arXiv subjects

B. Lanzoni

Publications and source records attributed to B. Lanzoni.

At least 91 records · Page 5Linked to original sources

arXiv2014

Probing the role of dynamical friction in shaping the BSS radial distribution. I - Semi-analytical models and preliminary N-body simulations

We present semi-analytical models and simplified $N$-body simulations with $10^4$ and $10^5$ particles aimed at probing the role of dynamical friction (DF) in determining the radial distribution of Blue Straggler Stars (BSSs) in globular clusters. The semi-analytical models show that DF (which is the only evolutionary mechanism at work) is responsible for the formation of a bimodal distribution with a dip progressively moving toward the external regions of the cluster. However, these models fail to reproduce the formation of the long-lived central peak observed in all dynamically evolved clusters. The results of $N$-body simulations confirm the formation of a sharp central peak, which remains as a stable feature over the time regardless of the initial concentration of the system. In spite of a noisy behavior, a bimodal distribution forms in many cases, with the size of the dip increasing as a function of time. In the most advanced stages the distribution becomes monotonic. These results are in agreement with the observations. Also the shape of the peak and the location of the minimum (which in most of the cases is within 10 core radii) turn out to be consistent with observational results. For a more detailed and close comparison with observations, including a proper calibration of the timescales of the dynamical processes driving the evolution of the BSS spatial distribution, more realistic simulations will be necessary.

astro-ph.GA↗

arXiv2014

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations

In this paper we describe a new UV-initiative HST project (GO-13297) that will complement the existing F606W and F814W database of the ACS Globular Cluster (GC) Treasury by imaging most of its clusters through UV/blue WFC3/UVIS filters F275W, F336W and F438W. This "magic trio" of filters has shown an uncanny ability to disentangle and characterize multiple-population (MP) patterns in GCs in a way that is exquisitely sensitive to C, N, and O abundance variations. Combination of these passbands with those in the optical also gives the best leverage for measuring helium enrichment. The dozen clusters that had previously been observed in these bands exhibit a bewildering variety of MP patterns, and the new survey will map the full variance of the phenomenon. The ubiquity of multiple stellar generations in GCs has made the formation of these cornerstone objects more intriguing than ever; GC formation and the origin of their MPs have now become one and the same problem. In the present paper we will describe the data base and our data reduction strategy, as well as the uses we intend to make of the final photometry, astrometry, and proper motions. We will also present preliminary color-magnitude diagrams from the data so far collected. These diagrams also draw on data from GO-12605 and GO-12311, which served as a pilot project for the present GO-13297.

astro-ph.SR↗

arXiv2014

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

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 vsini >40 km/s and about 20% has vsini >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.

astro-ph.SR↗

arXiv2014

WFPC2 UV survey of Galactic Globular Clusters. The Horizontal Branch temperature distribution

Ultraviolet observations are the best tool to study hot stellar populations which emit most of their light at short wavelengths. As part of a large project devoted to the characterization of the UV properties of Galactic globular clusters (GGCs), we collected mid/far UV and optical images with the WFPC2@HST for 31 GGCs. These clusters cover a wide range in metallicity and structural parameters, thus representing an ideal sample for comparison with theoretical models. Here we present the first results from an ongoing analysis aimed at deriving the temperature distribution of Horizontal Branch stars in GGCs.

astro-ph.SR↗

arXiv2014

Ceci n'est pas a globular cluster: the metallicity distribution of the stellar system Terzan 5

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]$\simeq0.25$ dex, accounting for 29% of the sample, a dominant sub-solar population at [Fe/H]$\simeq-0.30$ dex, corresponding to 62% of the total, and a minor (6%) metal-poor component at [Fe/H]$\simeq-0.8$ dex. Such a broad, multi-modal metallicity distribution demonstrates that Terzan 5 is not a genuine globular cluster but the remnant of a much more complex stellar system.

astro-ph.SR↗

arXiv2014

Radio timing and optical photometry of the black widow system PSR J1518+0204C in the globular cluster M5

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.

astro-ph.SR↗

arXiv2014

No evidence of chemical anomalies in the bimodal turnoff cluster NGC 1806 in the LMC

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 8 giant stars, members of the cluster, finding an average iron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic star-to-star variations in the abundances of light elements (Na, O, Mg, Al). Also, the (m_(F814W); m_(F336W)-m_(F814W)) 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.

astro-ph.SR↗

arXiv2014

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

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

astro-ph.SR↗

arXiv2014

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

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 =+21.0\pm4.6 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°). We found no evidence for the peak at \sim+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]\simeq+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° 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.

astro-ph.GA↗

arXiv2014

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

We present the helium abundance of the two metal-poor clusters M30 and NGC6397. Helium estimates have been obtained by using the high-resolution spectrograph FLAMES at the ESO Very Large Telescope and by measuring the HeI line at 4471 A in 24 and 35 horizontal branch stars in M30 and NGC6397, 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+-0.003 (sigma=0.021) for M30 and Y=0.241+-0.004 (sigma=0.023) NGC6397. 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 NGC6397 in agreement with the prediction that O-poor stars are formed by (He-enriched) gas polluted by the products of hot proton-capture reactions.

astro-ph.SR↗

arXiv2014

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

We report on the identification of the optical counterpart to Star1, the exotic object serendipitously discovered by Deutsch et al. in the core of the Galactic globular cluster NGC 6624. Star1 has been classified by Deutsch et al. as either a quiescent Cataclysmic Variable or a low-mass X-ray binary. Deutsch et al. proposed StarA as 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 Δm_F435W~0.7 mag and orbital period of 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.

astro-ph.SR↗

arXiv2014

Terzan 5: a pristine fragment of the Galactic Bulge?

Terzan 5 is a stellar system located in the inner Bulge of the Galaxy and has been historically catalogued as a globular cluster. However, recent photometric (Ferraro et al. 2009) and spectroscopic (Origlia et al. 2011; Origlia et al. 2013) investigations have shown that it hosts at least three stellar populations with different iron abundances (with a total spread of Delta[Fe/H]>1 dex) thus demonstrating that Terzan 5 is not a genuine globular cluster. In addition, the striking similarity between the chemical patterns of this system and those of its surrounding environment, the Galactic Bulge, from the point of view of both the metallicity distribution and the alpha-element enrichment, suggests that Terzan 5 could be a pristine fragment of the Bulge itself.

astro-ph.GA↗

arXiv2013

Blue Straggler masses from pulsation properties. I. The case of NGC6541

We used high-spatial resolution images acquired with the WFC3 on board HST to probe the population of variable blue straggler stars in the central region of the poorly studied Galactic globular cluster NGC 6541. The time sampling of the acquired multi wavelength (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 non adiabatic models. We found masses in the range 1.0-1.1Mo, with an average value of 1.06+-0.09 Mo (sigma = 0.04), significantly in excess of the cluster Main Sequence Turn Off mass (~ 0.75Mo). 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 Mo for most of the BSS population, in agreement with what discussed in a number of previous studies.

astro-ph.SR↗

arXiv2013

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

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.

astro-ph.GA↗

arXiv2013

The WFPC2 UV Survey: the BSS population in NGC 5824

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 (r_c \simeq 4.4 arcsec) and a concentration parameter c \simeq 2.74. We also present the first age determination for this cluster. From the comparison with isochrones, we have found t=13\pm0.5 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 \sim 20 arcsec, 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.

astro-ph.GA↗

arXiv2013

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

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 (α-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.

astro-ph.SR↗

arXiv2013

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

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.

astro-ph.GA↗

arXiv2013

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

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.

astro-ph.SR↗