arXiv subjects

B. Lanzoni

Publications and source records attributed to B. Lanzoni.

At least 109 records · Page 6Linked to original sources

arXiv2013

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

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.

astro-ph.GA↗

arXiv2013

New clues on the nature of the companion to PSR~J1740-5340 in NGC6397 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 Msun) remnant star, descending from a ~0.8 Msun progenitor which lost ~70-80 % of its original material because of mass transfer activity onto the pulsar.

astro-ph.SR↗

arXiv2013

The optical counterpart to the X-ray transient IGR J18245-2452 in the globular cluster M28

We report on the identification of the optical counterpart to the recently detected INTEGRAL transient IGR J18245-2452 in the Galactic globular cluster M28. From the analysis of a multi epoch HST dataset we have identified a strongly variable star 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 Halpha excess during the outburst state, suggestive of active accretion processes by the neutron star.

astro-ph.SR↗

arXiv2013

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

We present the identification of the companion star to the intermediate mass binary pulsar J1439-5501 obtained by means of ground-based deep images in the B, V and I bands, acquired with FORS2 mounted at the ESO-VLT. The companion is a massive white dwarf (WD) with B=23.57+-0.02, V=23.21+-0.01 and I=22.96+-0.01, located at only ~0.05" from the pulsar radio position. Comparing the WD location in the (B, B-V) and (V, V-I) Color-Magnitude diagrams with theoretical cooling sequences we derived a range of plausible combinations of companion masses (1<~Mcom<~1.3 Msun), distances (d<~1200 pc), radii (<~7.8 10^3 Rsun) and temperatures (T=31350^{+21500}_{-7400}). From the PSR mass function and the estimated mass range we also constrained the inclination angle i >~ 55 degrees and the pulsar mass (Mpsr <~2.2 Msun). The comparison between the WD cooling age and the spin down age suggests that the latter is overestimated by a factor of about ten.

astro-ph.SR↗

arXiv2013

FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30

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.

astro-ph.GA↗

arXiv2013

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

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.

astro-ph.SR↗

arXiv2013

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

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 17,000 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".

astro-ph.SR↗

arXiv2013

Ultraviolet properties of Galactic globular clusters with GALEX

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.

astro-ph.GA↗

arXiv2013

Terzan 5: a Fossil Remnant of the Galactic Bulge

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 (Δ[Fe/H]=0.5). Such a large difference has been measured only in ω 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.

astro-ph.GA↗

arXiv2012

Dynamical age differences among coeval star clusters as revealed by blue stragglers

Globular star clusters that formed at the same cosmic time may have evolved rather differently from a 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.

astro-ph.SR↗

arXiv2012

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 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 (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 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 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 (~ 0.02Msun) that completely filled its Roche Lobe.

astro-ph.SR↗

arXiv2012

Chemical and kinematical properties of BSSs and HB stars in NGC 6397

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.

astro-ph.SR↗

arXiv2012

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

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.

astro-ph.SR↗

arXiv2012

The BSS population in NGC 6229

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.

astro-ph.GA↗

arXiv2012

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

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.

astro-ph.SR↗

arXiv2012

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

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.

astro-ph.SR↗

arXiv2011

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

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.

astro-ph.SR↗

arXiv2010

Spectroscopy Unveils the Complex Nature of Terzan 5

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.

astro-ph.GA↗