dentary. The specimen is assigned to P. brasiliensis based on the absence of the postorbital bar, the shape and extension of the snout, the height of the horizontal ramus of the dentary, the position of the Meckelian groove, the morphology of the last postcanine (the only one with the crown partially preserved), and fitting size. The natural endocast is composed of sediment that filled in the nasal cavity. On the dorsal surface of the endocast, there is a longitudinal median sulcus formed by the median bony ridge. Laterally to this median sulcus, there is a longitudinal sulcus on each side formed by the lateral crests. The vomer is low and robust, tapering anteriorly and showing a dorsal groove along its entire length. Other aspects of the endocranial anatomy of UFRGS-PV-0543-T, including wide olfactory region and marked oval-shaped olfactory bulbs, are similar to those of other Late Triassic probainognathian cynodonts.Neste trabalho, descrevemos um novo espécime (UFRGS-PV-0543-T) do cinodonte não-mamaliaforme Prozostrodon brasiliensis, coletado na localidade Faixa Nova (Santa Maria, Rio Grande do Sul, Brasil), Triássico Superior, Zona de Associação de Hyperodapedon. O novo espécime é representado por uma porção do crânio (incluindo nasais fragmentados, lacrimais, pré-frontais, frontais, palatinos, vômer, e fragmentos da pré-maxila direita, maxila esquerda e pterigoide), que preserva o molde endocraniano natural da cavidade nasal, e um dentário direito fragmentado. O espécime é atribuído a P. brasiliensis baseado na ausência da barra pós-orbital, a forma e extensão do rostro, a altura do ramo horizontal do dentário, a posição do sulco Meckeliano, a morfologia do último pós-canino (o único com a coroa parcialmente preservada), e tamanho similar ao holótipo. O molde endocraniano natural é formado pelo sedimento que preencheu a cavidade nasal durante o processo de fossilização. Sobre a superfície dorsal desse molde, existem três sulcos longitudinais originados pela crista mediana e cristas laterais, presentes na superfície interna dos ossos que formam o teto da cavidade nasal. O vômer é baixo e robusto, afinado anteriormente e apresenta um sulco dorsal ao longo de todo o seu comprimento. Outros aspectos da anatomia endocraniana de UFRGS-PV-0543-T, incluindo ampla região olfatória e bulbos olfatórios ovais marcados, são semelhantes aos de outros cinodontes probainognátios do Triássico Superior.Fil: Kerber, Leonardo. Universidade Federal de Santa Maria; BrasilFil: Martinelli, Agustín Guillermo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Gusmão Rodrigues, Pablo. Universidade Federal do Rio Grande do Sul; BrasilFil: Ribeiro, Ana Maria. Museu de Ciencias Naturais; BrasilFil: Schultz, Cesar Leandro. Universidade Federal do Rio Grande do Sul; BrasilFil: Bento Soares, Marina. Universidade Federal do Rio de Janeiro; Brasi