Paper XV

GALA: an automatic tool for the abundance analysis of stellar spectra

Mucciarelli A.; Pancino E.; Lovisi L.; Ferraro F. R. ; Lapenna E.

GALA is a freely distributed Fortran code to derive automatically the atmospheric parameters (temperature, gravity, microturbulent velocity and overall metallicity) and abundances for individual species of stellar spectra using the classical method based on the equivalent widths of metallic lines. The abundances of individual spectral lines are derived by using the WIDTH9 code developed by R. L. Kurucz. GALA is designed to obtain the best model atmosphere, by optimizing temperature, surface gravity, microturbulent velocity and metallicity, after rejecting the discrepant lines. Finally, it computes accurate internal errors for each atmospheric parameter and abundance. The code permits to obtain chemical abundances and atmospheric parameters for large stellar samples in a very short time, thus making GALA an useful tool in the epoch of the multi-object spectrographs and large surveys. An extensive set of tests with both synthetic and observed spectra is performed and discussed to explore the capabilities and robustness of the code.

This paper has been published in the Astrophysical Journal (2013, ApJ, 766, 78). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1302.3618.

Paper XIV

The Horizontal Branch in the UV Colour Magnitude Diagrams. II. The case of M3, M13 and M79

Dalessandro E.; Salaris, M.; Ferraro F. R. ; Mucciarelli A.; Cassisi S.

We present a detailed comparison between far-UV/optical colour Magnitude Diagrams obtained with high-resolution Hubble Space Telescope data and suitable theoretical models for three Galactic Globular Clusters: M3, M13 and M79. These systems represent a classical example of clusters in the intermediate metallicity regime that, even sharing similar metal content and age, show remarkably different Horizontal Branch morphologies. As a consequence, the observed differences in the colour distributions of Horizontal Branch stars cannot be interpreted in terms of either first (metallicity) or a second parameter such as age. We investigate here the possible role of variations of initial Helium abundance (Y). Thanks to the use of a proper setup of far-UV filters, we are able to put strong constraints on the maximum Y (Y_max) values compatible with the data. We find differences Delta Y_max ~ 0.02-0.04 between the clusters with M13 showing the largest value (Y_max ~0.30) and M3 the smallest (Y_max ~ 0.27). In general we observe that these values are correlated with the colour extensions of their Horizontal Branches and with the range of the observed Na-O anti-correlations.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2013, MNRAS, 430, 459). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1212.4419.

Paper XVI

Infrared observations of the candidate double neutron star system PSR J1811-1736

Mignani R.P.; Corongiu A.; Pallanca C.; Ferraro F. R.

PSR J1811-1736 (P=104 ms) is an old (~1.89 Gyrs) binary pulsar (Porb=18.8 d) in a highly eccentric orbit (e=0.828) with an unidentified companion. Interestingly enough, the pulsar timing solution yields an estimated companion mass 0.93 Msol<M_C<1.5 Msol, compatible with that of a neutron star. As such, it is possible that PSR J1811-1736 is a double neutron star (DNS) system, one of the very few discovered so far. This scenario can be investigated through deep optical/infrared (IR) observations. We used J, H, K-band images, obtained as part of the UK Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS), and available in the recent Data Release 9 Plus, to search for its undetected companion of the PSR J1811-1736 binary pulsar. We detected a possible companion star to PSR J1811-1736 within the 3 sigma radio position uncertainty (1.32 arcsec), with magnitudes J=18.61+/- 0.07, H=16.65 +/- 0.03, and K=15.46+/-0.02. The star colours are consistent with either a main sequence (MS) star close to the turn-off or a lower red giant branch (RGB) star, at a pulsar distance of ~5.5 kpc and with a reddening of E(B-V)~4.9. The star mass and radius would be compatible with the constraints on the masses and orbital inclination of the binary system inferred from the mass function and the lack of radio eclipses near superior conjunction. Thus, it is possible that it is the companion to PSR J1811-1736. However, based on the star density in the field, we estimated a quite large chance coincidence probability of ~0.27 between the pulsar and the star, which makes the association unlikely. No other star is detected within the 3 sigma pulsar radio position down to J~20.5, H~19.4 and K~18.6, which would allow us to rule out a MS companion star earlier than a mid-to-late M spectral type.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2013, MNRAS, 430, 1008) A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1212.4801.

Paper XIX

FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30

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

We present spectroscopic observations acquired with FLAMES and XSHOOTER at the Very Large Telescope for a sample of 15 Blue Straggler Stars (BSSs) in the globular cluster (GC) M30. The targets have been selected to sample the two BSS sequences discovered, with 7 BSSs along the blue sequence and 8 along the red one. No difference in the kinematical properties of the two groups of BSSs has been found. In particular, almost all the observed BSSs have projected rotational velocity lower than ~30 km/s, with only one (blue) fast rotating BSS (>90 km/s), identified as a W UMa binary. This rotational velocity distribution is similar to those obtained in 47 Tucanae and NGC 6397, while M4 remains the only GC studied so far harboring a large fraction of fast rotating BSSs. All stars hotter than ~7800 K (regardless of the parent BSS sequence) show iron abundances larger than those measured from normal cluster stars, with a clearcut trend with the effective temperature. This behaviour suggests that particle trasport mechanisms driven by radiative levitation occur in the photosphere of these stars, as already observed for the BSSs in NGC 6397. Finally, 4 BSSs belonging to the red sequence (not affected by radiative levitation) show a strong depletion of [O/Fe], with respect to the abundance measured in Red Giant Branch and Horizontal Branch stars. This O-depletion is compatible with the chemical signature expected in BSSs formed by mass transfer processes in binary systems, in agreement with the mechanism proposed for the formation of BSSs in the red sequence.

This paper has been published in the Astrophysical Journal (2013, ApJ, 772,148). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1306.0839.

Paper XVII

The velocity dispersion profile of NGC 6388 from resolved-star spectroscopy: no evidence of a central cusp and new constraints on the BH mass

Lanzoni B.; Mucciarelli A.; Origlia L.; Bellazzini M.; Ferraro F. R.; Valenti E.; Miocchi P.; Dalessandro E.; Pallanca C.; Massari D.

By combining high spatial resolution and wide-field spectroscopy performed, respectively, with SINFONI and FLAMES at the ESO/VLT we measured the radial velocities of more than 600 stars in the direction of NGC 6388, a Galactic globular cluster which is suspected to host an intermediate-mass black hole. Approximately 55% of the observed targets turned out to be cluster members. The cluster velocity dispersion has been derived from the radial velocity of individual stars: 52 measurements in the innermost 2", and 276 stars located between 18" and 600". The velocity dispersion profile shows a central value of ~13 km/s, a flat behavior out to ~60" and a decreasing trend outwards. The comparison with spherical and isotropic models shows that the observed density and velocity dispersion profiles are inconsistent with the presence of a central black hole more massive than ~2000 Msol. These findings are at odds with recent results obtained from integrated light spectra, showing a velocity dispersion profile with a steep central cusp of 23-25 km/s at r<2" and suggesting the presence of a black hole with a mass of ~1.7x10^4 Msol (Lutzgendorf et al. 2011). We also found some evidence of systemic rotation with amplitude Arot ~8 km/s in the innermost 2" (0.13 pc), decreasing to Arot = 3.2 km/s at 18" <r< 160 ".

This paper has been published in the Astrophysical Journal (2013, ApJ, 769,107). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1304.2953.

Paper XVIII

UV observations of the globular cluster M10 from HST and GALEX. The BSS population.

Dalessandro E.; Ferraro F. R.; Lanzoni B.; Schiavon R.P.; O'Connel R.W; Beccari G.

We present a combination of high-resolution Hubble Space Telescope and wide-field ground-based and Galaxy Evolution Explorer data of the Galactic Globular Cluster M10 (NGC6254). By using this large data-set we determined the center of gravity of the cluster and we built its density profile from star counts over its entire radial extension. We find that the density profile is well reproduced by a single-mass King model with structural parameters c=1.41 and rc=41". We also studied the Blue Straggler Star population and its radial distribution. We count a total number of 120 BSS within the tidal radius. Their radial distribution is bimodal: highly peaked in the cluster center, decreasing at intermediate distances and rising again outwards. We discuss these results in the context of the dynamical clock scheme presented by Ferraro et al. (2012) and of recent results about the radial distribution of binary systems in this cluster.

This paper has been published in the Astrophysical Journal (2013, ApJ, 770, 45). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1305.0846.

Paper XX

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

Pallanca C.; Lanzoni B.; Dalessandro E.; Ferraro F. R.; Possenti A.; Salaris M.; Burgay M.

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 \pm 0.02, V = 23.21 \pm 0.01 and I = 22.96 \pm 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 Msol), distances (d < 1200 pc), radii (< 0.0078 Rsol) and temperatures (T =31350 {-7400, +21500} K). From the PSR mass function and the estimated mass range we also constrained the inclination angle i > 55 degree and the pulsar mass (Mpsr < 2.2Msol). The comparison between the WD cooling age and the spin down age suggests that the latter is overestimated by a factor of about ten.

This paper has been published in the Astrophysical Journal (2013, ApJ, 773,127). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1306.3787.

Paper XXI

The optical counterpart to the X-ray transient IGR J1824-24525 in the globular cluster M28

Pallanca C.; Dalessandro E.; Ferraro F. R.; Lanzoni B.; Beccari G.

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 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 Hα excess during the outburst state, suggestive of active accretion processes by the neutron star.

This paper has been published in the Astrophysical Journal (2013, ApJ, 773,122). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1306.6416.

Paper XXII

New clues on the nature of the companion to PSR J1740-5340 in NGC6397 from XSHOOTER spectroscopy

Mucciarelli A.; Salaris M.; Lanzoni B.; Pallanca C.; Dalessandro E.; Ferraro F. R.

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 \pm 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 Msol) remnant star, descending from a ~0.8 Msol progenitor which lost ~70-80% of its original material because of mass transfer activity onto the pulsar.

This paper has been published in the Astrophysical Journal (2013, ApJ, 772. L27). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1307.0919

Paper XXIII

Star count density profiles and structural parameters of 26 Galactic globular clusters

Miocchi P.; Lanzoni B.; Ferraro F. R.; Dalessandro E.; Vesperini E.; Pasquato M.; Beccari G.; Pallanca C.; Sanna N.

We used a proper combination of high-resolution HST observations and wide-field ground based data to derive the radial star density profile 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 is 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 fitted 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 distances; (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 what expected from simulations of cluster dynamical evolution and the ratio between these two radii well correlates 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.

This paper has been published in the Astrophysical Journal (2013, ApJ, 774, 151). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1307.6035

Paper XXIV

Deep multi-telescope photometry of NGC 5466. I. Blue Stragglers and binary systems

Beccari G.; Dalessandro E.; Lanzoni B.; Ferraro F. R.; Sollima, A.; Bellazzini M., Miocchi P.

We present a detailed investigation of the radial distribution of blue straggler star and binary populations in the Galactic globular cluster NGC 5466, over the entire extension of the system. We used a combination of data acquired with the ACS on board the Hubble Space Telescope, the LBC-blue mounted on the Large Binocular Telescope, and MEGACAM on the Canadian-France-Hawaii Telescope. Blue straggler stars show a bimodal distribution with a mild central peak and a quite internal minimum. This feature is interpreted in terms of a relatively young dynamical age in the framework of the "dynamical clock" concept proposed by Ferraro et al. (2012). The estimated fraction of binaries is 6-7% in the central region (r<90") and slightly lower (5.5%) in the outskirts, at r>200". Quite interestingly, the comparison with the results of Milone et al. (2012) suggests that also binary systems may display a bimodal radial distribution, with the position of the minimum consistent with that of blue straggler stars. If confirmed, this feature would give additional support to the scenario where the radial distribution of objects more massive than the average cluster stars is primarily shaped by the effect of dynamical friction. Moreover, this would also be consistent with the idea that the unperturbed evolution of primordial binaries could be the dominant BSS formation process in low-density environments.

This paper has been published in the Astrophysical Journal (2013, ApJ, 776, 60). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1308.5810

Paper XXV

Another brick in uderstanding chemical and kinematical properties of BSSs: NGC 6752

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

We used high-resolution spectra acquired with the multifiber facility FLAMES at the Very Large Telescope of the European Southern Observatory to investigate the chemical and kinematical properties of a sample of 22 Blue Straggler Stars (BSSs) and 26 red giant branch stars in the nearby globular cluster NGC 6752. We measured radial and rotational velocities and Fe, O and C abundances. According to radial velocities, metallicity and proper motions we identified 18 BSSs as likely cluster members. We found that all the BSSs rotate slowly (less than 40 km/s), similar to the findings in 47 Tucanae, NGC 6397 and M30. The Fe abundance analysis reveals the presence of 3 BSSs affected by radiative levitation (showing [Fe/H] significantly higher than that measured in "normal" cluster stars), confirming that element transport mechanisms occur in the photosphere of BSSs hotter than 8000 K. Finally, BSS C and O abundances are consistent with those measured in dwarf stars. No C and O depletion ascribable to mass transfer processes has been found on the atmospheres of the studied BSSs (at odds with previous results for 47 Tucanae and M30), suggesting the collisional origin for BSSs in NGC 6752 or that the CO-depletion is a transient phenomenon.

This paper has been published in the Astrophysical Journal (2013, ApJ, 778, 64). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1309.5234

Paper XXVI

HST absolute proper motions of NGC 6681 (M70) and the Sagittarius dwarf spherodial galaxy

Massari D.; Bellini A.; Ferraro F. R.; van der Marel R.P.; Anderson J.; Dalessandro E.; Lanzoni B.

We have measured absolute proper motions for the three populations intercepted in the direction of the Galactic globular cluster NGC 6681: the cluster itself, the Sagittarius dwarf spheroidal galaxy and the field. For this we used Hubble Space Telescope ACS/WFC and WFC3/UVIS optical imaging data separated by a temporal baseline of 5.464 years. Five background galaxies were used to determine the zero point of the absolute-motion reference frame. The resulting absolute proper motion of NGC 6681 is (mu_alpha cos(delta), mu_delta)= (1.58\pm0.18, -4.57\pm0.16) mas/yr. This is the first estimate ever made for this cluster. For the Sgr dSph we obtain (mu_alpha cos(delta), mu_delta)= (-2.54\pm0.18, -1.19\pm0.16) mas/yr, consistent with previous measurements and with the values predicted by theoretical models. The absolute proper motion of the Galaxy population in our field of view is (mu_alpha cos(delta), mu_delta)= (-1.21\pm0.27, -4.39\pm0.26) mas/yr. In this study we also use background Sagittarius Dwarf Spheroidal stars to determine the rotation of the globular cluster in the plane of the sky and find that NGC 6681 is not rotating significantly: v_rot=0.82\pm1.02 km/s at a distance of 1 arcmin from the cluster center.

This paper has been published in the Astrophysical Journal (2013, ApJ, 779, 81). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1310.2096

Paper XXVII

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

Dalessandro E.; Ferraro F. R.; Massari D.; Lanzoni B.; Miocchi P.; Beccari G.; Bellini A.; Sills A.; Sigurdsson S.; Mucciarelli A.; Lovisi L.

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.

This paper has been published in the Astrophysical Journal (2013, ApJ, 778, 135). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1310.2389

Paper XXVIII

The Terzan 5 puzzle: discovery of a third, metal-poor component

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

We report on the discovery of 3 metal-poor giant stars in Terzan 5, a complex stellar system in the the Galactic bulge, known to have two populations at [Fe/H]=-0.25 and +0.3. For these 3 stars we present new echelle spectra obtained with NIRSPEC at Keck II, which confirm their radial velocity membership and provide average [Fe/H]=-0.79 dex iron abundance and [alpha/Fe]=+0.36 dex enhancement. This new population extends the metallicity range of Terzan 5 0.5 dex more metal poor, and it has properties consistent with having formed from a gas polluted by core collapse supernovae.

This paper has been published in the Astrophysical Journal Letter (2013, ApJ, 779, L5). A PDF copy of the preprint is also available on the astro-ph server at this address arXiv:1311.1706