No evidence of mass segregation in the low mass Galactic globular cluster NGC 6101

We used a combination of Hubble Space Telescope and ground based data to probe the dynamical state of the low mass Galactic globular cluster NGC 6101. We have re-derived the structural parameters of the cluster by using star counts and we find that it is about three times more extended than thought before. By using three different indicators, namely the radial distribution of Blue Straggler Stars, that of Main Sequence binaries and the luminosity (mass) function, we demonstrated that NGC 6101 shows no evidence of mass segregation, even in the innermost regions. Indeed, both the BSS and the binary radial distributions fully resemble that of any other cluster population. In addition the slope of the luminosity (mass) functions does not change with the distance, as expected for non relaxed stellar systems. NGC 6101 is one of the few globulars where the absence of mass segregation has been observed so far. This result provides additional support to the use of the dynamical clock calibrated on the radial distribution of the Blue Stragglers as a powerful indicator of the cluster dynamical age.

Proper motions in Terzan 5: membership of the multi-iron sub-populations and first constrain to the orbit

By exploiting two sets of high-resolution images obtained with HST ACS/WFC over a baseline of ~10 years we have measured relative proper motions of ~70,000 stars in the stellar system Terzan 5. The results confirm the membership of the three sub-populations with different iron abudances discovered in the system. The orbit of the system has been derived from a first estimate of its absolute proper motion, obtained by using bulge stars as reference. The results of the integration of this orbit within an axisymmetric Galactic model exclude any external accretion origin for this cluster. Terzan 5 is known to have chemistry similar to the Galactic bulge; our findings support a kinematic link between the cluster and the bulge, further strengthening the possibility that Terzan 5 is the fossil remnant of one of the pristine clumps that originated the bulge.

GEMINI/GeMS observations unveil the structure of the heavily obscured globular cluster Liller 1

By exploiting the exceptional high-resolution capabilities of the near-IR camera GSAOI combined with the multi-conjugate adaptive optics system GeMS at the GEMINI South Telescope, we investigated the structural and physical properties of the heavily obscured globular cluster Liller 1 in the Galactic bulge. We have obtained the deepest and most accurate color-magnitude diagram published so far for this cluster, reaching Ks ~ 19 (below the main sequence turn-off level). We used these data to re-determine the center of gravity of the system, finding that it is located about 2.2 south-east from the literature value. We also built new star density and surface brightness profiles for the cluster, and re-derived its main structural and physical parameters (scale radii, concentration parameter, central mass density, total mass). We find that Liller 1 is significantly less concentrated (concentration parameter c=1.74) and less extended (tidal radius r_t=298 and core radius r_c=5.39) than previously thought. By using these newly determined structural parameters we estimated the mass of Liller 1 M_tot = 2.3 x 10^6 Msun (Mtot = 1.5 x 10^6 Msun for a Kroupa IMF), which is comparable to that of the most massive clusters in the Galaxy (omega Centauri and Terzan 5). Also Liller 1 has the second highest collision rate (after Terzan 5) among all star clusters in the Galaxy, thus confirming that it is an ideal environment for the formation of collisional objects (such as millisecond pulsars).

Evidence of tidal distortions and mass loss from the old open cluster NGC 6791

We present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC 6791. We used deep and wide-field data obtained with the Canada-France-Hawaii-Telescope covering a 2x2 square degrees area around the cluster. The two-dimensional density map obtained with the optimal matched filter technique shows a clear elongation and an irregular distribution starting from ~300 from the cluster center. At larger distances, two tails extending in opposite directions beyond the tidal radius are also visible. These features are aligned to both the absolute proper motion and to the Galactic center directions. Moreover, other overdensities appear to be stretched in a direction perpendicular to the Galactic plane. Accordingly to the behaviour observed in the density map, we find that both the surface brightness and the star count density profiles reveal a departure from a King model starting from ~600 from the center. These observational evidence suggest that NGC 6791 is currently experiencing mass loss likely due to gravitational shocking and interactions with the tidal field. We use this evidence to argue that NGC 6791 should have lost a significant fraction of its original mass. A larger initial mass would in fact explain why the cluster survived so long. Using available recipes based on analytic studies and N-body simulations, we derived the expected mass loss due to stellar evolution and tidal interactions and estimated the initial cluster mass to be M_ini=(1.5-4) x 10^5 M_sun.

First evidence of fully spatially mixed first and second generations in globular clusters: the case of NGC 6362

We present the first evidence of multiple populations in the Galactic globular cluster NGC 6362. We used optical and near-UV Hubble Space Telescope and ground based photometry, finding that both the sub giant and red giant branches are split in two parallel sequences in all color magnitude diagrams where the F336W filter (or U band) is used. This cluster is one of the least massive globulars (M_tot~5x10^4 M_sun) where multiple populations have been detected so far. Even more interestingly and at odds with any previous finding, we observe that the two identified populations share the same radial distribution all over the cluster extension. NGC 6362 is the first system where stars from different populations are found to be completely spatially mixed. Based on N-body and hydrodynamical simulations of multiple stellar generations, we argue that, to reproduce these findings, NGC 6362 should have lost up to the 80% of its original mass

The WFPC2 UV Survey: the BSS population in NGC 5824

We have used a combination of high-resolution Hubble Space Telescope WFPC2 and wide-field ground-based observations, in ultraviolet and optical bands, to study the blue straggler star population of the massive outer-halo globular cluster NGC 5824, over its entire radial extent. We have computed the center of gravity of the cluster and constructed the radial density profile, from detailed star counts. The profile is well reproduced by a Wilson model with a small core (r_c simeq 4.4 arcsec) and a concentration parameter c simeq 2.74. We also present the first age determination for this cluster. From the comparison with isochrones, we have found t=13pm0.5 Gyr. We discuss this result in the context of the observed age-metallicity relation of Galactic globular clusters. A total of 60 bright blue stragglers has been identified. Their radial distribution is found to be bimodal, with a central peak, a well defined minimum at r sim 20 arcsec, and an upturn at large radii. In the framework of the dynamical clock defined by Ferraro et al. (2012), this feature suggests that NGC 5824 is a cluster of intermediate dynamical age.

Double Blue Straggler sequences in GCs: the case of NGC 362

We used high-quality images acquired with the WFC3 on board the HST to probe the blue straggler star (BSS) population of the Galactic globular cluster NGC 362. We have found two distinct sequences of BSS: this is the second case, after M 30, where such a feature has been observed. Indeed the BSS location, their extension in magnitude and color and their radial distribution within the cluster nicely resemble those observed in M 30, thus suggesting that the same interpretative scenario can be applied: the red BSS sub-population is generated by mass transfer binaries, the blue one by collisions. The discovery of four new W UMa stars, three of which lying along the red-BSS sequence, further supports this scenario. We also found that the inner portion of the density profile deviates from a King model and is well reproduced by either a mild power-law (alpha -0.2) or a double King profile. This feature supports the hypothesis that the cluster is currently undergoing the core collapse phase. Moreover, the BSS radial distribution shows a central peak and monotonically decreases outward without any evidence of an external rising branch. This evidence is a further indication of the advanced dynamical age of NGC 362: in fact, together with M 30, NGC 362 belongs to the family of dynamically old clusters (Family III) in the dynamical clock classification proposed by Ferraro et al. (2012). The observational evidence presented here strengthens the possible connection between the existence of a double BSS sequence and a quite advanced dynamical status of the parent cluster.

The optical counterpart to the X-ray transient IGR J18245-2452 in the globular cluster M28

We report on the identification of the optical counterpart to the recently detected INTEGRAL transient IGR J18245-2452 in the Galactic globular cluster M28. From the analysis of a multi epoch HST dataset we have identified a strongly variable star positionally coincident with the radio and Chandra X-ray sources associated to the INTEGRAL transient. The star has been detected during both a quiescent and an outburst state. In the former case it appears as a faint, unperturbed main sequence star, while in the latter state it is about two magnitudes brighter and slightly bluer than main sequence stars. We also detected Halpha excess during the outburst state, suggestive of active accretion processes by the neutron star.

The optical companion to the intermediate mass millisecond pulsar J1439-5501 in the Galactic field

We present the identification of the companion star to the intermediate mass binary pulsar J1439-5501 obtained by means of ground-based deep images in the B, V and I bands, acquired with FORS2 mounted at the ESO-VLT. The companion is a massive white dwarf (WD) with B=23.57+-0.02, V=23.21+-0.01 and I=22.96+-0.01, located at only ~0.05 from the pulsar radio position. Comparing the WD location in the (B, B-V) and (V, V-I) Color-Magnitude diagrams with theoretical cooling sequences we derived a range of plausible combinations of companion masses (1<~Mcom<~1.3 Msun), distances (d<~1200 pc), radii (<~7.8 10^3 Rsun) and temperatures (T=31350^{+21500}_{-7400}). From the PSR mass function and the estimated mass range we also constrained the inclination angle i >~ 55 degrees and the pulsar mass (Mpsr <~2.2 Msun). The comparison between the WD cooling age and the spin down age suggests that the latter is overestimated by a factor of about ten.

The identification of the optical companion to the binary millisecond pulsar J0610-2100 in the Galactic field

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.