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We present an empirical calibration of the Mixing-Length free parameter ${\alpha}$ based on a homogeneous Infrared database of 28 Galactic globular clusters spanning a wide metallicity range (-2.15$, Comment: 15 pages, 3 figures, ApJ in press

Using HST/WFPC2 data, we have performed a comparative study of the Blue Straggler Star (BSS) populations in the central regions of the globular clusters AM 1, Eridanus, Palomar 3, and Palomar 4. Located at distances RGC > 50 kpc from the Galactic Centre, these are (together with Palomar 14 and NGC 2419) the most distant clusters in the Halo. We determine their colour-magnitude diagrams and centres of gravity. The four clusters turn out to have similar ages (10.5-11 Gyr), significantly smaller than those of the inner-Halo globulars, and similar metallicities. By exploiting wide field ground based data, we build the most extended radial density profiles from resolved star counts ever published for these systems. These are well reproduced by isotropic King models of relatively low concentration. BSSs appear to be significantly more centrally segregated than red giants in all globular clusters, in agreement with the estimated core and half-mass relaxation times which are smaller than the cluster ages. Assuming that this is a signature of mass segregation, we conclude that AM 1 and Eridanus are slightly dynamically more evolved than Pal 3 and Pal 4., Accepted for publication in ApJ, references updated

We present the first optical observations of the unique system J0737-3039 (composed of two pulsars, hereafter PSR-A and PSR-B). Ultra-deep optical observations, performed with the High Resolution Camera of the Advanced Camera for Surveys on board the Hubble Space Telescope could not detect any optical emission from the system down to m_F435W=27.0 and m_F606W=28.3. The estimated optical flux limits are used to constrain the three-component (two thermal and one non-thermal) model recently proposed to reproduce the XMM-Newton X-ray spectrum. They suggest the presence of a break at low energies in the non-thermal power law component of PSR-A and are compatible with the expected black-body emission from the PSR-B surface. The corresponding efficiency of the optical emission from PSR-A's magnetosphere would be comparable to that of other Myr-old pulsars, thus suggesting that this parameter may not dramatically evolve over a time-scale of a few Myr., 16 pages, 3 figures, ApJ accepted

We present ultraviolet (UV) integrated colors of 44 Galactic globular clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV and NUV bands. This database is the largest homogeneous catalog of UV colors ever published for stellar systems in our Galaxy. The proximity of GGCs makes it possible to resolve many individual stars even with the somewhat low spatial resolution of GALEX. This allows us to determine how the integrated UV colors are driven by hot stellar populations, primarily horizontal branch stars and their progeny. The UV colors are found to be correlated with various parameters commonly used to define the horizontal branch morphology. We also investigate how the UV colors vary with parameters like metallicity, age, helium abundance, and concentration. We find for the first time that GCs associated with the Sagittarius dwarf galaxy have (FUV – V) colors systematically redder than GGCs with the same metallicity. Finally, we speculate about the presence of an interesting trend, suggesting that the UV color of GCs may be correlated with the mass of the host galaxy, in the sense that more massive galaxies possess bluer clusters.

We have used optical images acquired with the Hubble Space Telescope to construct the first high-resolution extinction map in the direction of Terzan 5, a peculiar stellar system in the inner bulge of our Galaxy. The map has a spatial resolution of 8" X 8", over a total FoV of 200" X 200". The absorption clouds show a patchy structure on a typical scale of 20" and extinction variations as large as delta E(B-V) = 0.67 mag, especially in the direction of the center of the system. These correspond to an absolute color excess ranging from E(B-V)=2.15 mag, up to 2.82 mag. After the correction for differential reddening, two distinct red giant branches become clearly visible in the color magnitude diagram of Terzan 5 and they well correspond to the two sub-populations with different iron abundances recently discovered in this system., Comment: Published by the Astrophysical Journal Letters, Volume 755, Issue 2, article id. L32 (2012). 15 pages, 5 figures

We have used a combination of high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and wide-field ground-based observations in ultraviolet and optical bands to study the blue straggler star (BSS) population of the outer halo globular cluster NGC 6229 over its entire radial extent. A total of 64 bright BSS (with m 255≤ 21.30, corresponding to m 555≤ 20.75) has been identified. The BSS-projected radial distribution is found to be bimodal, with a high central peak, a well-defined minimum at intermediate radii (r∼ 40arcsec) and an upturn in the outskirts. From detailed star counts even in the very inner region, we compute the centre of gravity of the cluster and the most accurate and extended radial density profile ever published for this system. The profile is reasonably well reproduced by a standard King model with an extended core (r c≃ 9.5arcsec) and a modest value of the concentration parameter (c≃ 1.49). However, a deviation from the model is noted in the most external region of the cluster (at r > 250arcsec from the centre). This feature needs to be further investigated in order to assess the possible presence of a tidal tail in this cluster.

We present Galaxy Evolution Explorer (GALEX) data for 44 Galactic globular clusters (GCs) obtained during three GALEX observing cycles between 2004 and 2008. This is the largest homogeneous data set on the UV photometric properties of Galactic GCs ever collected. The sample selection and photometric analysis are discussed, and color-magnitude diagrams (CMDs) are presented. The blue and intermediate-blue horizontal branch is the dominant feature of the UV CMDs of old Galactic GCs. Our sample is large enough to display the remarkable variety of horizontal branch shapes found in old stellar populations. Other stellar types that are obviously detected are blue stragglers and post-core-He burning stars. The main features of UV CMDs of Galactic GCs are briefly discussed. We establish the locus of post-core-He burning stars in the UV CMD and present a catalog of candidate asymptotic giant branch (AGB), AGB-manque, post early-AGB, and post-AGB stars within our cluster sample.

We present Li and Fe abundances for 87 stars in the globular cluster M4, obtained by using high-resolution spectra collected with GIRAFFE at the Very Large Telescope. The targets range from the turn-off up to the red giant branch bump. The Li abundance in the turn-off stars is uniform, with an average value equal to A(Li)= 2.30 ± 0.02 dex (σ= 0.10 dex), consistent with the upper envelope of Li content measured in other globular clusters and in the halo field stars, confirming also for M4 the discrepancy with the primordial Li abundance predicted by Wilkinson Microwave Anisotropy Probe+ big bang nucleosynthesis (WMAP+BBNS). The global behaviour of A(Li) as a function of the effective temperature allows us to identify the two main drops in the Li evolution due to the first dredge-up and to the extra-mixing episode after the red giant branch bump. The measured iron content of M4 results to [Fe/H]=-1.10 ± 0.01 dex (σ= 0.07 dex), with no systematic offsets between dwarf and giant stars. The behaviour of the Li and Fe abundances along the entire evolutionary path is incompatible with theoretical models including pure atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account, able to inhibit the atomic diffusion. The measured value of A(Li) and its homogeneity in the turn-off stars allow us to put strong constraints on the shape of the Li profile inside the M4 turn-off stars. The global behaviour of A(Li) with the effective temperature can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing. One cannot reproduce the global trend that starts from the WMAP+BBNS A(Li) and adopts the turbulent mixing described by Richard, Michaud & Richer with the same efficiency as that used by Korn et al. to explain the Li content in NGC6397. In fact, such a solution is not able to well reproduce simultaneously the Li abundance observed in turn-off and red giant branch stars. However, the WMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned. We conclude that the cosmological Li discrepancy cannot be easily solved with the present, poor understanding of the turbulence in the stellar interiors, and a future effort to well understand the true nature of this non-canonical process is needed.

We present an empirical method to derive photometric metallicity, reddening and distance to old stellar populations by using a few major features of the Red Giant Branch (RGB) in near IR color magnitude diagrams. We combine the observed RGB features with a set of equations linking the global metallicity [M/H] to suitable RGB parameters (colors, magnitudes and slope), as calibrated from a homogeneous sample of Galactic Globular Clusters with different metallicities. This technique can be applied to efficiently derive the main population parameters of old stellar systems, in the view of using ground-based adaptive optics and space facilities to probe the stellar content of remote galaxies., 16 pages, 4 figures, accepted for pubblication on ApJ

We have used deep V and R images acquired at the ESO Very Large Telescope to identify the optical companion to the binary pulsar PSR J0610-2100, one of the black-widow millisecond pulsars recently detected by the Fermi Gamma-ray Telescope in the Galactic plane. We found a faint star (V~26.7) nearly coincident (\delta r ~0".28) with the pulsar nominal position. This star is visible only in half of the available images, while it disappears in the deepest ones (those acquired under the best seeing conditions), thus indicating that it is variable. Although our observations do not sample the entire orbital period (P=0.28 d) of the pulsar, we found that the optical modulation of the variable star nicely correlates with the pulsar orbital period and describes a well defined peak (R~25.6) at \Phi=0.75, suggesting a modulation due to the pulsar heating. We tentatively conclude that the companion to PSR J0610-2100 is a heavily ablated very low mass star (~ 0.02Msun) that completely filled its Roche Lobe., Comment: 17 pages, 5 figures - Accepted for pubblication in ApJ