A. Sollima

Metallicities, relative ages, and kinematics of stellar populations in ω centauri

We present results of an extensive spectroscopic survey of subgiant stars in the stellar system ω Cen. Using infrared Ca II triplet lines, we derived metallicities and radial velocities for more than 250 stars belonging to different stellar populations of the system. We find that the most metal-rich component, the anomalous subgiant branch (SGB-a), has a metallicity of [Fe/H] ~ -0.6, fully compatible with that determined along the anomalous red giant branch (RGB-a). Our analysis suggests that the age of this component and of the other metal-intermediate (-1.4 < [Fe/H] < -1.0) stellar populations of the system are all comparable to that of the dominant metal-poor population within 2 Gyr, regardless of the choice of helium abundance. These results impose severe constraints on the timescale of the enrichment process of this stellar system, excluding the possibility of an extended star formation period. The radial velocity analysis of the entire sample demonstrates that only metal-intermediate populations are kinematically cooler than the others.

The fraction of binary systems in the core of 13 low-density Galactic globular clusters

We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.

News from the Galactic suburbia: the chemical composition of the remote globular cluster NGC 2419

We present the chemical analysis of 49 giant stars of the globular cluster NGC 2419, using medium resolution spectra collected with the multi-object spectrograph DEIMOS@Keck. Previous analysis of this cluster revealed a large dispersion in the line strength of the infrared Ca ii triplet, suggesting an intrinsic star-to-star scatter in its Fe or Ca content. From our analysis, we assess that all the investigated stars share the same [Fe/H], [Ca/Fe] and [Ti/Fe] abundance ratios, while a large spread in Mg and K abundances is detected. The distribution of [Mg/Fe] is bimodal, with ∼40 per cent of the observed targets having subsolar [Mg/Fe], down to [Mg/Fe] ∼ −1 dex, a level of Mg deficiency never observed before in globular clusters. It is found that the large dispersion in Mg abundances is likely the main origin of the observed dispersion of the Ca ii triplet lines strengths (that can be erroneously interpreted in terms of Fe or Ca abundance scatter) because Mg plays a relevant role in the atmosphere of giant stars as an electron donor. A strong depletion in the Mg abundance leads to an increase of the line strength of the Ca ii triplet, due to the variation in the electronic pressure, at a constant Fe and Ca abundance. Finally, we detect an anti-correlation between Mg and K abundances, not easily explainable within the framework of the current nucleosynthesis models.

The calibration of the RGB Tip as a Standard Candle - Extension to Near Infrared colors and higher metallicity

New empirical calibrations of the Red Giant Branch Tip in the I, J, H and K bands based on two fundamental pillars, namely ω Centauri and 47 Tucanae, have been obtained by using a large optical and near infrared photometric database. Our best estimates give M TRGB = −4.05 ± 0.12, M TRGB

The Pure Noncollisional Blue Straggler Population in the Giant Stellar System ω Centauri

We have used high spatial resolution data from the Hubble Space Telescope (HST) and wide-field ground-based observations to search for blue straggler stars (BSSs) over the entire radial extent of the large stellar system ? Centauri. We have detected the largest population of BSSs ever observed in any stellar system. Even though the sample is restricted to the brightest portion of the BSS sequence, more than 300 candidates have been identified. BSSs are thought to be produced by the evolution of binary systems (formed either by stellar collisions or mass exchange in binary stars). Since systems like Galactic globular clusters (GGCs) and ? Cen evolve dynamically on timescales significantly shorter than their ages, binaries should have settled toward the center, showing a more concentrated radial distribution than the ordinary, less massive single stars. Indeed, in all GGCs that have been surveyed for BSSs, the BSS distribution is peaked at the center. Conversely, in ? Cen we find that the BSSs share the same radial distribution as the adopted reference populations. This is the cleanest evidence ever found that such a stellar system is not fully relaxed even in the central region. We further argue that the absence of central concentration in the BSS distribution rules out a collisional origin. Thus, the ? Cen BSSs are the purest and largest population of noncollisional BSSs ever observed. Our results allow the first empirical quantitative estimate of the production rate of BSSs via this channel. BSSs in ? Cen may represent the best local template for modeling the BSS populations in distant galaxies where they cannot be individually observed.

THE GLOBULAR CLUSTER NGC 2419: A CRUCIBLE FOR THEORIES OF GRAVITY*

We present the analysis of a kinematic data set of stars in the globular cluster NGC?2419, taken with the DEep Imaging Multi-Object Spectrograph at the Keck II telescope. Combined with a reanalysis of deep Hubble Space Telescope and Subaru Telescope imaging data, which provide an accurate luminosity profile of the cluster, we investigate the validity of a large set of dynamical models of the system, which are checked for stability via N-body simulations. We find that isotropic models in either Newtonian or Modified Newtonian Dynamics (MOND) are ruled out with extremely high confidence. However, a simple Michie model in Newtonian gravity with anisotropic velocity dispersion provides an excellent representation of the luminosity profile and kinematics of the cluster. The anisotropy profiles of these models ensure an isotropic center to the cluster, which progresses to extreme radial anisotropy toward the outskirts. In contrast, with MOND we find that Michie models that reproduce the luminosity profile either overpredict the velocity dispersion on the outskirts of the cluster if the mass-to-light ratio (M/L) is kept at astrophysically motivated values or else they underpredict the central velocity dispersion if the M/L is taken to be very small. We find that the best Michie model in MOND is a factor of ~104 less likely than the Newtonian model that best fits the system. A likelihood ratio of 350 is found when we investigate more general models by solving the Jeans equation with a Markov Chain Monte Carlo scheme. We verified with N-body simulations that these results are not significantly different when the MOND external field effect is accounted for. If the assumptions that the cluster is in dynamical equilibrium, spherical, not on a peculiar orbit, and possesses a single dynamical tracer population of constant M/L are correct, we conclude that the present observations provide a very severe challenge for MOND.

The Na-O anticorrelation in horizontal branch stars - III. 47 Tucanae and M 5

To check the impact of the multiple population scenario for globular clusters on their horizontal branch (HB), we presen t an analysis of the composition of 110 red HB (RHB) stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M 5. In 47 Tuc we found tight relations between the colours of the stars and their ab undances of p−capture elements. This strongly supports the idea that the H e content - which is expected to be closely correlated with the abundances of p−capture elements - is the third parameter (after overall metallicity and age) that determines the colour of HB stars. However, the range in He abundance must be small (�Y< 0.03) in 47 Tuc to reproduce our observations; this agrees with previous an alyses. There is possibly a correlation between the abundances of p− and n−capture elements in 47 Tuc. If confirmed, this might suggest t hat asymptotic giant branch stars of moderate mass contributed to the gas from which second-generation stars formed. Considering the selection effects in our sample (which does not include stars warmer than 11000 K and RR Lyrae variables, which were excluded because we could not obtain accurate abundances with the adopted observing procedure) is important to understand our result s for M 5. In this case, we find that, as expected, RHB stars are N a-poor and O-rich, and likely belong to the primordial population. The re is a clear correlation of the [Na/O] ratio and N abundance with colour along the BHB. A derivation of the He abundance for these stars yields a low value of Y = 0.22± 0.03. This is expected because HB stars of a putative He-rich population in this cluster shoul d be warmer than 11000 K, and would accordingly not have been sampled by our analysis. However, we need some additional source of scatter in the total mass loss of stars climbing up the red gian t branch to reproduce our results for M 5. Finally, we found a C-star on the HB of 47 Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M 5. These stars are among the brightest and coolest HB stars.

The RR Lyrae period–K-luminosity relation for globular clusters: an observational approach

The period-metallicity-K-band luminosity (PL K ) relation for RR Lyrae stars in 15 Galactic globular clusters and in the Large Magellanic Cloud (LMC) globular cluster Reticulum has been derived. It is based on accurate near-infrared (K) photometry combined with Two-Micron All-Sky Survey (2MASS) and other literature data. The PL K relation has been calibrated and compared with the previous empirical and theoretical determinations in literature. The zero point of the absolute calibration has been obtained from the K magnitude of RR Lyr whose distance modulus has been measured via trigonometric parallax with Hubble Space Telescope (HST). Using this relation, we obtain a distance modulus to the LMC of (m - M) 0 = 18.54 ± 0.15 mag, in good agreement with recent determinations based on the analysis of Cepheid variable stars.

The Na-O anticorrelation in horizontal branch stars - I. NGC 2808

The chemical composition of horizontal branch (HB) stars might help to clarify the formation history of individual globular clusters (GCs). We studied the Na-O anti-correlation from moderately high resolution spectra for 91 stars on the bimodal HB of NGC 1851; in addition we observed 13 stars on the lower red giant branch (RGB). In our HB sample, 35 stars are on the blue HB (BHB), one is an RR Lyrae, and 55 stars are on the red HB (RHB). The ratio of BHB to RHB stars is close to the total in the cluster (35 and 54%, respectively), while RR Lyrae variables are under-represented, (they are ∼12% of the NGC 1851 stars). We also derived abundances for He and N in BHB stars. For RHB stars we derived Ba abundances and a few interesting upper limits for N. The RHB stars clearly separate into two groups: the vast majority are O-rich and Na-poor, while about 10–15% are Na-rich and moderately O-poor. Most (but not all) Na-rich RHB stars are also Ba-rich and there is an overall correlation between Na and Ba abundances within the RHB. The group of Ba-rich RHB stars resides on the warmer edge and includes ∼10% of the RHB stars. We propose that they are the descendant of the stars on the RGB sequence with very red v − y colour. This sequence is known also to consist of Ba and perhaps CNO-rich stars and consistently includes ∼5–10% of the RGB stars of NGC 1851. However, the upper limit we obtain for N ([N/Fe] < 1.55) for one of the Ba-rich stars coupled with the low C-abundances for RGB Ba-rich stars from the literature suggests that the total CNO might not be particularly high ([(C+N+O)/Fe] ≤ 0.15). The other Na-rich RHB stars are also at the warm edge of the RHB and the only RR Lyrae is Na-rich and moderately O-poor. We also find a Na-O anticorrelation among BHB stars, partially overlapping that found among RHB stars, though generally BHB stars are more Na-rich and O-poor. However, there is no clear correlation between temperature and Na and O abundances within the BHB. The average He abundance in BHB stars is Y = 0.29 ± 0.05, which excludes a large population of extremely He-rich stars from our sample. N abundances are quite uniform at [N/Fe] = 1.16 ± 0.14 among BHB stars, with a small trend with temperature. This value is consistent with normal CNO abundance and excludes that BHB stars are very CNO-rich: this leaves an age spread of ∼1.5 Gyr as the only viable explanation for the split of the SGB. To help clarifying the formation history of NGC 1851, we computed synthetic HB’s trying to identify which HB stars are the descendant of the bright and faint subgiant branch (b-SGB and f-SGB) stars identified by Milone et al. (2008, ApJ, 673, 241), with respectively 2/ 3a nd 1/ 3o f the stars of NGC 1851. While most BHB stars likely descend from f-SGB stars and are older, and most RHB stars from b-SGB ones and are younger, the correspondence is probably not one-to-one. In particular, the Ba-rich RHB stars should be less massive than the remaining RHB stars, and the location of their progenitors on the SGB is uncertain. If they descend from f-SGB stars, number counts then require that RR Lyrae variables and possibly some mild BHB stars descend from b-SGB stars; this suggestion is supported by a few circumstantial facts. An investigation of the composition of a large enough sample of SGB stars is required to firmly establish these relations.

The Discovery of an Anomalous Subgiant Branch in the Color-Magnitude Diagram of ω Centauri

Using deep high-resolution multiband images taken with the Very Large Telescope and the Hubble Space Telescope, we discovered a new anomalous sequence in the color-magnitude diagram of ω Cen. This feature appears as a narrow well-defined subgiant branch (SGB-a), which merges into the main sequence of the dominant cluster population at a magnitude significantly fainter than the cluster turnoff (TO). The simplest hypothesis assumes that the new feature is the extension of the anomalous red giant branch (RGB-a) metal-rich population discovered by Lee et al. and Pancino et al. However, under this assumption the interpretation of the SGB-a does not easily fit into the context of a self-enrichment scenario within ω Cen. In fact, its TO magnitude, shape, and extension are not compatible with a young metal-rich population, as required by the self-enrichment process. The TO level of the SGB-a suggests indeed an age as old as the main cluster population, further supporting the extracluster origin of the most metal-rich stars, as suggested by Ferraro, Bellazzini, & Pancino. Only accurate measurements of radial velocities and metal abundances for a representative sample of stars will firmly establish whether or not the SGB-a is actually related to the RGB-a and will finally shed light on the origin of the metal-rich population of ω Cen.