F. R. Ferraro

Publications (201)327.55 Total impact

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

  E. Dalessandro, F. R. Ferraro, B. Lanzoni, R. P. Schiavon, R. W. O'Connell, G. Beccari
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  ABSTRACT: 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 r_c=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.
  05/2013;
• Article: GALA: an automatic tool for the abundance analysis of stellar spectra

  A. Mucciarelli, E. Pancino, L. Lovisi, F. R. Ferraro, E. Lapenna
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  ABSTRACT: 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.
  02/2013;
• Article: Ultraviolet properties of Galactic globular clusters with GALEX

  E. Dalessandro, R. P. Schiavon, F. R. Ferraro, S. T. Sohn, B. Lanzoni, R. W. O' Connell
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  ABSTRACT: We present ultraviolet (UV) integrated colors of 44 Galactic globular clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV and NUV bands. We find for the first time that GCs associated with the Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs with the same metallicity. M31 GCs show almost the same UV colors as GGCs, while M87 are systematically bluer. We speculate about the presence of an interesting trend, suggesting that the UV color of GCs may be correlated with the mass of the host galaxy, in the sense that more massive galaxies possess bluer clusters.
  01/2013;
• Article: Terzan 5: a Fossil Remnant of the Galactic Bulge

  D. Massari, F. R. Ferraro, E. Dalessandro, B. Lanzoni, A. Mucciarelli, L. Origlia
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  ABSTRACT: Terzan 5 is a stellar system located in the Galactic Bulge, at a distance of 5.9 kpc. Recent discoveries show that it hosts two stellar populations with different iron abundance ({\Delta}[Fe/H]=0.5). Such a large difference has been measured only in {\omega} Centauri in the Galactic halo. Moreover no anticorrelation is observed in Terzan 5, hence it is not a genuine globular cluster. The observed chemical patterns are strikingly similar to those observed in the Bulge stars. This suggests that Terzan 5 is a remnant fragment of the Galactic bulge.
  01/2013;
• Article: Dynamical age differences among coeval star clusters as revealed by blue stragglers.

  F R Ferraro, B Lanzoni, E Dalessandro, G Beccari, M Pasquato, P Miocchi, R T Rood, S Sigurdsson, A Sills, E Vesperini, M Mapelli, R Contreras, N Sanna, A Mucciarelli
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  ABSTRACT: Globular star clusters that formed at the same cosmic time may have evolved rather differently from the dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different 'dynamical ages'). Blue straggler stars have masses greater than those at the turn-off point on the main sequence and therefore must be the result of either a collision or a mass-transfer event. Because they are among the most massive and luminous objects in old clusters, they can be used as test particles with which to probe dynamical evolution. Here we report that globular clusters can be grouped into a few distinct families on the basis of the radial distribution of blue stragglers. This grouping corresponds well to an effective ranking of the dynamical stage reached by stellar systems, thereby permitting a direct measure of the cluster dynamical age purely from observed properties.
  Nature 12/2012; 492(7429):393-5. · 36.28 Impact Factor
• Article: Infrared observations of the candidate double neutron star system PSR J1811-1736

  R. P. Mignani, A. Corongiu, C. Pallanca, F. R. Ferraro
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  ABSTRACT: PSR J1811-1736 (P=104 ms) is an old (~1.89 Gyrs) binary pulsar (P_orb=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 M_{\odot}<M_C<1.5 M_{\odot}, 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.
  12/2012;
• Article: The identification of the optical companion to the binary millisecond pulsar J0610-2100 in the Galactic field

  C. Pallanca, R. P. Mignani, E. Dalessandro, F. R. Ferraro, B. Lanzoni, A. Possenti, M. Burgay, E. Sabbi
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  ABSTRACT: 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.
  06/2012;
• Article: Chemical and kinematical properties of BSSs and HB stars in NGC 6397

  L. Lovisi, A. Mucciarelli, B. Lanzoni, F. R. Ferraro, R. Gratton, E. Dalessandro, R. Contreras Ramos
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  ABSTRACT: We used three sets of 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 42 horizontal branch (HB) stars, 18 Blue Straggler Stars (BSSs) and 86 main sequence turn-off and sub-giant branch stars in the nearby globular cluster NGC 6397. We measured rotational velocities and Fe, O and Mg abundances. All the unevolved stars in our sample turn out to have low rotational velocites (v sin i< 10\kms), while HB stars and BSSs show a broad distribution, with values ranging from 0 to 70 \kms. For HB stars with T<10500 K there is a clear temperature-oxygen anti-correlation, that can be understood if the star position along the HB is mainly determined by the He content. The hottest BSSs and HB stars (with temperatures T>8200 K and T> 10500 K, respectively) also show significant deviations in their iron abundance with respect to the cluster metallicity (as traced by the unevolved stars, [Fe/H]=-2.12). While similar chemical patterns have been already observed in other hot HB stars, this is the first evidence ever collected for BSSs. We interprete these abundance anomalies as due to the metal radiative levitation, occurring in stars with shallow or no convective envelopes.
  05/2012;
• Source

  Available from: ArXiv

  Article: Constraining the optical emission from the double pulsar system J0737-3039

  F. R. Ferraro, R. P. Mignani, C. Pallanca, E. Dalessandro, B. Lanzoni, A. Pellizzoni, A. Possenti, M. Burgay, F. Camilo, N. D'Amico, A. G. Lyne, M. Kramer, R. N. Manchester
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  ABSTRACT: We present the first optical observations of the unique system J0737-3039 (composed of two pulsars, hereafter PSR-A and PSR-B). Ultra-deep optical observations, performed with the High Resolution Camera of the Advanced Camera for Surveys on board the Hubble Space Telescope could not detect any optical emission from the system down to m_F435W=27.0 and m_F606W=28.3. The estimated optical flux limits are used to constrain the three-component (two thermal and one non-thermal) model recently proposed to reproduce the XMM-Newton X-ray spectrum. They suggest the presence of a break at low energies in the non-thermal power law component of PSR-A and are compatible with the expected black-body emission from the PSR-B surface. The corresponding efficiency of the optical emission from PSR-A's magnetosphere would be comparable to that of other Myr-old pulsars, thus suggesting that this parameter may not dramatically evolve over a time-scale of a few Myr.
  02/2012;
• Source

  Available from: Giacomo Beccari

  Article: The BSS population in NGC 6229

  N. Sanna, E. Dalessandro, B. Lanzoni, F. R. Ferraro, G. Beccari, R. T. Rood
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  ABSTRACT: 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 (BSS) population of the outer-halo globular cluster NGC 6229, over its entire radial extent. A total of 64 bright BSS (with m255 6 21.30, corresponding to m555 6 20.75) has been identified. The BSS projected radial distribution is found to be bimodal, with a high central peak, a well defined minimum at intermediate radii (r \sim 40"), and an upturn in the outskirts. From detailed star counts even in the very inner region, we compute the centre of gravity of the cluster and the most accurate and extended radial density profile ever published for this system. The profile is reasonably well reproduced by a standard King model with an extended core (rc \simeq 9.5") and a modest value of the concentration parameter (c \simeq 1.49). However, a deviation from the model is noted in the most external region of the cluster (at r > 250" from the centre). This feature needs to be further investigated in order to assess the possible presence of a tidal tail in this cluster.
  02/2012;
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  Available from: ArXiv

  Article: The Unimodal Distribution Of Blue Straggler Stars in M75 (NGC 6864)

  R. Contreras Ramos, F. R. Ferraro, E. Dalessandro, B. Lanzoni, R. T. Rood
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  ABSTRACT: We have used a combination of multiband high-resolution and wide-field ground-based observations to image the Galactic globular cluster M75 (NGC 6864). The extensive photometric sample covers the entire cluster extension, from the very central regions out to the tidal radius, allowing us to determine the center of gravity and to construct the most extended star density profile ever published for this cluster. We also present the first detailed star counts in the very inner regions. The star density profile is well re-produced by a standard King model with core radius r_c ~ 5.4" and intermediate-high concentration c ~ 1.75. The present paper presents a detailed study of the BSS population and its radial distribution. A total number of 62 bright BSSs (with m_F255W < 21, corresponding to m_F555W < 20) has been identified, and they have been found to be highly segregated in the cluster core. No significant upturn in the BSS frequency has been observed in the outskirts of M75, in contrast to several other clusters studied with the same technique. This observational fact is quite similar to what has been found in M79 (NGC 1904) by Lanzoni et al. (2007a). Indeed the BSS radial distributions in the two clusters is qualitatively very similar, even if in M75 the relative BSS frequency seems to decrease significantly faster than in M79: indeed it decreases by a factor of 5 (from 3.4 to 0.7) within 1 r_c. Such evidence indicate that the vast majority of the cluster heavy stars (binaries) have already sunk to the core.
  01/2012;
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  Available from: ArXiv

  Article: Blood ties: the real nature of the LMC binary globular clusters NGC 2136 and NGC 2137

  A. Mucciarelli, L. Origlia, F. R. Ferraro, M. Bellazzini, B. Lanzoni
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  ABSTRACT: We have used a sample of high-resolution spectra obtained with the multi-fiber facility FLAMES at the Very Large Telescope of the European Southern Observatory, to derive the kinematical and chemical properties of the two young Large Magellanic Cloud globular clusters NGC 2136 and NGC 2137. These two clusters represent a typical example of LMC cluster pair suspected to be bound in a binary system: indeed the cluster centers of gravity have an angular separation of less than 1.4 arcmin in the sky. The spectral analysis of seven giants in NGC 2136 and four in NGC 2137 reveals that the two clusters share very similar systemic radial velocities, namely Vrad=271.5\pm0.4 km/s (sigma=1.0 km/s) and Vrad=270.6\pm0.5 km/s (sigma=0.9 km/s) for NGC 2136 and NGC 2137, respectively, and they have also indistinguishable abundance patterns. The iron content is [Fe/H]=-0.40\pm0.01 dex (sigma=0.03 dex) for NGC 2136 and -0.39\pm0.01 dex (sigma=0.01 dex) for NGC 2137, while the [alpha/Fe] ratios are roughly solar in both clusters. These findings suggest that the two clusters are gravitationally bound and that they formed from the fragmentation of the same molecular cloud that was chemically homogeneous. This is the first firm confirmation of the binary nature of a LMC cluster pair. The most likely fate of this system is to merge into a single structure in a time-scale comparable with its orbital period.
  01/2012;
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  Available from: Giacomo Beccari

  Article: The binary fraction in the globular cluster M10 (NGC 6254): comparing core and outer regions

  E. Dalessandro, B. Lanzoni, G. Beccari, A. Sollima, F. R. Ferraro, M. Pasquato
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  ABSTRACT: We study the binary fraction of the globular cluster M10 (NGC 6254) as a function of radius from the cluster core to the outskirts, by means of a quan- titative analysis of the color distribution of stars relative to the fiducial main sequence. By taking advantage of two data-sets, acquired with the Advanced Camera for Survey and the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have studied both the core and the external regions of the cluster. The binary fraction is found to decrease from 14% within the core, to 1.5% in a region between 1 and 2 half-mass radii from the cluster centre. Such a trend and the derived values are in agreement with previous results ob- tained in clusters of comparable total magnitude. The estimated binary fraction is sufficient to account for the suppression of mass segregation observed in M10, without any need to invoke the presence of an intermediate-mass black hole in its centre.
  08/2011;
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  Available from: L. Monaco

  Article: The subgiant branch of omega Cen seen through high-resolution spectroscopy. I. The first stellar generation in omega Centauri?

  E. Pancino, A. Mucciarelli, L. Sbordone, M. Bellazzini, L. Pasquini, L. Monaco, F. R. Ferraro
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  ABSTRACT: We analysed high-resolution UVES spectra of six stars belonging to the subgiant branch of omega Centauri, and derived abundance ratios of 19 chemical elements (namely Al, Ba, C, Ca, Co, Cr, Cu, Fe, La, Mg, Mn, N, Na, Ni, Sc, Si, Sr, Ti, and Y). A comparison with previous abundance determinations for red giants provided remarkable agreement and allowed us to identify the sub-populations to which our targets belong. We found that three targets belong to a low-metallicity population at [Fe/H]~-2.0 dex, [alpha/Fe]~+0.4 dex and [s/Fe]~0 dex. Stars with similar characteristics were found in small amounts by past surveys of red giants. We discuss the possibility that they belong to a separate sub-population that we name VMP (very metal-poor, at most 5% of the total cluster population), which - in the self-enrichment hypothesis - is the best-candidate first stellar generation in omega Cen. Two of the remaining targets belong to the dominant metal-poor population (MP) at [Fe/H]~-1.7 dex, and the last one to the metal-intermediate (MInt) one at [Fe/H]~-1.2 dex. The existence of the newly defined VMP population could help to understand some puzzling results based on low-resolution spectroscopy (Sollima et al., Villanova et al.) in their age differences determinations, because the metallicity resolution of these studies was probably not enough to detect the VMP population. The VMP could also correspond to some of the additional substructures of the subgiant-branch region found in the latest HST photometry (Bellini et al.). After trying to correlate chemical abundances with substructures in the subgiant branch of omega Cen, we found that the age difference between the VMP and MP populations should be small (0+/-2 Gyr), while the difference between the MP and MInt populations could be slightly larger (2+/-2~Gyr). Comment: 15 pages, 14 figures, accepted for publication by A&A
  12/2010;
• Article: Lithium abundance in the globular cluster M4: from the turn‐off to the red giant branch bump★

  A. Mucciarelli, M. Salaris, L. Lovisi, F. R. Ferraro, B. Lanzoni, S. Lucatello, R. G. Gratton
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  ABSTRACT: We present Li and Fe abundances for 87 stars in the globular cluster M4, obtained by using high-resolution spectra collected with GIRAFFE at the Very Large Telescope. The targets range from the turn-off up to the red giant branch bump. The Li abundance in the turn-off stars is uniform, with an average value equal to A(Li)= 2.30 ± 0.02 dex (σ= 0.10 dex), consistent with the upper envelope of Li content measured in other globular clusters and in the halo field stars, confirming also for M4 the discrepancy with the primordial Li abundance predicted by Wilkinson Microwave Anisotropy Probe+ big bang nucleosynthesis (WMAP+BBNS). The global behaviour of A(Li) as a function of the effective temperature allows us to identify the two main drops in the Li evolution due to the first dredge-up and to the extra-mixing episode after the red giant branch bump. The measured iron content of M4 results to [Fe/H]=−1.10 ± 0.01 dex (σ= 0.07 dex), with no systematic offsets between dwarf and giant stars.The behaviour of the Li and Fe abundances along the entire evolutionary path is incompatible with theoretical models including pure atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account, able to inhibit the atomic diffusion. The measured value of A(Li) and its homogeneity in the turn-off stars allow us to put strong constraints on the shape of the Li profile inside the M4 turn-off stars. The global behaviour of A(Li) with the effective temperature can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing. One cannot reproduce the global trend that starts from the WMAP+BBNS A(Li) and adopts the turbulent mixing described by Richard, Michaud & Richer with the same efficiency as that used by Korn et al. to explain the Li content in NGC 6397. In fact, such a solution is not able to well reproduce simultaneously the Li abundance observed in turn-off and red giant branch stars. However, the WMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned.We conclude that the cosmological Li discrepancy cannot be easily solved with the present, poor understanding of the turbulence in the stellar interiors, and a future effort to well understand the true nature of this non-canonical process is needed.
  Monthly Notices of the Royal Astronomical Society 12/2010; 412(1):81 - 94. · 4.90 Impact Factor
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  Available from: Giacomo Beccari

  Article: Spectroscopy Unveils the Complex Nature of Terzan 5

  L. Origlia, R. M. Rich, F. R. Ferraro, B. Lanzoni, M. Bellazzini, E. Dalessandro, A. Mucciarelli, E. Valenti, G. Beccari
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  ABSTRACT: We present the chemical abundance analysis of 33 red giant stars belonging to the complex stellar system Terzan 5. We confirm the discovery of two stellar populations (Ferraro et al. 2009, Nature, 462,483) with distinct iron abundances: a relatively metal-poor component with [Fe/H]=-0.25 +/- 0.07 r.m.s., and another component with [Fe/H]=+0.27 +/- 0.04 r.m.s., exceeding in metallicity any known Galactic globular cluster. The two populations also show different [alpha/Fe] abundance ratios. The metal-poor component has an average [alpha/Fe]=+0.34 +/- 0.06 r.m.s., consistent with the canonical scenario for rapid enrichment by core collapse supernovae (SNe). The metal-rich component has [alpha/Fe]=+0.03 +/-i 0.04 r.m.s., suggesting that the gas from which it formed was polluted by both type II and type Ia SNe on a longer timescale. Neither of the two populations shows evidence of the [Al/Fe] over [O/Fe] anti-correlation, that is typically observed in Galactic globular clusters. Because these chemical abundance patterns are unique, we propose that Terzan 5 is not a true globular cluster, but a stellar system with a much more complex history of star formation and chemical enrichment.
  The Astrophysical Journal Letters 12/2010; · 5.53 Impact Factor
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  Available from: ArXiv

  Article: Lithium abundance in the globular cluster M4: from the Turn-Off to the RGB Bump

  A. Mucciarelli, M. Salaris, L. Lovisi, F. R. Ferraro, L. Lanzoni, S. Lucatello, R. G. Gratton
  [show abstract] [hide abstract]
  ABSTRACT: We present Li and Fe abundances for 87 stars in the GC M4,obtained with GIRAFFE high-resolution spectra. The targets range from the TO up to the RGB Bump. The Li abundance in the TO stars is uniform, with an average value A(Li)=2.30+-0.02 dex,consistent with the upper envelope of Li content measured in other GCs and in the Halo stars,confirming also for M4 the discrepancy with the primordial Li abundance predicted by WMAP+BBNS. The iron content of M4 is [Fe/H]=-1.10+-0.01 dex, with no systematic offsets between dwarf and giant stars.The behaviour of the Li and Fe abundance along the entire evolutionary path is incompatible with models with atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account,able to inhibit the atomic diffusion.The measured A(Li) and its homogeneity in the TO stars allow to put strong constraints on the shape of the Li profile inside the M4 TO stars. The global behaviour of A(Li) with T_{eff} can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing.One cannot reproduce the global trend starting from the WMAP+BBNS A(Li) and adopting the turbulent mixing described by Richard et al.(2005) with the same efficiency used by Korn et al.(2006) to explain the Li content in NGC6397. Such a solution is not able to well reproduce simultaneously the Li abundance observed in TO and RGB stars.Otherwise, theWMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned. The cosmological Li discrepancy cannot be easily solved with the present,poor understanding of the turbulence in the stellar interiors and a future effort to well understand the true nature of this non-canonical process is needed. Comment: Accepted for publication in the MNRAS
  10/2010;
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  Available from: ArXiv

  Article: The optical companion to the binary millisecond pulsar J1824-2452H in the globular cluster M28

  C. Pallanca, E. Dalessandro, F. R. Ferraro, B. Lanzoni, R. T. Rood, A. Possenti, N. D'Amico, P. C. Freire, I. Stairs, S. M. Ransom, S. Begin
  [show abstract] [hide abstract]
  ABSTRACT: 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 variability (~ 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. Comment: accepted for publication on ApJ, 17 pages, 5 figures
  10/2010;
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  Available from: ArXiv

  Article: The peculiar Horizontal Branch of NGC 2808

  E. Dalessandro, M. Salaris, F. R. Ferraro, S. Cassisi, B. Lanzoni, R. T. Rood, F. Fusi Pecci, E. Sabbi
  [show abstract] [hide abstract]
  ABSTRACT: 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 multimodal 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. Comment: Accepted for publication in the MNRAS (12 pages, 11 figures, 1 table)
  08/2010;
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  Available from: Giacomo Beccari

  Article: Fast Rotating Blue Stragglers in the Globular Cluster M4

  L. Lovisi, A. Mucciarelli, F. R. Ferraro, S. Lucatello, B. Lanzoni, E. Dalessandro, G. Beccari, R. T. Rood, A. Sills, F. Fusi Pecci, R. Gratton, and G. Piotto
  [show abstract] [hide abstract]
  ABSTRACT: We have used high-resolution spectra obtained with the spectrograph FLAMES at the European Southern Observatory Very Large Telescope to determine the kinematical properties and the abundance patterns of 20 blue straggler stars (BSSs) in the globular cluster (GC) M4. We found that ~40% of the measured BSSs are fast rotators (with rotational velocities >50 km s–1). This is the largest frequency of rapidly rotating BSSs ever detected in a GC. In addition, at odds with what has been found in 47 Tucanae, no evidence of carbon and/or oxygen depletion has been revealed in the sample of 11 BSSs for which we were able to measure the abundances. This could be due to either low statistics, or a different BSS formation process acting in M4.
  The Astrophysical Journal Letters 07/2010; 719(2):L121. · 5.53 Impact Factor