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 degre
es 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.
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 po
sitionally 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.
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.
We report on the optical identification of the companion star to the eclipsing millisecond pulsar PSR J1824-2452H in the galactic globular cluster M28 (NGC 6626). This star is at only 0.2 from the nominal position of the pulsar and it shows optical v
ariability (~ 0.25 mag) that nicely correlates with the pulsar orbital period. It is located on the blue side of the cluster main sequence, ~1.5 mag fainter than the turn-off point. The observed light curve shows two distinct and asymmetric minima, suggesting that the companion star is suffering tidal distortion from the pulsar. This discovery increases the number of non-degenerate MSP companions optically identified so far in globular clusters (4 out of 7), suggesting that these systems could be a common outcome of the pulsar recycling process, at least in dense environments where they can be originated by exchange interactions.
We present an accurate analysis of the peculiar Horizontal Branch (HB) of the massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and optical images of the central region of the cluster obtained with HST. We confirm the multim
odal distribution of stars along the HB: 4 sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modeling is possible for three HB groups by assuming different helium abundances in the range 0.24 < Y < 0.4 that are consistent with the multiple populations observed in the Main Sequence; (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so called blue-hook region. These objects are probably hot-flashers , stars that peel off the red giant branch before reaching the tip and ignite helium at high effective temperatures. Both of these conclusions are based on the luminosity of the HB in the optical and UV bands and do not depend on specific assumptions about mass loss.
Terzan 5 is a globular cluster-like stellar system in the Galactic Bulge which has been recently found to harbor two stellar populations with different iron content and probably different ages (Ferraro et al. 2009). This discovery suggests that Terza
n 5 may be the relic of a primordial building block which contributed to the formation of the Galactic Bulge. Here we present a re-determination of the structural parameters (center of gravity, density and surface brightness profiles, total luminosity and mass) of Terzan 5, as obtained from the combination of high-resolution (ESO-MAD and HST ACS-WFC) and wide-field (ESO-WFI) observations. We find that Terzan 5 is significantly less concentrated and more massive than previously thought. Still it has the largest collision rate of any stellar aggregate in the Galaxy. We discuss the impact of these findings on the exceptional population of millisecond pulsars harbored in this stellar system.
Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxys history. Blue stragglers are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been
proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently, the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M30. This globular cluster is thought to have undergone core collapse, during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences arise from the cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.
Globular star clusters are compact and massive stellar systems old enough to have witnessed the entire history of our Galaxy, the Milky Way. Although recent results suggest that their formation may have been more complex than previously thought, they
still are the best approximation to a stellar population formed over a relatively short time scale (less than 1 Gyr) and with virtually no dispersion in the iron content. Indeed, only one cluster-like system (omega Centauri) in the Galactic halo is known to have multiple stellar populations with a significant spread in iron abundance and age4,5. Similar findings in the Galactic bulge have been hampered by the obscuration arising from thick and varying layers of interstellar dust. Here we report that Terzan 5, a globular-cluster-like system in the Galactic bulge, has two stellar populations with different iron content and ages. Terzan 5 could be the surviving remnant of one of the primordial building blocks that are thought to merge and form galaxy bulges.
By combining high-resolution HST and wide-field ground based observations, in ultraviolet and optical bands, we study the Blue Stragglers Star (BSS) population of the galactic globular cluster M5 (NGC 5904) from its very central regions up to its per
iphery. The BSS distribution is highly peaked in the cluster center, decreases at intermediate radii and rises again outward. Such a bimodal distribution is similar to those previously observed in other globular clusters (M3, 47Tucanae, NGC6752). As for these clusters, dynamical simulations suggest that, while the majority of BSS in M5 could be originated by stellar collisions, a significant fraction (20-40%) of BSS generated by mass transfer processes in primordial binaries is required to reproduce the observed radial distribution. A candidate BSS has been detected beyond the cluster tidal radius. If confirmed, this could represent an interesting case of an evaporating BSS.
