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The VISCACHA (VIsible Soar photometry of star Clusters in tApii and Coxi HuguA†) Survey is an ongoing project based on deep and spatially resolved photometric observations of Magellanic Cloud star clusters, collected using the SOuthern Astrophysical Research (SOAR) telescope together with the SOAR Adaptive Module Imager. So far we have used >300h of telescope time to observe ∼150 star clusters, mostly with low mass (M < 104M⊙) on the outskirts of the LMC and SMC. With this high-quality data set, we homogeneously determine physical properties using deep colour-magnitude diagrams (ages, metallicities, reddening, distances, mass, luminosity and mass functions) and structural parameters (radial density profiles, sizes) for these clusters which are used as a proxy to investigate the interplay between the Magellanic Clouds and their evolution. We present the VISCACHA survey and its initial results, based on our first two papers. The project’s long term goals and expected legacy to the community are also addressed.
When we use optical isochrone-fitting solutions from the literature to 2mass CMDs, they are often not the best solutions in the infrared domain. We analysed 10 open clusters with 2mass, nine of them previously studied with optical photometry (NGC 1245, NGC 1342, NGC 1502, NGC 2104, NGC 2204, NGC 2243, NGC 2281, NGC 6709 and NGC 744) and one using integrated spectroscopy (BH 132). The study involved the classical (by eye) and a semi-automated method of isochrone fitting. We used the solutions of the first method as input for the second, looking for refined solutions. The semi-automated method uses a synthetic color–magnitude diagram (CMD), based on different Padova isochrones, to compare with the observed CMDs by means of likelihood statistics. The derived astrophysical parameters are age, distance and reddening values. The present results show better fits than those implied by the optical values. We also show that the semi-automated method decreases the parameter uncertainties.
We present photometry for the globular cluster NGC 6642 using the F606W and F814W filters with the ACS/WFC third-generation camera aboard the Hubble Space Telescope. The colour–magnitude diagram shows sources reaching ≈ 6 mag below the turnoff in mF606W. Theoretical isochrone fitting was performed and evolutionary parameters were obtained, including the metallicity [Fe/H] = −1.80 ± 0.2 dex and age, log(age/yr) = 10.14 ± 0.05. We confirm that NGC 6642 is located in the Galactic bulge, at a distance of d⊙ = 8.05±0.66 kpc and suffers from a reddening of E(B − V) = 0.46 ± 0.02 mag. These values are in general agreement with those of previous authors. Completeness-corrected luminosity and mass functions were obtained for different annuli centred on NGC 6642. Their spatial variation indicates the existence of mass segregation and depletion of low-mass stars. Most striking is the inverted shape of the mass function itself, with an increase in stellar numbers as a function of increasing mass. This has been observed previously in other globular clusters and is also the result of N-body simulations of stellar systems which have reached ≃90% of their lifetime and are subjected to strong tidal effects. We thus conclude that NGC 6642 is a very old, highly evolved globular cluster. Its current location close to perigalacticon, at only 1.4 kpc from the Galactic Centre, may contribute to this high level of dynamical evolution and stellar depletion.
We present the distribution of Galactic bulge globular clusters and a method based on simultaneous detection of field and cluster horizontal branches to derive the cluster distances. This method has the advantage of being independent of both reddening and the reddening law, RV = AV/E(B−V). The vast majority of clusters projected in the direction of the Galactic bulge are located on the near side of the Galactic Center. Deviations from the reddening law do not seem to be responsible for this peculiarity. We need to introduce a peculiar, steep dependence of the absolute horizontal-branch magnitude with metallicity in the metal-rich regime if we want to reproduce a symmetrical distribution. Instead, if the observed distribution is correct, we expect a rather large number of bulge globular clusters are still to be discovered.
To study the evolution of binary star clusters, we have imaged seven systems in the Small Magellanic Cloud with the SOAR 4m telescope using B and V filters. The sample contains pairs with well-separated components (d < 30 pc) as well as systems that apparently have merged, as evidenced by their unusual structures. By employing isochrone fitting to their color–magnitude diagrams, we have determined reddening values, ages and metallicities, and by fitting King models to their radial stellar-density profiles we estimated core radii. Disturbances of the density profiles are interpreted as evidence of interactions. Properties such as the distances between their components and their age differences are addressed in terms of the timescales involved, to assess the physical connection of the system. In two cases, the age difference is more than 50 Myr, which suggests a chance alignment, capture or sequential star formation.
We present flux-calibrated integrated spectra in the optical spectral range of Galactic open clusters (GOCs) and Magellanic Cloud (MC) stellar clusters (SCs) obtained at CASLEO (Argentina). The SC parameters were derived using the equivalent-width (EW) method and the template-matching procedure by comparing the line strengths and continuum distribution of the cluster spectra with those of template spectra with known parameters. MC cluster reddening values were also estimated by interpolation between the available extinction maps. The derived ages for the GOCs range from 3 Myr to 4 Gyr, while those of the MC SCs vary from 3 Myr to 7 Gyr. E(B−V) colour-excess values in the MCs appear to be all lower than 0.17 mag, while those of the GOCs range from 0.00 to 2.40 mag. The present data led us to upgrade the spectral libraries of reference spectra or templates of solar and MC metallicities.
The metallicity distribution and abundance ratios of the Galactic bulge are reviewed. Issues raised by different groups in recent work, in particular the high metallicity end, a comparison between the oxygen abundances derived from different indicators, the [OI] 630nm and IR OH lines, and the issue of measuring giants vs. dwarfs, are discussed. Finally, abundances in bulge globular clusters are briefly described.
Washington CCD photometry of intermediate-age clusters in the Small Magellanic Cloud (SMC) is presented. The cluster age distribution in the SMC suggests formation epochs at 3 and 6 Gyr, respectively. Recent star formation is confined to the central body of the SMC. The chemical evolution of the SMC appears to be best represented by bursty models.
We present a set of star cluster integrated spectra based on observations from the ultraviolet to the near-infrared. Mean template spectra were built following a ranking of clusters according to age and metallicity, which were taken from recent studies and put on consistent scales. The complete spectral database including star clusters, galaxies and stars (templates and individuals) is available through the CDS.
We have mainly estimated ages and foreground interstellar reddening values for 15 SMC concentrated star clusters, from integrated spectra in the range (3600–6800) Å. Most of the sample are young blue objects (6–60 Myr), while L28, NGC 643 and L 114 are found to be intermediate-age clusters (1–6 Gyr). The present data also constitute a spectral library at the metallicity level of SMC clusters.
We report on a survey of 25 candidate old LMC clusters. Washington photometry was obtained on the CTIO 0.9m in < 1h, reaching below the turnoff. Ages based on the magnitude difference δT1 between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates all turned out to be of intermediate age (1–3 Gyr). We confirm that there was apparently no cluster formation in the LMC from 3–9 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at ~1.5 Gyrs ago. We also determine ages for the surrounding fields, as well as metallicities for both the clusters and fields from the color of the giant branch. In most cases the stellar population of each cluster is quite similar to that of its field. The mean metallicity for the intermediate age outer disk clusters is −0.65. A few clusters stand out in the age-metallicity relation in the sense that they are intermediate age clusters at relatively low metallicity. In the northern part of the LMC disk 3 fields all have a secondary clump ~0.45 mag fainter than the dominant clump, suggesting a component located behind the LMC at a distance comparable to that of the SMC.
Observational evidences of dust in the nuclear region of AGNs are substantial (Rudy 1984, ApJ, 284, 33; Jones et al. 1984, PASP, 96, 692). The ionization cones observed in several Seyfert galaxies has been interpreted as shadowing effects by a dust obscuring torus which hides the broad emission line region (BLR) and the central source (Wilson 1992; Storchi-Bergmann, Mulchaey and Wilson 1992, ApJ 395, L73). A large sample of optical and far-IR data for IRAS Seyfert galaxies has been analysed together with dust emission models (Bonatto and Pastoriza 1993), where it has been concluded that the same dust emission model can be applied to both Seyfert types. In order to further study the effects of dust in the spectra of active galactic nuclei, we have obtained spectrophotometry of 21 IRAS Seyfert galaxies in the range 3500–7200 Å and analyse them in conjuction with their IRAS fluxes. The stellar population type is derived from comparisons with normal galaxy templates using dilution effects in the K CaII line as discriminator. For 55% of the sample the population is of late type. For the rest, blue continua due to recent star formation and/or power-law may amount up to 30% at 4000Å. We conclude that the bulge stellar populations of IRAS Seyfert galaxies are similar to those of normal spirals, except that they are more reddened by E(B-V)i ∼ 0.20. Population-subtracted emission line ratios indicate on average stronger reddening for the narrow-line region (E(B-V)l ∼ 0.8. From photoionization models a power-law index for the ionizing continuum α=1.5, and a metallicity larger than solar are obtained. The most luminous IRAS galaxy of the sample (IRAS555) is discuss in detail: in order to be compatible with the observed IRAS fluxes and the optical stellar continuum, the ionizing continuum must be reddened by AV > 10 magnitudes. Consequently a dust structure in this galaxy appears to be increasingly affecting stars and gas towards the galaxy center.
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