arXiv subjects

M. Bellazzini

Publications and source records attributed to M. Bellazzini.

At least 199 records · Page 11Linked to original sources

arXiv2003

The multiple stellar population in omega Centauri: spatial distribution and structural properties

We present a detailed analysis of the spatial distribution of the stellar populations in the Galactic globular cluster omega Centauri. Taking advantage of the large photometric catalog published by Pancino et al. (2000), we confirm that metal-rich populations have a spatial distribution which is significantly different from the metal-poor dominant population. In particular: (i) the different sub-populations have different centroids and (ii) the metal-poor population is elongated along the E-W direction, while the metal-rich populations are oriented along the orthogonal direction, i.e., N-S. The evidence presented here can partially explain the weird spatial metallicity segregation found by Jurcsik (1998), and further supports the hyphothesis that different sub-populations in omega Centauri might have had different origins.

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arXiv2003

Building up the Galactic Halo: The Sagittarius dSph and the globular cluster M22

The Sagittarius dwarf spheroidal (Sgr) and a few paculiar Galactic Globular Clusters (GGC) can be considered as building blocks of theGalactic Halo. We present a series of results based on a wide field photometry of the Sgr system and the GGC M22. In particular for Sgr we present: (1) the detection of the RGB-bump, a feature that together with the shape of the RGB can be used to constrain the allowed range of age and metallicity and (2) the detection of a clear blue horizontal branch which probes the existance of a metal poor population. In the case of M22 we present clear evidence that differential reddening is the primary cause for the observed spread of the evolutionary sequences.

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arXiv2003

Tracing the Sgr Stream with 2MASS. Detection of Stream stars around Outer Halo Globular Clusters

We use infrared Color Magnitude Diagrams from the 2-Micron All-Sky Survey (2MASS) to search for stars belonging to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) around selected Galactic globular clusters. Statistically significant detections are presented for the cases of Pal 12 and NGC 4147, strongly supporting the idea that these clusters are associated with the Sgr Stream and that they were previous members of the Sgr dSph galaxy.

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arXiv2003

A near-infrared spectroscopic screening of the red giant populations in omega Centauri

Near-infrared spectra of 21 giants in omega Centauri, spanning the whole range of metallicities observed in this cluster, are presented. This work is part of a coordinated photometric and spectroscopic campaign in the optical and in the infrared, aimed at studying the complex stellar population of omega Centauri and understanding its formation and chemical evolution. By analyzing the several CO and OH molecular bands and atomic lines in the spectra of the selected giants, metal abundances and abundance ratios have been obtained. The existence of three major metallicity regimes at [Fe/H]=-1.6, -1.2 and [Fe/H]<-0.5 has been confirmed. The most metal-rich stars in our sample show a lower (if any) alpha-enhancement when compared to the more metal-poor components, suggesting that they should have formed in a medium significantly polluted by type Ia supernova ejecta. Isotopic carbon abundances have been also inferred, providing an average 13C/12C=4, which clearly indicates that extra-mixing processes occurred in the stellar interiors during the ascent on the Red Giant Branch.

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arXiv2002

The Andromeda Project. I. Deep HST-WFPC2 V,I photometry of 16 fields toward the disk and the halo of the M31 galaxy. Probing the stellar content and metallicity distribution

HST-WFPC2 F555W and F814W photometry were obtained for 16 fields of the luminous nearby spiral galaxy M31, sampling the stellar content of the disk and the halo at different distances from the center, from ~ 20 to ~ 150 arcmin (i.e. ~ 4.5 to 35 kpc), down to limiting V and I magnitudes of ~ 27. The Color-Magnitude diagrams (CMD) show the presence of complex stellar populations, including an intermediate age/young population and older populations with a wide range of metallicity. Those fields superposed on the disk of M31 generally show a blue plume of stars which we identify with main sequence members. Accordingly, the star formation rate over the last 0.5 Gyr appears to have varied dramatically with location in the disk. All the CMDs show a prominent Red Giant Branch (RGB) with a descending tip in the V band, characteristic of metallicity higher than 1/10 Solar. A red clump is detected in all of the fields, and a weak blue horizontal branch is frequently present. The metallicity distributions (MDs), obtained by comparison of the RGB stars with globular cluster templates, are basically similar in all the sampled fields: they all show a long, albeit scantly populated metal-poor tail and a main component at [Fe/H] ~ -0.6. However, some differences also exist, e.g. in some fields a very metal-rich ([Fe/H] >= -0.2) component is present. Whereas the fraction of metal-poor stars seems to be approximately constant in all fields, the fraction of very-metal-rich stars varies with position and seems to be more prominent in those fields superposed on the disk and/or with the presence of streams or substructures. This might indicate and possibly trace interaction effects with some companion, e.g. M32.

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arXiv2002

The Draco and Ursa Minor dwarf spheroidals. A comparative study

We present (V,I) photometry of two wide (~25 X 25 arcmin^2) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided ((m-M)_0(UMi)=19.41 \pm 0.12 and (m-M)_0(Dra)=19.84 \pm 0.14) and a comparative study of their evolved stellar population is presented. We detect for the first time the RGB-bump in the Luminosity Function of UMi (V_{RGB}^{Bump}=19.40\pm 0.06) while the feature is not detected in Draco. Photometric metallicity distributions are obtained for the two galaxies and an accurate analysis to determine the intrinsic metallicity spread is performed by means of artificial stars experiments. (Abridged) We demonstrate that the inner region of UMi is significantly structured, at odds with what expected for a system in dynamical equilibrium. In particular we show that the main density peak of UMi is off-centered with respect to the center of symmetry of the whole galaxy and it shows a much lower ellipticity with respect to the rest of the galaxy. Moreover, UMi stars are shown to be clustered according to two different characteristic clustering scales, as opposite to Draco, which instead has a very symmetric and smooth density profile. The possible consequences of this striking structural difference on our ideas about galaxy formation are briefly discussed. Combining our distance modulus with the more recent estimates of the total luminosity of UMi, we find that the mass to light (M/L) ratio of this galaxy may be as low as M/L ~ 7, a factor 5-10 lower than current estimates.

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arXiv2002

First detection of the RGB-bump in the Sagittarius dSph

We present V, I photometry of the Sagittarius Dwarf Spheroidal galaxy (Sgr) for a region of ~ 1^{circ} times 1^{circ}, centered on the globular cluster M 54. This catalog is the largest database of stars (~500,000) ever obtained for this galaxy. The wide area covered allows us to measure for the first time the position of the RGB-bump, a feature that has been identified in most Galactic globular clusters and only recently in a few galaxies of the Local Group. The presence of a single-peaked bump in the RGB differential Luminosity Function confirms that there is a dominant population in Sgr (Pop A). The photometric properties of the Pop A RGB and the position of the RGB bump have been used to constrain the range of possible ages and metallicities of this population. The most likely solution lies in the range -0.6 < [M/H] <= -0.4 and 4 Gyr <= age <= 8 Gyr.

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arXiv2002

Discovery of an accreted stellar system within the globular cluster omega Centauri

Recent wide-field photometric surveys (Lee et al.; Pancino et al.) have shown the existence of a previously unknown metal-rich ([Fe/H] -0.6) stellar population in the galactic globular cluster omega Centauri. The discovery of this new component, which comprises only a small percentage (~5%) of the entire cluster population, has added a new piece to the already puzzling picture of the star formation and chemical evolution of this stellar system. In this Letter we show that stars belonging to the newly discovered metal-rich population have a coherent bulk motion with respect to the other cluster stars, thus demonstrating that they formed in an independent self-gravitating stellar system. This is the first clear-cut evidence that extreme metal-rich stars were part of a small stellar system (a satellite of omega Centauri?) that has been accreted by the main body of the cluster. In this case, we are witnessing an in vivo example of hierarchical merging on the sub-galactic scale.

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arXiv2002

High resolution spectroscopy of metal rich giants in omega Cen: first indication of SNe Ia enrichment

We have obtained high-resolution, high S/N spectra for six red giants in omega Cen: three belonging to the recently discovered, metal-rich Red Giant Branch (RGB-a, Pancino et al. 2000) and three to the metal intermediate population (RGB-MInt). Accurate Iron, Copper and alpha-elements (Ca and Si) abundances have been derived and discussed. In particular, we have obtained the first direct abundance determination based on high-resolution spectroscopy for the RGB-a population, <[Fe/H]>=-0.60+/-0.15. Although this value is lower than previous estimates based on Calcium triplet measurements, we confirm that this population is the most metal rich in omega Cen. In addition, we have found a significant difference in the alpha-elements enhancement of the two populations. The three RGB-MInt stars have the expected overabundance, typical of halo and globular clusters stars: <[alpha/Fe]>=+0.29+/-0.01. The three RGB-a stars show, instead, a significantly lower alpha-enhancement: <[alpha/Fe]>=+0.10+/-0.04. We have also detected an increasing trend of [Cu/Fe] with metallicity, similar to the one observed for field stars by Sneden et al. (1991). The observational facts presented in this letter, if confirmed by larger samples of giants, are the first indication that supernovae type Ia ejecta have contaminated the medium from which the metal rich RGB-a stars have formed. The implications for current scenarios on the formation and evolution of omega Cen are briefly discussed.

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arXiv2002

Deep HST-WFPC2 photometry of NGC 288. II. The Main Sequence Luminosity Function

The Main Sequence Luminosity Function (LF) of the Galactic globular cluster NGC 288 has been obtained using deep WFPC2 photometry. We have employed a new method to correct for completeness and fully account for bin-to-bin migration due to blending and/or observational scatter. The effect of the presence of binary systems in the final LF is quantified and is found to be negligible. There is a strong indication of the mass segregation of unevolved single stars and clear signs of a depletion of low mass stars in NGC 288 with respect to other clusters. The results are in good agreement with the prediction of theoretical models of the dynamical evolution of NGC 288 that take into account the extreme orbital properties of this cluster.

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arXiv2001

Deep HST-WFPC2 photometry of NGC 288. I. Binary Systems and Blue Stragglers

We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the Main Sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn(1997). We have unequivocally detected a significant population of binary systems which has a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (r<1 r_h = 1.6 r_c) the binary fraction (f_b) lies in the range 0.08--0.38 regardless of the assumed distribution of mass ratios, F(q). The most probable f_b lies between 0.10 and 0.20 depending on the adopted F(q). On the other hand, in the outer region (r> 1 r_h), f_b must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of Blue Straggler Stars (BSS) is exceptionally high, suggesting that the BSS production mechanism via binary evolution can be very efficient. A large population of BSS is possible even in low density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSS in the Color Magnitude Diagram is not compatible with a rate of BSS production which has been constant in time, if it is assumed that all the BSS are formed by the merging of two stars.

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arXiv2001

The Bologna Key Project on Omega Cen

In this contribution we schematically summarize a number of ongoing, coordinated spectro-photometric projects devoted to the study of the stellar population properties in Omega Cen. All of these investigations are part of a global comprehensive project led by our team to address the complex formation history of stars in this cluster.

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arXiv2001

Wide Field Imaging of the Sextans Dwarf Spheroidal Galaxy

We present wide field multiband photometry of the very low surface brightness dwarf Spheroidal galaxy (dSph) Sextans. We have found convincing evidence for the presence of multiple stellar populations in this galaxy. In particular we discovered: (a) a Blue Horizontal Branch tail kinked over the instability strip; (b) a clearly bimodal distribution in color of the Red Giant Branch (RGB) stars and (c) a double RGB-bump. All these features suggest that (at least) two components are present in the old stellar population that dominates the galaxy: a main one with [Fe/H]~-1.8 and a minor component around [Fe/H]~-2.5. Similar evidence has been recently found by Majewski et al. (1999) for the Sculptor dwarf Spheroidal, suggesting that multiple star formation episodes are common also in the most nearby dSphs that ceased their star formation activity at very early epochs.

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arXiv2001

Age as the Second Parameter in NGC 288 / NGC 362? I. Turnoff Ages: a Purely Differential Comparison

We present deep V,I photometry of the globular clusters NGC 288, NGC 362 and NGC 1851 obtained during a single observational run under strictly homogeneous conditions. We use the bimodal horizontal branch (HB) of NGC 1851 as a ``bridge'' to obtain the optimum relative match between the HBs of NGC 288 and NGC 362. In this way we can effectively remove the uncertainties associated with distance, reddening and inhomogeneities in the absolute calibration, thus obtaining a very robust, purely differential estimate of the age difference between these two clusters. According to the bridge test, NGC 288 is found to be older than NGC 362 by 2 +- 1 Gyr. This result is fully confirmed also by all classical differential age diagnostics, either based on the luminosity or color of the main sequence turnoff point.

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arXiv2001

Age as the Second Parameter in NGC 288/NGC 362? II. The Horizontal Branch Revisited

We revisit the "second parameter" pair of globular clusters NGC 288/NGC 362 on the basis of theoretical models for red giant branch (RGB) and horizontal branch (HB) stars. The results of the most extensive set of RGB/HB simulations computed so far for these clusters are presented for two different metallicities. Using several different analytical mass loss formulae for RGB stars, we derive relative "HB morphology ages." We compare them with the relative main-sequence turnoff ages derived by application of the "bridge test" by Bellazzini et al. (2001), who found that NGC 288 is 2+/-1 Gyr older than NGC 362. We find that adoption of a higher metallicity ([Fe/H] ~ -1.2), as favored by the Carretta & Gratton metallicity scale, makes age a much more plausible second parameter candidate for this pair than is the case when a lower metallicity ([Fe/H] ~ -1.5), closer to the Zinn & West scale, is adopted. However, while the different HB morphology of these two clusters can be reproduced by canonical HB models with [Fe/H] ~ -1.2 and an age difference of 2 Gyr, this explanation is not without difficulty. In particular, we confirm previous suggestions that canonical models are unable to reproduce the detailed HB morphology of NGC 288 at its red end, for as yet unknown reasons. Finally, we show that the mass dispersion on the HB of NGC 362 is substantially larger than for NGC 288, and suggest that there is a correlation between the mass dispersion on the HB phase and the central density of globular clusters. This is presumably related to the way environmental effects affect RGB mass loss. [Abridged]

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arXiv2001

RR Lyrae Variables in the Globular Clusters of M31: a First Detection of Likely Candidates

The purpose of this paper is to show that RR Lyrae variables exist and can be detected in M31 globular clusters. We report on the first tentative identification of RR Lyrae candidates in four globular clusters of the Andromeda galaxy, i.e. G11, G33, G64 and G322. Based on HST-WFPC2 archive observations in the F555W and F814W filters spanning a total interval of about 5 consecutive hours we find evidence for 2, 4, 11 and 8 RR Lyrae variables of both ab and c Bailey types in G11, G33, G64 and G322, respectively. Several more candidates can be found by relaxing slightly the selection criteria. These numbers are quite consistent with the horizontal branch morphology exhibited by the four clusters, starting from the very blue HB in G11, and progressively moving to redder HBs in G64, G33 and G322.

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arXiv2001

Multiple stellar population in the Sextans dwarf spheroidal galaxy?

We present wide field (33 X 34 arcmin^2) multiband (BVI) CCD photometry (down to I <= 20.5) of the very low surface brightness dwarf Spheroidal (dSph) galaxy Sextans. In the derived Color Magnitude Diagrams we have found evidences suggesting the presence of multiple stellar populations in this dSph. In particular we discovered: {\it (i)} a Blue Horizontal Branch (HB) tail that appears to lie on a brighter sequence with respect to the prominent Red HB and the RR Lyrae stars, very similar to what found by Majewski et al. (1999) for the Sculptor dSph; {\it (ii)} hints of a bimodal distribution in color of the RGB stars; {\it (iii)} a double RGB-bump. All these features suggest that (at least) two components are present in the old stellar population of this galaxy: a main one with [Fe/H]~ -1.8 and a minor component around [Fe/H]<~ -2.3. The similarity with the Sculptor case may indicate that multiple star formation episodes are common also in the most nearby dSphs that ceased their star formation activity at very early epochs.

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arXiv2001

A Survey of M31 Globular Clusters using WFPC2 on board HST

We report new, as yet unpublished HST/WFPC2 V, I photometry of 9 globular clusters in M31. These are part of a total sample of 19 clusters on M31 with well observed CMDs from HST. The clusters have the full range of horizontal branch morphologies from blue HB to red clump, but none are found with an extreme (blue only) horizontal branch. Plots of HB type vs [Fe/H] are similar to those of Galactic clusters, including a hint of second parameter clusters. Sixteen halo fields adjacent to the observed clusters included in our images have been analysed. The M31 halo metallicity distribution peaks at [Fe/H]=--0.7 with a tail toward low metallicity, resembling the halo of NGC 5128; metal rich giants appear to be centrally concentrated.

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