F. R. Ferraro

The Ital-FLAMES survey of the Sagittarius dwarf spheroidal galaxy - I. Chemical abundances of bright RGB stars

We present iron and α element (Mg, Ca, Ti) abundances for a sample of 15 Red Giant Branch stars belonging to the main body of the Sagittarius dwarf Spheroidal galaxy. Abundances have been obtained from spectra collected using the high resolution spectrograph FLAMES-UVES mounted at the VLT. Stars of our sample have a mean metallicity of [Fe/H] = -0.41 ± 0.20 with a metal-poor tail extending to [Fe/H] = -1.52. The α element abundance ratios are slightly subsolar for metallicities higher than [Fe/H]? -1, suggesting a slow star formation rate. The [α/Fe] of stars having [Fe/H] < -1 are compatible to what observed in Milky Way stars of comparable metallicity.

Discovery of Another Peculiar Radial Distribution of Blue Stragglers in Globular Clusters: The Case of 47 Tucanae*

We have used the high-resolution Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope (HST) and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) in the globular cluster 47 Tuc spanning the entire radial extent of the cluster. The BSS distribution is highly peaked in the cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii. The observed distribution closely resembles that discovered in M3 by Ferraro and coworkers. To date, complete BSS surveys covering the full radial extent (from HST for the center and wide-field CCD, ground-based observations for the exterior) have only been performed for these two clusters. Both show a bimodal radial distribution despite their different dynamical properties. BSS surveys covering the full spatial extent of more globular clusters are clearly required to determine how common bimodality is and what its consequences are for theories of BSS formation and cluster dynamics.

The radial distribution of blue straggler stars and the nature of their progenitors

The origin of blue straggler stars (BSS) in globular clusters (GCs) is still not fully understood: they can form from stellar collisions, or through mass transfer in isolated, primordial binaries (PBs). In this paper we use the radial distribution of BSS observed in four GCs (M3, 47 Tuc, NGC 6752 and ω Cen) to investigate which formation process prevails. We find that both channels co-exist in all the considered GCs. The fraction of mass-transfer (collisional) BSS with respect to the total number of BSS is around ∼0.4‐0.5 (∼0.5‐0.6) in M3, 47 Tuc and NGC 6752. The case of ω Cen is peculiar with an underproduction of collisional BSS. The relative lack of collisional BSS in ω Cen can be understood if mass segregation has not yet driven to the core a sizeable number of PBs, which dominate stellar collisions through threeand four-body processes. The spatial distribution of BSS provides strong hints to their origin: the BSS in the cluster outskirts form almost exclusively from mass transfer in PBs, whereas the BSS found close to the cluster core most likely have a collisional origin.

The central density cusp of the Sagittarius dwarf spheroidal galaxy

We present an analysis of the density profile in the central region of the Sagittarius dwarf spheroidal galaxy. A strong density enhancement of Sgr stars is observed. The position of the peak of the detected cusp is indistinguishable from the centre of M54. The photometric properties of the cusp are fully compatible with those observed in the nuclei of dwarf elliptical galaxies, indicating that the Sgr dSph would appear as a nucleated galaxy independently of the presence of M54 at its centre.

The red giant branch tip and bump of the Leo II dwarf spheroidal galaxy

We present V and I photometry of a 9.4 x 9.4 arcmin 2 field centred on the dwarf spheroidal galaxy Leo II. The tip of the red giant branch (TRGB) is identified at I TRGB = 17.83 ± 0.03 and adopting ([M/H]) = -1.53 ± 0.2 from the comparison of RGB stars with Galactic templates, we obtain a distance modulus (m - M) 0 = 21.84 ± 0.13, corresponding to a distance D = 233 ± 15 kpc. Two significant bumps have been detected in the luminosity function of the RGB. The fainter bump (Bl, at V = 21.76 ± 0.05) is the RGB bump of the dominant stellar population while the actual nature of the brightest one (B2, at V = 21.35 ± 0.05) cannot be firmly assessed on the basis of the available data; it may be due to the asymptotic giant branch clump of the main population or it may be a secondary RGB bump. The luminosity of the main RGB bump (Bl) suggests that the majority of RGB stars in Leo II belong to a population that is ≥4 Gyr younger than the classical Galactic globular clusters. The stars belonging to the He-burning red clump are shown to be significantly more centrally concentrated than RR Lyrae and blue horizontal branch stars, probing the existence of an age/metallicity radial gradient in this remote dwarf spheroidal.

Probing the Red Giant Branch Phase Transition: Near-Infrared Photometry of Six Intermediate-Age Large Magellanic Cloud Clusters*

This is the first of a series of papers devoted to a global study of the photometric properties of the red stellar sequences in a complete sample of the Large Magellanic Cloud clusters, by means of near infrared array photometry. Deep J,H,Ks photometry and accurate Color Magnitude Diagrams down to K=18.5, i.e. 1.5 mag below the red He-clump, for six intermediate age clusters (namely NGC1987, NGC2108, NGC2190, NGC2209, NGC2231, NGC2249) are presented. A quantitative estimate of the population ratios (by number and luminosity) between Red Giant Branch and He-clump stars for each target cluster is provided and discussed in the framework of probing the so-called Red Giant Branch phase transition (RGB Ph-T). By using the Elson & Fall s-parameter as an age indicator, the observed RGB population shows a sharp enhancement (both in number and luminosity) at s=36. Obviously, the corresponding absolute age strictly depends on the details of theoretical models adopted to calibrate the s-parameter. Curiously, the currently available calibrations of the s-parameter in term of age based on canonical (by Elson & Fall 1988) and overshooting (Girardi et al. 1995) models provide ages that well agree within 10%, suggesting that the full development of the Red Giant Branch occurs at t=700 Myr and be a relatively fast event (delta t=300 Myr). However, the RGB Ph-T epoch derived from the overshooting calibration of the s-parameter turns out to be significantly earlier than the epoch provided by the recent evolutionary tracks by Girardi et al. (2000). A new calibration of the s-parameter based on high quality Color Magnitude Diagrams and updated models is urged to address the origin of this discrepancy and finally establish the epoch of the RGB Ph-T.This is the first of a series of papers devoted to a global study of the photometric properties of the red stellar sequences in a complete sample of the Large Magellanic Cloud clusters, by means of near-infrared array photometry. Deep J, H, Ks photometry and accurate color-magnitude diagrams down to K ≈ 18.5, i.e., ≈1.5 mag below the red He clump, for six intermediate-age clusters (namely, NGC 1987, NGC 2108, NGC 2190, NGC 2209, NGC 2231, NGC 2249) are presented. A quantitative estimate of the population ratios (by number and luminosity) between red giant branch (RGB) and He-clump stars for each target cluster is provided and discussed in the framework of probing the so-called RGB phase transition (Ph-T). By using the Elson & Fall s-parameter as an age indicator, the observed RGB population shows a sharp enhancement (in both number and luminosity) at s = 36. Obviously, the corresponding absolute age strictly depends on the details of theoretical models adopted to calibrate the s-parameter. Curiously, the currently available calibrations of the s-parameter in terms of age based on canonical (by Elson & Fall) and overshooting (Girardi and coworkers) models provide ages that well agree within 10%, suggesting that the full development of the RGB occurs at t ≈ 700 Myr and is a relatively fast event (δt ≈ 300 Myr). However, the RGB Ph-T epoch derived from the overshooting calibration of the s-parameter turns out to be significantly earlier than the epoch provided by the recent evolutionary tracks by Girardi and coworkers. A new calibration of the s-parameter based on high-quality color-magnitude diagrams and updated models is urged to address the origin of this discrepancy and finally establish the epoch of the RGB Ph-T.

Near-infrared photometry and spectroscopy of NGC 6539 and UKS 1 : two intermediate metallicity bulge globular clusters

Using the son of ISAAC (Infrared Spectrometer And Array Camera) imager at the European Southern Observatory New Technology Telescope and the near-infrared spectrographs on Keck II, we have obtained J, K images and echelle spectra covering the range 1.5-1.8 HIT) for the intermediate metallicity bulge globular clusters NGC 6539 and UKS 1. We find [Fe/H] = -0.76 and -0.78, respectively, and an average α-enhancement of +0.44 and +0.31 dex, consistent with previous measurements of metal-rich bulge clusters, and favouring the scenario of rapid chemical enrichment. We also measure very low 12 C/ 13 C 4.5 ± 1 isotopic ratios in both clusters, suggesting that extra-mixing mechanisms due to cool bottom processing are at work during evolution along the red giant branch. Finally, we measure accurate radial velocities of = +31 ± 4 and = +57 ± 6 km s -1 and velocity dispersions of 8 and 11 km s -1 for NGC 6539 and UKS 1, respectively.

Near-infrared photometry of four metal-rich bulge globular clusters: NGC 6304, 6569, 6637 and 6638

We present high-quality near-infrared (near-IR) photometry of four bulge metal-rich globular clusters, namely NGC 6304, 6569, 6637 and 6638. By using the observed colour-magnitude diagrams we derived photometric estimates of the cluster reddening and distance. We performed a detailed analysis of the red giant branch (RGB), presenting a complete description of its morphologic parameters and evolutionary features (bump and tip). Photometric estimates of the cluster metallicity were obtained using the updated data set (published by our group) linking metal abundance to a variety of near-IR indices measured along the RGB. The detection of the RGB bump and tip is also presented and briefly discussed.

Near-IR photometry of five Galactic Globular Clusters

We present near-infrared J and K observations of giant stars in five metal-poor Galactic Globular Clusters (namely M3, M5, M 10, M 13 and M92) obtained at the Telescopio Nazionale Galileo (TNG). This database has been used to determine the main photometric properties of the red giant branch (RGB) from the (K, J - K) and, once combined with the optical data, in the (K, V - K) Color Magnitude Diagrams. A set of photometric indices (the RGB colors at fixed magnitudes) and the major RGB evolutionary features (slope, bump, tip) have been measured. The results have been compared with the relations obtained by Ferraro et al. (2000) and with the theoretical expectations, showing a very good agreement.

Discovery of a stellar system in the background of 47 Tucanae - A new cluster of the Small Magellanic Cloud?

We report on the discovery of a stellar system in the background of the Galactic globular cluster 47 Tucanae (NGC 104), located 14.8 North-West of the cluster center. The object, whose apparent diameter is D ≃ 30, is partially resolved into stars on the available CCD images, reaching a limiting magnitude of V ∼ 22.5, and is detected as a significant (more than 5σ) overdensity of blue stars (B - V < 0.7). The color magnitude diagram of the system, its characteristic projected size and its position in the sky suggest that it is an intermediate-old age cluster belonging to the Small Magellanic Cloud, whose outskirts lie in the background of 47 Tuc. Although less likely, the possibility that the object is an unknown dwarf galaxy in the outskirts of the Local Group cannot completely he ruled out by the present data.