C. Pallanca

STAR COUNT DENSITY PROFILES AND STRUCTURAL PARAMETERS OF 26 GALACTIC GLOBULAR CLUSTERS

We used an appropriate combination of high-resolution Hubble Space Telescope observations and wide-field, ground-based data to derive the radial stellar density profiles of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system, a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This collection of data represents the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13, and M62 is not confirmed; (2) the majority of clusters in our sample are fit equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distance; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ~0.3 for about 80% of the clusters and a secondary peak at ~0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with that expected from simulations of cluster dynamical evolution and the ratio between these two radii correlates well with an empirical dynamical-age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.

THE OPTICAL COUNTERPART TO THE X-RAY TRANSIENT IGR J1824–24525 IN THE GLOBULAR CLUSTER M28*

We report on the identification of the optical counterpart to the recently detected INTEGRAL transient IGR J1824–24525 in the Galactic globular cluster M28. From analysis of a multi-epoch Hubble Space Telescope data set, we have identified a strongly variable star positionally coincident with the radio and Chandra X-ray sources associated with 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 Hα excess during the outburst state, suggestive of active accretion processes by the neutron star.

New Clues on the Nature of the Companion to PSR J1740?5340 in NGC?6397 from XSHOOTER Spectroscopy

By using XSHOOTER spectra acquired at the ESO Very Large Telescope, we have studied the surface chemical composition of the companion star to the binary millisecond pulsar PSR J1740–5340 in the globular cluster NGC 6397. The measured abundances of Fe, Mg, Al and Na confirm that the star belongs to the cluster. On the other hand, the measured surface abundance of nitrogen ([N/Fe] = +0.53 ± 0.15 dex) combined with the carbon upper limit ([C/Fe] < –2 dex) previously obtained from UVES spectra allow us to put severe constraints on its nature, strongly suggesting that the pulsar companion is a deeply peeled star. In fact, the comparison with theoretical stellar models indicates that the matter currently observed at the surface of this star has been processed by the hydrogen-burning CN-cycle at equilibrium. In turn, this evidence suggests that the pulsar companion is a low mass (~0.2 M ☉) remnant star, descending from a ~0.8 M ☉ progenitor which lost ~70%-80% of its original material because of mass transfer activity onto the pulsar.

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 PSR J0610−2100, one of the black-widow millisecond pulsars recently detected by the Fermi Gammaray Space Telescope in the Galactic plane. We found a faint star (V ∼ 26.7) nearly coincident (δr ∼ 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 days) 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 Φ = 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 (≈0.02 M� ) that completely filled its Roche lobe.

Optical Identification of He White Dwarfs Orbiting Four Millisecond Pulsars in the Globular Cluster 47 Tucanae

We used ultra-deep UV observations obtained with the Hubble Space Telescope to search for optical companions to binary millisecond pulsars (MSPs) in the globular cluster 47 Tucanae. We identified four new counterparts (to MSPs 47TucQ, 47TucS, 47TucT and 47TucY) and confirmed those already known (to MSPs 47TucU and 47TucW). In the color magnitude diagram, the detected companions are located in a region between the main sequence and the CO white dwarf cooling sequences, consistent with the cooling tracks of He white dwarfs of mass between 0.15 Msun and 0.20 Msun. For each identified companion, mass, cooling age, temperature and pulsar mass (as a function of the inclination angle) have been derived and discussed. For 47TucU we also found that the past accretion history likely proceeded in a sub-Eddington rate. The companion to the redback 47TucW is confirmed to be a non degenerate star, with properties particularly similar to those observed for black widow systems. Two stars have been identified within the 2-sigma astrometric uncertainty from the radio positions of 47TucH and 47TucI, but the available data prevent us from firmly assessing whether they are the true companions of these two MSPs.

Probing the MSP Prenatal Stage: The Optical Identification of the X-Ray Burster EXO 1745-248 in Terzan 5

We report on the optical identification of the neutron star burster EXO 1745-248 in Terzan 5. The identification was performed by exploiting Hubble Space Telescope/Advanced Camera for Surveys images acquired in Directors Discretionary Time shortly after (approximately one month) the Swift detection of the X-ray burst. The comparison between these images and previous archival data revealed the presence of a star that is currently brightened by ∼3 mag, consistent with expectations during an X-ray outburst. The centroid of this object well agrees with the position, in the archival images, of a star located in the turn-off/sub-giant-branch region of Terzan 5. This supports the scenario that the companion should have recently filled its Roche Lobe. Such a system represents the prenatal stage of a millisecond pulsar, an evolutionary phase during which heavy mass accretion on the compact object occurs, thus producing X-ray outbursts and re-accelerating the neutron star. Based on observations (GO 14061, GO 12933, GO 9799) with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

CONSTRAINING THE TRUE NATURE OF AN EXOTIC BINARY IN THE CORE OF NGC 6624

We report on the identification of the optical counterpart to Star1, the exotic object serendipitously discovered by Deutsch et al. in the core of the Galactic globular cluster NGC 6624. Star1 has been classified by Deutsch et al. as either a quiescent cataclysmic variable or a low-mass X-ray binary. Deutsch et al. proposed StarA as a possible optical counterpart to this object. We used high-resolution images obtained with the Hubble Space Telescope to perform a variability analysis of the stars close to the nominal position of Star1. While no variability was detected for StarA, we found another star, referred to here as COM{sub S}tar1, showing a clear sinusoidal light modulation with amplitude Δm {sub F435W} ∼ 0.7 mag and an orbital period of P {sub orb} ∼ 98 minutes. The shape of the light curve is likely caused by strong irradiation by the primary heating of one hemisphere of the companion, thus suggesting a quite hot primary.

Testing multimass dynamical models of star clusters with real data: mass segregation in three Galactic globular clusters

We present the results of the analysis of deep photometric data for a sample of three Galactic globular clusters (NGC5466, NGC6218 and NGC6981) with the aim of estimating their degree of mass segregation and testing the predictions of analytic dynamical models. The adopted dataset, composed by both Hubble Space Telescope and ground based data, reaches the low-mass end of the mass functions of these clusters from the center up to their tidal radii allowing to derive their radial distribution of stars with different masses. All the analysed clusters show evidence of mass segregation with the most massive stars more concentrated than low-mass ones. The structures of NGC5466 and NGC6981 are well reproduced by multimass dynamical models adopting a lowered-Maxwellian distribution function and the prescription for mass segregation given by Gunn & Griffin (1979). Instead, NGC6218 appears to be more mass segregated than model predictions. By applying the same technique to mock observations derived from snapshots selected from suitable N-body simulations we show that the deviation from the behaviour predicted by these models depends on the particular stage of dynamical evolution regardless of initial conditions.

Searching in the dark: the dark mass content of the Milky Way globular clusters NGC288 and NGC6218

We present an observational estimate of the fraction and distribution of dark mass in the innermost region of the two Galactic globular clusters NGC 6218 (M12) and NGC 288. Such an assessment has been made by comparing the dynamical and luminous mass profiles derived from an accurate analysis of the most extensive spectroscopic and photometric surveys performed on these stellar systems. We find that non-luminous matter constitutes more than 60% of the total mass in the region probed by our data (R<1.6 arcmin~r_h) in both clusters. We have carefully analyzed the effects of binaries and tidal heating on our estimate and ruled out the possibility that our result is a spurious consequence of these effects. The dark component appears to be more concentrated than the most massive stars suggesting that it is likely composed of dark remnants segregated in the cluster core.

MIKiS: The Multi-instrument Kinematic Survey of Galactic Globular Clusters. I. Velocity Dispersion Profiles and Rotation Signals of 11 Globular Clusters

We present the first results of the Multi-Instrument Kinematic Survey of Galactic Globular Clusters, a project aimed at exploring the internal kinematics of a representative sample of Galactic globular clusters from the radial velocity of individual stars, covering the entire radial extension of each system. This is achieved by exploiting the formidable combination of multi-object and integral field unit spectroscopic facilities of the ESO Very Large Telescope. As a first step, here we discuss the results obtained for 11 clusters from high and medium resolution spectra acquired through a combination of FLAMES and KMOS observations. We provide the first kinematical characterization of NGC 1261 and NGC 6496. In all the surveyed systems, the velocity dispersion profile declines at increasing radii, in agreement with the expectation from the King model that best fits the density/luminosity profile. In the majority of the surveyed systems we find evidence of rotation within a few half-mass radii from the center. These results are in general overall agreement with the predictions of recent theoretical studies, suggesting that the detected signals could be the relic of significant internal rotation set at the epoch of the clusters formation.