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M. Bellazzini

Publications and source records attributed to M. Bellazzini.

At least 109 records · Page 6Linked to original sources
arXiv2014

The extreme chemistry of multiple stellar populations in the metal-poor globular cluster NGC 4833

Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal branch (HB). We present the abundance analysis for a large sample of 78 red giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, La, Nd. This is the first extensive study of this cluster from high resolution spectroscopy. On the scale of our survey, the metallicity of NGC 4833 is [Fe/H]=-2.015+/-0.004+/-0.084 dex (rms=0.014 dex) from 12 stars observed with UVES, where the first error is from statistics and the second one refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous at better than 6%. On the other hand, the light elements involved in proton-capture reactions at high temperature show the large star-to-star variations observed in almost all GCs studied so far. The Na-O anticorrelation in NGC 4833 is quite extended, as expected from the high temperatures reached by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with extreme [O/Na] ratios. More striking is the finding that large star-to-star variations are seen also for Mg, which spans a range of more than 0.5 dex in this GC. Depletions in Mg are correlated to the abundances of O and anti-correlated with Na, Al, and Si abundances. This pattern suggests the action of nuclear processing at unusually high temperatures, producing the extreme chemistry observed in the stellar generations of NGC 4833. This extreme changes are also seen in giants of the much more massive GCs M 54 and omega Cen, and our conclusion is that NGC 4833 has probably lost a conpicuous fraction of its original mass due to bulge shocking, as also indicated by its orbit.

astro-ph.GA↗
arXiv2013

Dwarfs walking in a row. The filamentary nature of the NGC3109 association

We re-consider the association of dwarf galaxies around NGC3109, whose known members were NGC3109, Antlia, Sextans A and Sextans B, based on a new updated list of nearby galaxies and the most recent data. We find that the original members of the NGC3109 association, together with the recently discovered and adjacent dwarf irregular Leo P, form a very tight and elongated configuration in space. All these galaxies lie within ~100 kpc of a line that is ~1070 kpc long, from one extreme (NGC3109) to the other (Leo P), and they show a gradient in the Local Group standard of rest velocity with a total amplitude of 43 km/s Mpc and a r.m.s. scatter of just 16.8 km/s. It is shown that the reported configuration is exceptional given the known dwarf galaxies in the Local Group and its surroundings. We conclude that (a) Leo P is likely an additional member of the NGC3109 association, and (b) the association is highly ordered in space and velocity, and it is very elongated, suggesting that it has been originated by a tidal interaction or it was accreted as a filamentary sub-structure.

astro-ph.CO↗
arXiv2013

NGC 5694: another foster son of the Galactic Halo

We present the results of the analysis of high-resolution spectra obtained with UVES-FLAMES@VLT for six red giant branch stars in the outer-halo metal-poor ([Fe/H]I=-1.98 and [Fe/H]II=-1.83) Galactic globular cluster NGC 5694, which has been suggested as a possible incomer by Lee et al. (2006) based on the anomalous chemical composition of a single cluster giant. We obtain accurate abundances for a large number of elements and we find that: (a) the six target stars have the same chemical composition within the uncertainties, except for Na and Al; (b) the average cluster abundance of α elements (with the only exception of Si) is nearly solar, at odds with typical halo stars and globular clusters of similar metallicity; (c) Y, Ba, La and Eu abundances are also significantly lower than in Galactic field stars and star clusters of similar metallicity. Hence we confirm the Lee et al. classification of NGC 5694 as a cluster of extra-galactic origin. We provide the first insight on the Na-O and Mg-Al anti-correlations in this cluster: all the considered stars have very similar abundance ratios for these elements, except one that has significantly lower [Na/Fe] and [Al/Fe] ratios, suggesting that some degree of early self-enrichment has occurred also in this cluster.

astro-ph.SR↗
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↗
arXiv2013

NGC 362: another globular cluster with a split red giant branch

We obtained FLAMES GIRAFFE+UVES spectra for both first and second-generation red giant branch (RGB) stars in the globular cluster (GC) NGC 362 and used them to derive abundances of 21 atomic species for a sample of 92 stars. The surveyed elements include proton-capture (O, Na, Mg, Al, Si), alpha-capture (Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr, Ba, La, Ce, Nd, Eu, Dy). The analysis is fully consistent with that presented for twenty GCs in previous papers of this series. Stars in NGC 362 seem to be clustered into two discrete groups along the Na-O anti-correlation, with a gap at [O/Na] 0 dex. Na-rich, second generation stars show a trend to be more centrally concentrated, although the level of confidence is not very high. When compared to the classical second-parameter twin NGC 288, with similar metallicity, but different horizontal branch type and much lower total mass, the proton-capture processing in stars of NGC 362 seems to be more extreme, confirming previous analysis. We discovered the presence of a secondary RGB sequence, redder than the bulk of the RGB: a preliminary estimate shows that this sequence comprises about 6% of RGB stars. Our spectroscopic data and literature photometry indicate that this sequence is populated almost exclusively by giants rich in Ba, and probably rich in all s-process elements, as found in other clusters. In this regards, NGC 362 joins previously studied GCs like NGC 1851, NGC 6656 (M 22), and NGC 7089 (M 2).

astro-ph.GA↗
arXiv2013

The double RGB in M 2: C, N, Sr and Ba abundances

The globular cluster M 2 has a photometrically detected double red giant branch (RGB) sequence. We investigate here the chemical differences between the two RGBs in order to gain insight in the star formation history of this cluster. The low-resolution spectra, covering the blue spectral range, were collected with the MODS spectrograph on the LBT, and analyzed via spectrum synthesis technique. The high quality of the spectra allows us to measure C, N, Ba, and Sr abundances relative to iron for 15 RGB stars distributed along the two sequences. We add to the MODS sample C and N measurements for 35 additional stars belonging to the blue RGB sequence, presented in Lardo et al. (2012). We find a clear separation between the two groups of stars in s-process elements as well as C and N content. Both groups display a C-N anti-correlation and the red RGB stars are on average richer in C and N with respect to the blue RGB. Our results reinforce the suggestion that M2 belongs to the family of globular clusters with complex star formation history, together with Omega Cen, NGC 1851, and M 22.

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

Evidence for multiple populations in the massive globular cluster NGC 2419 from deep uVI LBT photometry

We present accurate wide-field uVI photometry of the remote and massive Galactic globular cluster NGC 2419, aimed at searching for the u-V color spread along the Red Giant Branch (RGB) that is generally interpreted as the photometric signature of the presence of multiple populations in globular clusters. Focusing on the RGB stars in the magnitude range 19.8 0.0 lie preferentially on the blue side.

astro-ph.SR↗
arXiv2013

Globular clusters seen by Gaia

We present a simulation of twelve globular clusters with different concentration, distance, and background population, whose properties are transformed into Gaia observables with the help of the lates Gaia science performances prescriptions. We adopt simplified crowding receipts, based on five years of simulations performed by DPAC (Data Processing and Analysis Consortium) scientists, to explore the effect of crowding and to give a basic idea of what will be made possible by Gaia in the field of Galactic globular clusters observations.

astro-ph.GA↗
arXiv2013

NGC5694: another extra-galactic globular cluster

We discuss the chemical composition of six giant stars of the outer Halo globular cluster NGC5694, through the analysis of UVES@FLAMES high-resolution spectra. The cluster has an average iron content [Fe/H]=--1.83+-0.01, solar-scaled [alpha/Fe] ratios and a very low Ba abundance ([Ba/Fe]=--0.71+-0.06). These anomalous abundance patterns are different from those observed in other Halo globular clusters but similar to those of the metal-poor stars in typical dwarf spheroidal galaxies. These findings suggest an extra-galactic origin for NGC5694, likely from a dwarf spheroidal galaxy.

astro-ph.GA↗
arXiv2013

The strange case of NGC2419: stellar populations, chemical composition, dynamics

A few years ago we started an observational campaign aimed at the thorough study of the massive and remote globular cluster NGC2419. We have used the collected data, e.g., to test alternative theories of gravitation, to constrain the stellar M/L ratio by direct analysis of the observed luminosity function, and to search for Dark Matter within the cluster. Here we present some recent results about (a) the peculiar abundance pattern that we observed in a sample of cluster giants, and (b) newly found photometric evidence for the presence of multiple populations in the cluster. In particular, from new deep and accurate uVI LBT photometry, we find that the color spread on the Red Giant Branch is significantly larger than the observational errors both in V-I and u-V, and that the stars lying to the blue of the RGB ridge line are more concentrated toward the center of the cluster than those lying to the red of the ridge line.

astro-ph.GA↗
arXiv2012

The horizontal branch morphology of M31 globular clusters. Extreme second parameter effect in outer halo clusters

We use deep, high quality colour magnitude diagrams obtained with the Hubble Space Telescope to compute a simplified version of the Mironov index [SMI; B/(B+R)] to parametrize the horizontal branch (HB) morphology for 23 globular clusters in the M31 galaxy (Sample-A), all located in the outer halo at projected distances between 10 kpc and 100 kpc. This allows us to compare them with their Galactic counterparts, for which we estimated the SMI exactly in the same way, in the SMI vs. [Fe/H] plane. We find that the majority of the considered M31 clusters lie in a significantly different locus, in this plane, with respect to Galactic clusters lying at any distance from the center of the Milky Way. In particular they have redder HB morphologies at a given metallicity, or, in other words, clusters with the same SMI value are ~0.4 dex more metal rich in the Milky Way than in M31. We discuss the possible origin of this difference and we conclude that the most likely explanation is that many globular clusters in the outer halo of M31 formed ~1-2 Gyr later than their counterparts in the outer halo of the Milky Way, while differences in the cluster-to-cluster distribution of He abundance of individual stars may also play a role. The analysis of another sample of 25 bright M31 clusters (eighteen of them with M_V<= -9.0, Sample-B), whose SMI estimates are much more uncertain as they are computed on shallow colour magnitude diagrams, suggests that extended blue HB tails can be relatively frequent among the most massive M31 globular clusters, possibly hinting at the presence of multiple populations.

astro-ph.GA↗
arXiv2012

News from the Galactic suburbia: the chemical composition of the remote globular cluster NGC 2419

We present the chemical analysis of 49 giant stars of the globular cluster NGC 2419, using medium resolution spectra collected with the multi-object spectrograph DEIMOS@Keck. Previous analysis of this cluster revealed a large dispersion in the line strength of the infrared Ca II triplet, suggesting an intrinsic star-to-star scatter in its Fe or Ca content. From our analysis, we assess that all the investigated stars share the same [Fe/H], [Ca/Fe] and [Ti/Fe] abundance ratios, while a large spread in Mg and K abundances is detected. The distribution of [Mg/Fe] is bimodal, with ~40% of the observed targets having subsolar [Mg/Fe], down to [Mg/Fe] ~ -1 dex, a level of Mg-deficiency never observed before in globular clusters. It is found that the large dispersion in Mg abundances is likely the main origin of the observed dispersion of the Ca II triplet lines strengths (that can be erroneously interpreted in terms of Fe or Ca abundance scatter) because Mg plays a relevant role in the atmosphere of giant stars as an electron donor. A strong depletion in the Mg abundance leads to an increase of the line strength of the Ca II triplet, due to the variation in the electronic pressure, at a constant Fe and Ca abundance. Finally, we detect an anti-correlation between Mg and K abundances, not easily explainable within the framework of the current nucleosynthesis models.

astro-ph.GA↗
arXiv2012

The Gaia spectrophotometric standard stars survey. I. Preliminary results

We describe two ground based observing campaigns aimed at building a grid of approximately 200 spectrophotometric standard stars (SPSS), with an internal ~1% precision and tied to Vega within ~3%, for the absolute flux calibration of data gathered by Gaia, the ESA astrometric mission. The criteria for the selection and a list of candidates are presented, together with a description of the survey strategy and the adopted data analysis methods. We also discuss a short list of notable rejected SPSS candidates and difficult cases, based on identification problems, literature discordant data, visual companions, and variability. In fact, all candidates are also monitored for constancy (within \pm5 mmag, approximately). In particular, we report on a CALSPEC standard, 1740346, that we found to be a delta Scuti variable during our short-term monitoring (1-2 h) campaign.

astro-ph.IM↗
arXiv2012

A comparison between the stellar and dynamical masses of six globular clusters

We present the results of a comprehensive analysis of the structure and kinematics of six Galactic globular clusters. By comparing the results of the most extensive photometric and kinematical surveys available to date with suited dynamical models, we determine the stellar and dynamical masses of these stellar systems taking into account for the effect of mass segregation, anisotropy and unresolved binaries. We show that the stellar masses of these clusters are on average smaller than those predicted by canonical integrated stellar evolution models because of the shallower slope of their mass functions. The derived stellar masses are found to be also systematically smaller than the dynamical masses by ~40%, although the presence of systematics affecting our estimates cannot be excluded. If confirmed, this evidence can be linked to an increased fraction of retained dark remnants or to the presence of a modest amount of dark matter.

astro-ph.GA↗
arXiv2012

The Luminosity Function and stellar Mass to Light ratio of the massive globular cluster NGC2419

We used archival Hubble Space Telescope WFC3 images to obtain the Luminosity Function of the remote globular cluster NGC2419 from two magnitudes above the Horizontal Branch level down to \sim3.0 magnitudes below the Turn Off point (to M_I\sim6.4), approximately covering the range of initial stellar masses 0.5 M_sun<= m <= 0.9 M_sun. The completeness-corrected Luminosity Function does not display any change of shape over the radial range covered by the WFC3 data, out to ~6 core radii (r_c), or, equivalently, to ~2 half-light radii. The Luminosity Function in this radial range is also identical to that obtained from ground based data at much larger distances from the cluster centre (12r_c<= R<= 22r_c), in the magnitude range in which the two distributions overlap (M_I<= 4.0). These results support the conclusion by Dalessandro et al. that there is no significant mass segregation among cluster stars, hence the stellar mass-to-light ratio remains constant with distance from the cluster centre. We fitted the observed Luminosity Function with theoretical counterparts with the proper age and metallicity from different sets of stellar evolution models and we consistently derive a total V band mass-to-light ratio 1.2<= M/L_V<= 1.7, by extrapolating to the Hydrogen burning limit, with a best-fit value M/L_V=1.5 +/- 0.1. On the other hand, assuming that there are no cluster stars with m<= 0.3 M_sun, we establish a robust lower limit M/L_V> 0.8. These estimates provide useful constraints for dynamical models of the cluster that were forced to consider the stellar mass-to-light ratio as a (nearly) free parameter.

astro-ph.GA↗
arXiv2012

Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies

We report preliminary results concerning the detailed chemical composition of metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter UfDSphs). The abundances have been determined thanks to spectra obtained with X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios slightly lower to what is measured in field halo star of the same metallicity.We did not find extreme abundances in our Hercules stars as the one found by Koch for his 2 Hercules stars. The synthesis of the neutron capture elements Ba and Sr seems to originate from the same nucleosynthetic process in operation during the early stages of the galactic evolution.

astro-ph.GA↗