A Double Main Sequence in the Globular Cluster NGC 6397

High-precision multi-band HST photometry reveals that the main sequence (MS) of the globular cluster NGC 6397 splits into two components, containing ~30% and ~70% of the stars. This double sequence is consistent with the idea that the cluster hosts two stellar populations: (i) a primordial population that has a composition similar to field stars, and containing ~30% of the stars, and (ii) a second generation with enhanced sodium and nitrogen, depleted carbon and oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the color difference between the two sequences across a variety of color baselines and find that the second sequence is anomalously faint in m_F336W. Theoretical isochrones indicate that this could be due to NH depletion.Comment: 19 pages, 11 figures, accepted for pubblication in Ap

The Gradients in the 47 Tuc Red Giant Branch Bump and Horizontal Branch are Consistent With a Centrally-Concentrated, Helium-Enriched Second Stellar Generation

We combine ground and space-based photometry of the Galactic globular cluster 47 Tuc to measure four independent lines of evidence for a helium gradient in the cluster, whereby stars in the cluster outskirts would have a lower initial helium abundance than stars in and near the cluster core. First and second, we show that the red giant branch bump (RGBB) stars exhibit gradients in their number counts and brightness. With increased separation from the cluster center, they become more numerous relative to the other red giant (RG) stars. They also become fainter. For our third and fourth lines of evidence, we show that the horizontal branch (HB) of the cluster becomes both fainter and redder for sightlines farther from the cluster center. These four results are respectively detected at the 2.3$\sigma$, 3.6$\sigma$, 7.7$\sigma$ and 4.1$\sigma$ levels. Each of these independent lines of evidence is found to be significant in the cluster-outskirts; closer in, the data are more compatible with uniform mixing. Our radial profile is qualitatively consistent with but quantitatively tighter than previous results based on CN absorption. These observations are qualitatively consistent with a scenario wherein a second generation of stars with modestly enhanced helium and CNO abundance formed deep within the gravitational potential of a cluster of previous generation stars having more canonical abundances.Comment: 20 pages, 6 figures, 1 table, submitted to The Astrophysical Journa

The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon

The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon acknowledges that metallicity alone is not enough to describe the HB morphology of all GCs. In particular, the outer Galactic halo contains GCs with redder HBs at a given metallicity than are found inside the Solar circle. Thus, at least a second parameter is required to characterize HB morphology. Here we analyze the median color difference between the HB and the red giant branch (RGB), d(V-I), measured from HST ACS photometry of 60 GCs within ~20 kpc of the Galactic Center. Analysis of this homogeneous data set reveals that, after the influence of metallicity has been removed, the correlation between d(V-I) and age is stronger than that of any other parameter considered. Expanding the sample to include HST photometry of the 6 most distant Galactic GCs lends additional support to the correlation between d(V-I) and age. This result is robust with respect to the adopted metallicity scale and the method of age determination, but must bear the caveat that high quality, detailed abundance information is not available for a significant fraction of the sample. When a subset of GCs with similar metallicities and ages are considered, a correlation between d(V-I) and central luminosity density is exposed. With respect to the existence of GCs with anomalously red HBs at a given metallicity, we conclude that age is the second parameter and central density is most likely the third. Important problems related to HB morphology in GCs, notably multi-modal distributions and faint blue tails, remain to be explained. (Abridged)Comment: Accepted for publication in ApJ; 49 pages, 19 figure

Near-IR period-luminosity relations for pulsating stars in ω\omega Centauri (NGC 5139)

$\omega$ Centauri (NGC 5139) hosts hundreds of pulsating variable stars of different types, thus representing a treasure trove for studies of their corresponding period-luminosity (PL) relations. Our goal in this study is to obtain the PL relations for RR Lyrae, and SX Phoenicis stars in the field of the cluster, based on high-quality, well-sampled light curves in the near-infrared (IR). $\omega$ Centauri was observed using VIRCAM mounted on VISTA. A total of 42 epochs in $J$ and 100 epochs in $K_{\rm S}$ were obtained, spanning 352 days. Point-spread function photometry was performed using DoPhot and DAOPHOT in the outer and inner regions of the cluster, respectively. Based on the comprehensive catalogue of near-IR light curves thus secured, PL relations were obtained for the different types of pulsators in the cluster, both in the $J$ and $K_{\rm S}$ bands. This includes the first PL relations in the near-IR for fundamental-mode SX Phoenicis stars. The near-IR magnitudes and periods of Type II Cepheids and RR Lyrae stars were used to derive an updated true distance modulus to the cluster, with a resulting value of $(m-M)_0 = 13.708 \pm 0.035 \pm 0.10$ mag, where the error bars correspond to the adopted statistical and systematic errors, respectively. Adding the errors in quadrature, this is equivalent to a heliocentric distance of $5.52\pm 0.27$ kpc.Comment: 10 pages, 8 figures, accepted for publication in A&

Mergers of multi-metallic globular clusters: The role of dynamics

Hubble Space Telescope observations of globular clusters (GCs) in the Antennae galaxy show clusters of clusters, or regions in the galaxy that span hundreds of parsecs, where many of the GCs are doomed to collide, and eventually merge. Several such objects appear likely to present a significant range in ages, hence possibly metallicities, and their merger could plausibly lead to multi-metallic GCs. Here we explore this process with direct-summation N-body simulations with GPU hardware. Our results reveal that colliding GCs with different metallicities and ages can produce a GC with multiplicity and occupation fractions not unlike those observed in multi-metallic clusters. In our simulations, the merged clusters have a phase with a larger amount of flattening than average, as a consequence of rapid rotation- thus suggesting that relatively recent mergers may play a role in producing highly flattened, multi-metallic clusters. We additionally explore the role of the King parameter of the cluster in the occupation fractions with a set of 160 direct-summation simulations and find that for equal size clusters the King parameter of the progenitor clusters determines the occupation fractions in the merger product, while in unequal size mergers the size of the clusters dominates the distribution of stars in the new GC. In particular, we find that the observed distribution of populations in Omega Cen can be described to some extent with our dynamical models.Comment: Accepted for publication MNRA

MCAO near-IR photometry of the Globular Cluster NGC 6388: MAD observations in crowded fields

Deep photometry of crowded fields, such as Galactic Globular Clusters, is severely limited by the actual resolution of ground-based telescopes. On the other hand, the Hubble Space Telescope does not provide the near-infrared (NIR) filters needed to allow large color baselines. In this work we aim at demonstrating how ground based observations can reach the required resolution when using Multi-Conjugated Adaptive Optic (MCAO) devices in the NIR, such as the experimental infrared camera (MAD) available on the VLT. This is particularly important since these corrections are planned to be available on all ground--based telescopes in the near future. We do this by combining the infrared photometry obtained by MAD/VLT with ACS/HST optical photometry of our scientific target, the bulge globular cluster NGC 6388, in which we imaged two fields. In particular, we constructed color-magnitude diagrams with an extremely wide color baseline in order to investigate the presence of multiple stellar populations in this cluster. From the analysis of the external field, observed with better seeing conditions, we derived the deepest optical-NIR CMD of NGC 6388 to date. The high-precision photometry reveals that two distinct sub-giant branches are clearly present in this cluster. We also use the CMD from the central region to estimate the distance ((m-M)=15.33) and the reddening (E(B-V)=0.38) for this cluster. We estimate the age to be ~11.5+/- 1.5 Gyr. The large relative-age error reflects the bimodal distribution of the SGB stars. This study clearly demonstrates how MCAO correction in the NIR bands implemented on ground based telescopes can complement the high-resolution optical data from HST.Comment: 9 pages, 10 figures. Accepted for publication on A. &

The second and third parameters of the Horizontal Branch in Globular Clusters

The Horizontal Branch (HB) second parameter of Globular Clusters (GCs) is a major open issue in stellar evolution. Large photometric and spectroscopic databases allow a re-examination of this issue. We derive median and extreme (90% of the distribution) colours and magnitudes of stars along the HB for about a hundred GCs. We transform these into median and extreme masses of stars on the HB taking into account evolutionary effects, and compare these masses with those expected at the tip of the Red Giant Branch to derive the total mass lost by the stars. A simple linear dependence on metallicity of this total mass lost explains well the median colours of HB stars. Adopting this mass loss law as universal, we find that age is the main second parameter. However, at least a third parameter is clearly required. The most likely candidate is the He abundance, which might be different in GCs stars belonging to the different stellar generations whose presence was previously derived from the Na-O and Mg-Al anticorrelations. Variations in the median He abundance allow explaining the extremely blue HB of some GCs; such variations are correlated with the R-parameter. Suitable He abundances allow deriving ages from the HB which are consistent with those obtained from the Main Sequence. Small corrections to these latter ages are then proposed, producing a tight age-metallicity relation for disk and bulge GCs. Star-to-star variations in the He content explain the extension of the HB. There is a strong correlation between this extension and the interquartile of the Na-O anticorrelation. The main driver for the variations in the He-content within GCs seems the total cluster mass. 47 Tuc and M3 exhibit exceptional behaviours; however, they can be accommodated in a scenario for the formation of GCs that relates their origin to cooling flows generated after very large episodes of star formation.Comment: 30 pages, 31 figures. In press on Astronomy and Astrophysics, version after language editin

Revisiting Delta Y/Delta Z from multiple main sequences in Globular Clusters: insight from nearby stars

For nearby K dwarfs, the broadening of the observed Main Sequence at low metallicities is much narrower than expected from isochrones with the standard helium-to-metal enrichment ratio DY/DZ=2. Though the latter value fits well the Main Sequence around solar metallicity, and agrees with independent measurements from HII regions as well as with theoretical stellar yields and chemical evolution models, a much higher DY/DZ~10 is necessary to reproduce the broadening observed for nearby subdwarfs. This result resembles, on a milder scale, the very high DY/DZ estimated from the multiple Main Sequences in Omega Cen and NGC 2808. Although not "inverted" as in omega Cen, where the metal-rich Main Sequence is bluer than the metal-poor one, the broadening observed for nearby subdwarfs is much narrower than stellar models predict for a standard helium content. We use this empirical evidence to argue that a revision of lower Main Sequence stellar models, suggested from nearby stars, could significantly reduce the helium content inferred for the subpopulations of those globular clusters. A simple formula based on empirically calibrated homology relations is constructed, for an alternative estimate of DY/DZ in multiple main sequences. We find that, under the most favourable assumptions, the estimated helium content for the enriched populations could decrease from Y~0.4 to as low as Y~0.3.Comment: 15 pages, 12 figures, in press on MNRA

The ACS Survey of Galactic Globular Clusters. XII. Photometric Binaries along the Main-Sequence

The fraction of binary stars is an important ingredient to interpret globular cluster dynamical evolution and their stellar population. We investigate the properties of main-sequence binaries measured in a uniform photometric sample of 59 Galactic globular clusters that were observed by HST WFC/ACS as a part of the Globular Cluster Treasury project. We measured the fraction of binaries and the distribution of mass-ratio as a function of radial location within the cluster, from the central core to beyond the half-mass radius. We studied the radial distribution of binary stars, and the distribution of stellar mass ratios. We investigated monovariate relations between the fraction of binaries and the main parameters of their host clusters. We found that in nearly all the clusters, the total fraction of binaries is significantly smaller than the fraction of binaries in the field, with a few exceptions only. Binary stars are significantly more centrally concentrated than single MS stars in most of the clusters studied in this paper. The distribution of the mass ratio is generally flat (for mass-ratio parameter q>0.5). We found a significant anti-correlation between the binary fraction in a cluster and its absolute luminosity (mass). Some, less significant correlation with the collisional parameter, the central stellar density, and the central velocity dispersion are present. There is no statistically significant relation between the binary fraction and other cluster parameters. We confirm the correlation between the binary fraction and the fraction of blue stragglers in the cluster.Comment: 43 Pages, 52 figures, accepted for publication in A&