Paper XXIX

The WFPC2 UV Survey: the BSS population in NGC 5824

Sanna N.; Dalessandro E.; Ferraro F. R.; Lanzoni B.; Miocchi P.; O'Connell R.W.

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 (rc ~ 4.4") and a concentration parameter c ~2.74. We also present the first age determination for this cluster. From the comparison with isochrones, we have found t=13\pm 0.5 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 ~20", 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.

This paper has been published in the Astrophysical Journal (2014, ApJ, 780,90). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1311.1332

Paper XXXI

Constraining the true nature of an exotic binary in the core of NGC 6624

Dalessandro E.; Pallanca C.; Ferraro F.R.; Lanzoni B.; Castglione C.; Vignali C.; Fiorentino G.

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 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, here named COM_Star1, showing a clear sinusoidal light modulation with amplitude \Delta m_F435W~0.7 mag and orbital period P_orb~98 min. The shape of the light curve is likely caused by strong irradiation by the primary heating one hemisphere of the companion, thus suggesting a quite hot primary.

This paper has been published in the Astrophysical Journal (2014, ApJ, 784, L29). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1401.7779

Paper XXXIII

An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters

Origlia L.; Ferraro F.R.; Fabbri S.; Fusi Pecci F.; Dalessandro E.; Rich R.M.; Valenti E.

The main aim of the present work is to derive an empirical mass-loss (ML) law for Population II stars in first and second ascent red giant branches. We used the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8 micron range of a carefully chosen sample of 15 Galactic globular clusters spanning the entire metallicity range and sampling the vast zoology of horizontal branch (HB) morphologies. We complemented the IRAC photometry with near-infrared data to build suitable color-magnitude and color-color diagrams and identify mass-losing giant stars. We find that while the majority of stars show colors typical of cool giants, some stars show an excess of mid-infrared light that is larger than expected from their photospheric emission and that is plausibly due to dust formation in mass flowing from them. For these stars, we estimate dust and total (gas + dust) ML rates and timescales. We finally calibrate an empirical ML law for Population II red and asymptotic giant branch stars with varying metallicity. We find that at a given red giant branch luminosity only a fraction of the stars are losing mass. From this, we conclude that ML is episodic and is active only a fraction of the time, which we define as the duty cycle. The fraction of mass-losing stars increases by increasing the stellar luminosity and metallicity. The ML rate, as estimated from reasonable assumptions for the gas-to-dust ratio and expansion velocity, depends on metallicity and slowly increases with decreasing metallicity. In contrast, the duty cycle increases with increasing metallicity, with the net result that total ML increases moderately with increasing metallicity, about 0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars, we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.

This paper has been published in Astronomy & Astrophysics (2014, A&A, 562, A136). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1403.4096

Paper XXXII

The helium abundance in the metal-poor globular clusters M30 and NGC6397

Mucciarelli A.; Lovisi L.; Lanzoni B.; Ferraro F.R.

We present the helium abundance of the two metal-poor clusters M30 and NGC 6397. Helium estimates have been obtained by using the high-resolution spectrograph FLAMES at the ESO Very Large Telescope and by measuring the He I line at 4471 A in 24 and 35 horizontal branch stars in M30 and NGC 6397, respectively. This sample represents the largest dataset of He abundances collected so far in metal-poor clusters. The He mass fraction turns out to be Y=0.252 \pm 0.003 (sigma=0.021) for M30 and Y=0.241 \pm 0.004 (sigma=0.023) NGC 6397. These values are fully compatible with the cosmological abundance, thus suggesting that the horizontal branch stars are not strongly enriched in He. The small spread of the Y distributions are compatible with those expected from the observed main sequence splitting. Finally, we find an hint of a weak anticorrelation between Y and [O/Fe] in NGC 6397 in agreement with the prediction that O-poor stars are formed by (He-enriched) gas polluted by the products of hot proton-capture reactions.

This paper has been published in the Astrophysical Journal (2014, ApJ, 786, 14). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1403.0595

Paper XXXIV

Chemical and kinematical properties of Galactic bulge stars surrounding the stellar system Terzan 5

Massari D.; Mucciarelli A.; Ferraro F.R.; Origlia L.; Rich R.M.; Lanzoni B.; Dalessandro E.; Ibata R.; Lovisi L.; Bellazzini M.; Reitzel D.

As part of a study aimed at determining the kinematical and chemical properties of Terzan 5, we present the first characterization of the bulge stars surrounding this puzzling stellar system. We observed 615 targets located well beyond the tidal radius of Terzan 5 and we found that their radial velocity distribution is well described by a Gaussian function peaked at <v_rad>=+21.0\pm 4.6 km/s and with dispersion sigma_v=113.0\pm2.7 km/s. This is the one of the few high-precision spectroscopic survey of radial velocities for a large sample of bulge stars in such a low and positive latitude environment (b=+1.7 deg). We found no evidence for the peak at <v_rad>~+200 km/s found in Nidever et al. (2012). The strong contamination of many observed spectra by TiO bands prevented us from deriving the iron abundance for the entire spectroscopic sample, introducing a selection bias. The metallicity distribution was finally derived for a sub-sample of 112 stars in a magnitude range where the effect of the selection bias is negligible. The distribution is quite broad and roughly peaked at solar metallicity ([Fe/H] ~ +0.05 dex) with a similar number of stars in the super-solar and in the sub-solar ranges. The population number ratios in different metallicity ranges agree well with those observed in other low-latitude bulge fields suggesting (i) the possible presence of a plateau for |b|<4 deg for the ratio between stars in the super-solar (0<[Fe/H]<0.5 dex) and sub-solar (-0.5<[Fe/H]<0 dex) metallicity ranges; (ii) a severe drop of the metal-poor component ([Fe/H]<-0.5) as a function of Galactic latitude.

This paper has been published in the Astrophysical Journal (2014, ApJ, 791, 101). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1403.4096

Paper XXXV

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

Dalessandro E.; Massari D.; Bellazzini M.; Miocchi P.; Mucciarelli A.; Salaris M.; Cassisi S.; Ferraro F.R.; Lanzoni B.

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.

This paper has been published in the Astrophysical Journal Letter (2014, ApJL, 791, L4). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1403.4096

Paper XXXVI

No Evidence of Chemical Anomalies in the Bimodal Turnoff Cluster NGC 1806 in the Large Magellanic Cloud

Mucciarelli A. ; Dalessandro E.; Ferraro F.R.; Origlia, L.; Lanzoni B.

We have studied the chemical composition of NGC 1806, a massive, intermediate-age globular cluster that shows a double main sequence turnoff. We analyzed a sample of high-resolution spectra (secured with FLAMES at the Very Large Telescope) for eight giant stars, members of the cluster, finding an average iron content of [Fe/H] = -0.60 \pm 0.01 dex and no evidence of intrinsic star-to-star variations in the abundances of light elements (Na, O, Mg, Al). Also, the (mF 814W ; mF 336W-mF 814W ) color-magnitude diagram obtained by combining optical and near-UV Hubble Space Telescope photometry exhibits a narrow red giant branch, thus ruling out intrinsic variations of C and N abundances in the cluster. These findings demonstrate that NGC 1806 does not harbor chemically distinct sub-populations, at variance with what was found in old globular clusters. In turn, this indicates that the double main sequence turnoff phenomenon cannot be explained in the context of the self-enrichment processes usually invoked to explain the chemical anomalies observed in old globulars. Other solutions (i.e., stellar rotation, merging between clusters, or collisions with giant molecular clouds) should be envisaged to explain this class of globulars.

This paper has been published in the Astrophysical Journal Letter (2014, ApJL, 793, L6). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1409.0259

Paper XXXVII

Binary pulsars studies with multiwavelength sky surveys - I. Companion star identification

Mignani R.P.; Corongiu A.; Pallanca C.; Oates S. R.; Yershov V.N.; Breeveld A.A.; Page M.J.; Ferraro F.R.; Possenti A.; Jackson A.C.

The identification of the stellar companions to binary pulsars is key to studying the evolution of the binary system and how this is influenced by the interactions between the two stars. For only a fraction of the known binary pulsars, the stellar companions have been identified. Here, we used 11 source catalogues available from multiwavelength (ultraviolet, optical, infrared) imaging sky surveys to search for the stellar companions of a sample of 144 field binary pulsars (i.e. not in globular clusters) selected from the Australia Telescope National Facility data base (version 1.48) and from the public list of gamma-ray pulsars detected by Fermi. We found positional associations in at least one source catalogue for 22 pulsars, of which 10 are detected in γ -rays by Fermi, including 15 millisecond pulsars. For six pulsars in our compilation, we confirm their identifications. For another seven pulsars that had yet not been identified, we examine potential identifications. In particular, we identified a likely companion star candidate to PSR J2317+1439, whereas for both PSR B1953+29 and PSR J1935+1726 the companion star identification is more uncertain. Follow-up observations of these three pulsars are needed to settle the proposed identifications. For the remaining nine pulsars that had been already identified, we provide additional spectral information in at least one of the surveys' spectral bands, which we will use to better constrain the stars' spectral energy distributions.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2014, MNRAS, 443, 2223).

Paper XXXVIII

Ceci N'est Pas a Globular Cluster: The Metallicity Distribution of the Stellar System Terzan 5

Massari D.; Mucciarelli A.; Ferraro F.R.; Origlia L.; Rich R.M.; Lanzoni B.; Dalessandro E.; Valenti E.; Ibata R.; Lovisi L.; Bellazzini M.; Reitzel D.

We present new determinations of the iron abundance for 220 stars belonging to the stellar system Terzan 5 in the Galactic bulge. The spectra have been acquired with FLAMES at the Very Large Telescope of the European Southern Observatory and DEIMOS at the Keck II Telescope. This is by far the largest spectroscopic sample of stars ever observed in this stellar system. From this dataset, a subsample of targets with spectra unaffected by TiO bands was extracted and statistically decontaminated from field stars. Once combined with 34 additional stars previously published by our group, a total sample of 135 member stars covering the entire radial extent of the system has been used to determine the metallicity distribution function of Terzan 5. The iron distribution clearly shows three peaks: a super-solar component at [Fe/H]=0.25 dex, accounting for ~29% of the sample, a dominant sub-solar population at [Fe/H]=-0.30 dex, corresponding to ~62% of the total, and a minor (6%) metal-poor component at [Fe/H]=-0.8 dex. Such a broad, multi-modal metallicity distribution demon- strates that Terzan 5 is not a genuine globular cluster but the remnant of a much more complex stellar system.

This paper has been published in the Astrophysical Journal (2014, ApJ, 795, 22). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1409.1682

Paper XXXIX

Radio Timing and Optical Photometry of the Black Widow System PSR J1518+0204C in the Globular Cluster M5

Pallanca C.; Ransom S.M.; Ferraro F.R.; Dalessandro E.; Lanzoni B.; Hessels J.W.T.; Stairs I.; Freire P.C.C.

We report on the determination of astrometric, spin and orbital parameters for PSR J1518+0204C, a "black widow" binary millisecond pulsar in the globular cluster M5. The accurate position and orbital parameters obtained from radio timing allowed us to search for the optical companion. By using WFC3/HST images we identified a very faint variable star (m_F390W > 24.8, m_F606W > 24.3, m_F814W > 23.1) located at only 0.25" from the pulsar's timing position. Due to its strong variability, this star is visible only in a sub-sample of images. However, the light curve obtained folding the available data with the orbital parameters of the pulsar shows a maximum at the pulsar inferior conjunction and a possible minimum at the pulsar superior conjunction. Furthermore, the shape of the optical modulation indicates a heating process possibly due to the pulsar wind. This is the first identification of an optical companion to a black widow pulsar in the dense stellar environment of a globular cluster.

This paper has been published in the Astrophysical Journal (2014, ApJ, 795, 29). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1409.1424

Paper XL

Dynamical Friction in Multi-component Evolving Globular Clusters

Alessandrini E.; Lanzoni B.; Miocchi P.; Ciotti L.; Ferraro F.R.

We use the Chandrasekhar formalism and direct N-body simulations to study the effect of dynamical friction on a test object only slightly more massive than the field stars, orbiting a spherically symmetric background of particles with a mass spectrum. The main goal is to verify whether the dynamical friction time (t_DF) develops a non-monotonic radial-dependence that could explain the bimodality of the Blue Straggler radial distributions observed in globular clusters. In these systems, in fact, relaxation effects lead to a mass and velocity radial segregation of the different mass components, so that mass-spectrum effects on t_DF are expected to be dependent on radius. We find that, in spite of the presence of different masses, t_DF is always a monotonic function of radius, at all evolutionary times and independently of the initial concentration of the simulated cluster. This because the radial dependence of t_DF is largely dominated by the total mass density profile of the background stars (which is monotonically decreasing with radius). Hence, a progressive temporal erosion of the BSS population at larger and larger distances from the cluster center remains the simplest and the most likely explanation of the shape of the observed BSS radial distributions, as suggested in previous works. We also confirm the theoretical expectation that approximating a multi-mass globular cluster as made of (averaged) equal-mass stars can lead to significant overestimates of t_DF within the half-mass radius.

This paper has been published in the Astrophysical Journal (2014, ApJ, 795, 169). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1409.4987

Paper XLI

Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. I. Sample Selection, Data Reduction and NGC 7078 Results

Bellini A., et al.

We present the first study of high-precision internal proper motions (PMs) in a large sample of globular clusters, based on Hubble Space Telescope (HST) data obtained over the past decade with the ACS/WFC, ACS/HRC, and WFC3/UVIS instruments. We determine PMs for over 1.3 million stars in the central regions of 22 clusters, with a median number of ~60,000 stars per cluster. These PMs have the potential to significantly advance our understanding of the internal kinematics of globular clusters by extending past line-of-sight (LOS) velocity measurements to two- or three-dimensional velocities, lower stellar masses, and larger sample sizes. We describe the reduction pipeline that we developed to derive homogeneous PMs from the very heterogeneous archival data. We demonstrate the quality of the measurements through extensive Monte-Carlo simulations. We also discuss the PM errors introduced by various systematic effects, and the techniques that we have developed to correct or remove them to the extent possible. We provide in electronic form the catalog for NGC 7078 (M 15), which consists of 77,837 stars in the central 2.'4. We validate the catalog by comparison with existing PM measurements and LOS velocities, and use it to study the dependence of the velocity dispersion on radius, stellar magnitude (or mass) along the main sequence, and direction in the plane of the sky (radial/tangential). Subsequent papers in this series will explore a range of applications in globular-cluster science, and will also present the PM catalogs for the other sample clusters.

This paper has been published in the Astrophysical Journal (2014, ApJ, 797, 115). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1410.5820

Paper XLII

Spinning like a Blue Straggler: the population of fast rotating Blue Straggler stars in Omega Centauri

Mucciarelli A.; Lovisi L.; Ferraro F.R.; Dalessandro E.; Lanzoni B.; Monaco L.

By using high-resolution spectra acquired with FLAMES-GIRAFFE at the ESO/VLT, we measured radial and rotational velocities for 110 Blue Straggler stars (BSSs) in omega Centauri, the globular cluster-like stellar system harboring the largest known BSS population. According to their radial velocities, 109 BSSs are members of the system. The rotational velocity distribution is very broad, with the bulk of BSSs spinning at less than ~40 km/s (in agreement with the majority of such stars observed in other globular clusters) and a long tail reaching ~200 km/s. About 40% of the sample has v_e sini>40 km/s and about 20% has v_e sini >70 km/s. Such a large fraction is very similar to the percentage of of fast rotating BSSs observed in M4. Thus, omega Centauri is the second stellar cluster, beyond M4, with a surprisingly high population of fast spinning BSSs. We found a hint of a radial behaviour of the fraction of fast rotating BSSs, with a mild peak within one core radius, and a possibile rise in the external regions (beyond four core radii). This may suggest that recent formation episodes of mass transfer BSSs occurred preferentially in the outskirts of omega Centauri, or that braking mechanisms able to slow down these stars are least efficient in lowest density environments.

This paper has been published in the Astrophysical Journal (2014, ApJ, 797, 43). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1410.2275

Paper XLIII

NLTE effects on AGB stars in 47 Tucanae

Lapenna E.; Mucciarelli A.; Lanzoni B.; Ferraro F.R.; Dalessandro E.; Origlia L.; Massari D.

We present the iron abundance of 24 asymptotic giant branch (AGB) stars members of the globular cluster 47 Tucanae, obtained with high-resolution spectra collected with the FEROS spectrograph at the MPG/ESO-2.2m Telescope. We find that the iron abundances derived from neutral lines (with mean value [Fe I/H]=-0.94\pm 0.01, sigma=0.08 dex) are systematically lower than those derived from single ionized lines ([Fe II/H]=-0.83\pm 0.01, sigma= 0.05 dex). Only the latter are in agreement with those obtained for a sample of red giant branch (RGB) cluster stars, for which Fe I and Fe II lines provide the same iron abundance. This finding suggests that Non Local Thermodynamical Equilibrium (NLTE) effects driven by overionization mechanisms are present in the atmosphere of AGB stars and significantly affect Fe I lines, while leaving Fe II features unaltered. On the other hand, the very good ionization equilibrium found for RGB stars indicates that these NLTE effects may depend on the evolutionary stage. We discuss the impact of this finding both on the chemical analysis of AGB stars, and on the search for evolved blue stragglers.

This paper has been published in the Astrophysical Journal (2014, ApJ, 797, 124). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1410.3841

Paper XXXX

Blue Straggler masses from pulsation properties. I. The case of NGC 6541

Fiorentino G.; Lanzoni B.; Dalessandro E.; Ferraro F.R.; Bono G.; Marconi >

We used high spatial resolution images acquired with the Wide Field Camera 3 on board Hubble Space Telescope to probe the population of variable blue straggler stars (BSSs) in the central region of the poorly studied Galactic globular cluster NGC 6541. The time sampling of the acquired multiwavelength (F390W, F555W, and F814W) data allowed us to discover three WUMa stars and nine SX Phoenicis. Periods, mean magnitudes, and pulsation modes have been derived for the nine SX Phoenicis, and their masses have been estimated by using pulsation equations obtained from linear nonadiabatic models. We found masses in the range 1.0-1.1 Msol, with an average value of 1.06 \pm 0.09 Msol (sigma = 0.04), significantly in excess of the cluster main-sequence turn-off mass (~0.75 Msol). A mild trend between mass and luminosity seems also to be present. The computed pulsation masses turn out to be in very good agreement with the predictions of evolutionary tracks for single stars, indicating values in the range ~1.0-1.2 Msol for most of the BSS population, in agreement with what was discussed in a number of previous studies.

This paper has been published in the Astrophysical Journal (2014, ApJ, 783, 34). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1312.0338