arXiv subjects

E. Pancino

Publications and source records attributed to E. Pancino.

At least 181 records · Page 10Linked to original sources

arXiv2007

Na-O Anticorrelation and Horizontal Branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra

We present an analysis of FLAMES-Giraffe spectra for several bright giants in NGC 6441, to investigate the presence and extent of the Na-O anticorrelation in this anomalous globular cluster. The field of NGC 6441 is very crowded, with severe contamination by foreground (mainly bulge) field stars. Appropriate membership criteria were devised to identify a group of 25 likely cluster members among the about 130 stars observed. Combined with the UVES data obtained with the same observations, high dispersion abundance analyses are now available for a total of 30 stars in NGC 6441, 29 of them having data for both O and Na. The spectra were analyzed by a standard line analysis procedure; care was taken to minimize the impact of the differential interstellar reddening throughout the cluster, and to extract reliable information from crowded, and moderately high S/N (30-70), moderately high resolution (R ~ 23,000) spectra. NGC 6441 is very metal-rich ([Fe/H]=$-0.34\pm 0.02\pm0.04$ dex). There is no clear sign of star-to-star scatter in the Fe-peak elements. The alpha-elements Mg, Si, Ca, and Ti are overabundant by rather large factors, suggesting that the cluster formed from material enriched by massive core collapse SNe. The O-Na anticorrelation is well defined, with about 1/4 of the stars being Na-rich and O-poor. One of the stars is a Ba-rich and moderately C-rich star. The distribution of [Na/O] ratios among RGB stars in NGC 6441 appears similar to the distribution of colors of stars along the horizontal branch. The fraction of Na-poor, O-rich stars found in NGC 6441 agrees well with that of stars on the red horizontal branch of this cluster (in both cases about 80%), with a sloping distribution toward lower values of [O/Na] (among RGB stars and bluer colors (among HB stars).

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arXiv2007

The chemical evolution of Omega Centauri's progenitor system

Chemical evolution models are presented for the anomalous globular cluster Omega Centauri. After demonstrating that the chemical features of Omega Cen can not be reproduced in the framework of the closed-box self-enrichment scenario, we discuss a model in which this cluster is the remnant of a dwarf spheroidal galaxy evolved in isolation and then swallowed by the Milky Way. Both infall of primordial matter and metal-enriched gas outflows have to be considered in order to reproduce the stellar metallicity distribution function, the age-metallicity relation and several abundance ratios. Yet, as long as an ordinary stellar mass function and standard stellar yields are assumed, we fail by far to get the enormous helium enhancement required to explain the blue main sequence (and, perhaps, the extreme horizontal branch) stellar data. Rotating models of massive stars producing stellar winds with large helium excesses at low metallicities have been put forward as promising candidates to solve the `helium enigma' of Omega Cen (Maeder & Meynet, 2006, A&A, 448, L37). However, we show that for any reasonable choice of the initial mass function the helium-to-metal enrichment of the integrated stellar population is unavoidably much lower than 70 and conclude that the issue of the helium enhancement in Omega Cen still waits for a satisfactory explanation. We briefly speculate upon possible solutions.

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arXiv2006

Deep FORS1 observations of the double Main Sequence of omega Centauri

We present the results of a deep photometric survey performed with FORS1@VLT aimed at investigating the complex Main Sequence structure of the stellar system omega Centauri. We confirm the presence of a double Main Sequence and identify its blue component (bMS) over a large field of view up to 26' from the cluster center. We found that bMS stars are significantly more concentrated toward the cluster center than the other "normal" MS stars. The bMS morphology and its position in the CMD have been used to constrain the helium overabundance required to explain the observed MS morphology.

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arXiv2006

Na-O Anticorrelation And HB. III. The abundances of NGC 6441 from FLAMES-UVES spectra

The aim of the present work is to determine accurate metallicities for a group of red giant branch stars in the field of the bulge Globular Cluster NGC 6441. This is the third paper of a series resulting from a large project aimed at determining the extent of the Na-O anticorrelation among Globular Cluster stars and exploring its relationship with HB morphology. We present an LTE abundance analysis of these objects, based on data gathered with the FLAMES fiber facility and the UVES spectrograph at VLT2. Five of the thirteen stars observed are members of the cluster. The average Fe abundance for these five stars is [Fe/H]=$-0.39\pm 0.04\pm 0.05$~dex, where the first error bar includes the uncertainties related to star-to-star random errors, and the second one the systematic effects related to the various assumptions made in the analysis.The overall abundance pattern is quite typical of Globular Clusters, with an excess of the $α-$elements and of Eu. There is evidence that the stars of NGC 6441 are enriched in Na and Al, while they have been depleted of O and Mg, due to H-burning at high temperatures, in analogy with extensive observations for other Globular Clusters: in particular, one star is clearly Na and Al-rich and O and Mg-poor. We obtained also quite high V abundances, but it is possible that this is an artifact of the analysis, since similar large V abundances are derived also for the field stars. These last are all more metal-rich than NGC 6441 and probably belong to the bulge population.

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arXiv2005

The pure non-collisional Blue Straggler population in the giant stellar system omega Centauri

We have used high spatial resolution data from the Hubble Space Telescope and wide-field ground-based observations to search for blue straggler stars (BSS) over the entire radial extent of the large stellar system omega Centauri. We have detected the largest population of BSS ever observed in any stellar system. Even though the sample is restricted to the brightest portion of the BSS sequence, more than 300 candidates have been identified. BSS are thought to be produced by the evolution of binary systems (either formed by stellar collisions or mass exchange in binary stars). Since systems like Galactic globular clusters (GGC) and omega Cen evolve dynamically on time-scales significantly shorter than their ages, binaries should have settled toward the center, showing a more concentrated radial distribution than the ordinary, less massive single stars. Indeed, in all GGCs which have been surveyed for BSS, the BSS distribution is peaked at the center. Conversely, in omega Cen we find that the BSS share the same radial distribution as the adopted reference populations. This is the cleanest evidence ever found that such a stellar system is not fully relaxed even in the central region. We further argue that the absence of central concentration in the BSS distribution rules out a collisional origin. Thus, the omega Cen BSS are the purest and largest population of non-collisional BSS ever observed. Our results allow the first empirical quantitative estimate of the production rate of BSS via this channel. BSS in omega Cen may represent the best local template for modeling the BSS populations in distant galaxies where they cannot be individually observed.

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arXiv2005

The Ital-FLAMES survey of the Sagittarius dwarf Spheroidal galaxy. I. Chemical abundances of bright RGB stars

We present iron and $α$ element (Mg, Ca, Ti) abundances for a sample of 15 Red Giant Branch stars belonging to the main body of the Sagittarius dwarf Spheroidal galaxy. Abundances have been obtained from spectra collected using the high resolution spectrograph FLAMES-UVES mounted at the VLT. Stars of our sample have a mean metallicity of [Fe/H]=-0.41$\pm$0.20 with a metal poor tail extending to [Fe/H]=-1.52. The $α$ element abundance ratios are slightly subsolar for metallicities higher than [Fe/H]\gtsima-1, suggesting a slow star formation rate. The [$α$/Fe] of stars having [Fe/H]$<$-1 are compatible to what observed in Milky Way stars of comparable metallicity.

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arXiv2005

Metallicities, relative ages and kinematics of stellar populations in omega Centauri

We present results of an extensive spectroscopic survey of Subgiant stars in the stellar system omega Centauri. Using infrared CaII triplet lines, we derived metallicities and radial velocities for more than 250 stars belonging to different stellar populations of the system. We find that the most metal rich component, the anomalous Sub Giant Branch (SGB-a), has a metallicity of [Fe/H] -0.6 fully compatible with that determined along the anomalous Red Giant Branch (RGB-a). Our analysis suggests that the age of this component and of the other metal-intermediate (-1.4 < [Fe/H] < -1.0) stellar populations of the system are all comparable to that of the dominant metal poor population within 2 Gyr regardless of any choice of helium abundance. These results impose severe constraints on the time-scale of the enrichment process of this stellar system, excluding the possibility of an extended star formation period. The radial velocity analysis of the entire sample demonstrates that only the metal-intermediate populations are kinematically cooler than the others.

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arXiv2005

Discovery of a stellar system in the background of 47 Tucanae

We report on the discovery of a stellar system in the background of the Galactic globular cluster 47 Tucanae (NGC 104), located 14.8' North-West of the cluster center. The object, whose apparent diameter is D~30'', is partially resolved into stars on the available CCD images, reaching a limiting magnitude of V~22.5, and is detected as a significant (more than 5 sigma) overdensity of blue stars (B-V<0.7). The color magnitude diagram of the system, its characteristic projected size and its position in the sky suggest that it is an intermediate-old age cluster belonging to the Small Magellanic Cloud, whose outskirts lie in the background of 47 Tuc. Although less likely, the possibility that the object is an unknown dwarf galaxy in the outskirts of the Local Group cannot completely be ruled out by the present data.

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arXiv2004

On the discrete nature of the Red Giant Branch of omega Centauri

We report the results of an extensive VLT high-resolution imaging campaign of the central region of the giant globular cluster omega Centauri. More than 100,000 stars have been measured in the inner 9'x 9' region of the cluster. On the basis of multiband Colour Magnitude Diagrams (CMD), we confirm the existence of multiple stellar populations along the Red Giant Branch (RGB). Moreover, thanks to the high-precision of this dataset, we conclude that the RGB does not present a smooth and continuous distribution, but shows a discrete structure: besides the metal-poor and the extreme metal-rich population (RGB-a), the existence of three metal intermediate populations is shown.

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arXiv2004

The central density cusp of the Sagittarius dwarf spheroidal galaxy

We present an analysis of the density profile in the central region of the Sagittarius dwarf spheroidal galaxy. A strong density enhancement of Sgr stars is observed. The position of the peak of the detected cusp is indistinguishable from the center of M54. The photometric properties of the cusp are fully compatible with those observed in the nuclei of dwarf elliptical galaxies, indicating that the Sgr dSph would appear as a nucleated galaxy independently of the presence of M54 at its center.

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arXiv2004

Metal rich stars in omega Cen: preliminary FLAMES GTO results

I present preliminary results for a sample of ~700 red giants in omega Cen, observed during the Ital-FLAMES Consortium GTO time in May 2003, for the Bologna Project on omega Cen. Preliminary Fe and Ca abundances confirm previous results: while the metal-poor and intermediate populations show a normal halo alpha-enhancement of [alpha/Fe]=$+0.3, the most metal-rich stars show a significantly lower [alpha/Fe]=+0.1. If the metal-rich stars have evolved within the cluster in a process of self-enrichment, the only way to lower their alpha-enhancement would be SNe type Ia intervention.

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arXiv2004

The distance to the Sgr dwarf spheroidal galaxy from the Red Giant Branch Tip

We derived the distance to the central region of the Sagittarius dwarf spheroidal galaxy from the Red Giant Branch Tip. The obtained distance modulus is $(m-M)_0=17.10\pm0.15$, corresponding to a heliocentric distance $D=26.30\pm1.8$ Kpc. This estimate is in good agreement with the distance obtained from RR Lyrae stars of the globular cluster M~54, located in the core of the Sgr galaxy, once the most accurate estimate of the cluster metallicity and the most recent calibration of the $M_V(RRLy) vs.[Fe/H]$ relation are adopted.

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arXiv2004

The calibration of the RGB Tip as a Standard Candle: Extension to Near Infrared colors and higher metallicity

New empirical calibrations of the Red Giant Branch Tip in the I,J,H and K bands based on two fundamental pillars, namely omega Centauri and 47 Tucanae, have been obtained by using a large optical and near infrared photometric database. Our best estimates give M_I(TRGB)= -4.05 pm 0.12, M_J(TRGB)= -5.20 pm 0.16, M_H(TRGB)= -5.94 pm 0.18 and M_K(TRGB) = -6.04 pm 0.16 at [M/H]=-1.5 (omega Cen) and M_I(TRGB)= -3.91 pm 0.13, M_J(TRGB)= -5.47 pm 0.25, M_H(TRGB)= -6.35 pm 0.30 and M_K(TRGB)= -6.56 pm 0.20 at [M/H]=-0.6 (47 Tuc). With these new empirical calibrations we also provide robust relations of the TRGB magnitude in I, J, H and K bands as a function of the global metallicity ([M/H]). It has also been shown that our calibrations self-consistently provide a distance modulus of the Large Magellanic Cloud in good agreement with the standard value ((m-M)_0 = 18.50)).

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arXiv2004

Discovery of an anomalous Sub Giant Branch in the Color Magnitude Diagram of omega Centauri

Using deep high-resolution multi-band images taken with the Very Large Telescope and the Hubble Space Telescope, we discovered a new anomalous sequence in the Color Magnitude Diagram of omega Cen. This feature appears as a narrow, well-defined Sub Giant Branch (SGB-a), which merges into the Main Sequence of the dominant cluster population at a magnitude significantly fainter than the cluster Turn-Off (TO). The simplest hypothesis assumes that the new feature is the extension of the anomalous Red Giant Branch (RGB-a) metal-rich population discovered by Lee et al. (1999) and Pancino et al. (2000). However, under this assumption the interpretation of the SGB-a does not easily fit into the context of a self-enrichment scenario within omega Cen. In fact, its TO magnitude, shape and extension are not compatible with a young, metal-rich population, as required by the self-enrichment process. The TO level of the SGB-a suggests indeed an age as old as the main cluster population, further supporting the extra-cluster origin of the most metal rich stars, as suggested by Ferraro, Bellazzini & Pancino (2002). Only accurate measurements of radial velocities and metal abundances for a representative sample of stars will firmly establish whether or not the SGB-a is actually related to the RGB-a and will finally shed light on the origin of the metal rich population of omega Cen.

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arXiv2004

IR photometric properties of Red Giants in omega Cen

We present a near-infrared J and K photometric catalog containing more than 73,000 stars in the central region of the giant Globular Cluster omega Centauri. This is the largest IR data-set ever published for this cluster and has been used to completely characterize the morphology and the properties of the Red Giant Branch (RGB). In particular, we concentrated our attention on (i) the anomalous RGB (RGB-a), recently discovered in this cluster and (ii) the RGB of the dominant metal poor population (RGB-MP) in both the infrared (K,J-K) and optical-infrared (K,V-K) color magnitude diagrams. The full set of morphological parameters and photometric indices has been measured and compared with the empirical relations by Ferraro et al. (2000). We find that the detailed photometric properties of the RGB-a are in full agreement with the recent spectroscopic metallicity estimates, that place it at the metal-rich extreme of the stellar population mix in omega Centauri.

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arXiv2004

Wide Field Photometry of the Galactic Globular Cluster M22

We present wide field photometry of the Galactic Globular Cluster M~22 in the B, V and I passbands for more than 186,000 stars. The study is complemented by the photometry in two narrowband filters centered on H$_α$ and the adjacent continuum, and by infrared J, H and K magnitudes derived from the 2 MASS survey for $\sim$2000 stars. Profiting from this huge database, we completely characterized the evolved stellar sequences of the cluster by determining a variety of photometric parameters, including new photometric estimates of the mean metallicity, reddening and distance to the cluster. In particular, from our multi-wavelength analysis, we re-examined the long-standing metallicity spread problem in M~22. According to our dataset, we conclude that most of the observed width of the red giant branch must be due to differential reddening, which amounts to a maximum of $ΔE(B-V)\simeq0.06$, although the presence of a small metallicity spread cannot be completely ruled out. More specifically, the maximum metallicity spread allowed by our data is of the order of $Δ$[Fe/H]$\simeq 0.1÷0.2$ dex, i.e., not much more than what allowed by the photometric errors. Finally, we identified most of the known variable stars and peculiar objects in our field of view. In particular, we find additional evidence supporting previous optical identifications of the central star of the Planetary Nebula IRAS 18333-2357, which is associated with M~22.

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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

Metal Rich Stars in omega Centauri

I present some preliminary results from a high-resolution spectroscopic observing campaign conducted with UVES at the ESO VLT, devoted to the study of the newly discovered, metal-rich red giant branch (RGB-a) in omega Centauri. In particular, I derive and discuss accurate abundances of iron-peak elements, alpha-elements and s-process elements. The main results discussed in this contribution are: (i) the RGB-a is the most metal-rich component of the RGB stellar mix in omega Cen, with [Fe/H]=-0.62+/-0.06; (ii) the RGB-a has a lower alpha-enhancement compared to the other red giants, possibly due to SNe type Ia pollution and (iii) the s-process elements overabundance of the RGB-a is similar to that of the other red giants in omega Cen, and unusually high for globular cluster stars, due to pollution by low mass AGB stars.

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