E. Pancino

A Step toward the Calibration of the Red Giant Branch Tip as a Standard Candle

The absolute I magnitude of the tip of the red giant branch (M) is one of the most promising standard candles actually used in astrophysics as a fundamental pillar for the cosmological distance scale. With the aim of improving the observational basis of its calibration, we have obtained an accurate estimate of the M for the globular cluster ω Centauri, based (1) on the largest photometric database ever assembled for a globular cluster, by Pancino and colleagues, and (2) on a direct distance estimate for ω Cen, recently obtained by Thompson and colleagues from a detached eclipsing binary. The derived value M = -4.04 ± 0.12 provides, at present, the most accurate empirical zero point for the calibration of the M- relation at ~ -1.7. We also derive a new empirical M-[Fe/H] relation based on the large IR data set of red giants in Galactic globular clusters recently presented by Ferraro and colleagues. This database (extending up to [Fe/H] = -0.2) covers a more appropriate metallicity range for extragalactic applications than previous empirical calibrations (which were limited to [Fe/H] ≤ -0.7). The proposed relation is in excellent agreement with the newly determined zero point.

Metallicities, relative ages, and kinematics of stellar populations in ω centauri

We present results of an extensive spectroscopic survey of subgiant stars in the stellar system ω Cen. Using infrared Ca II 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 subgiant 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 the choice of helium abundance. These results impose severe constraints on the timescale 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 metal-intermediate populations are kinematically cooler than the others.

High-Resolution Spectroscopy of Metal-rich Giants in ω Centauri: First Indication of Type Ia Supernova Enrichment*

We have obtained high-resolution, high signal-to-noise ratio spectra for six red giants in ? Centauri: three belong to the recently discovered metal-rich red giant branch (RGB-a; as defined by Pancino et al.) and three to the metal-intermediate population (RGB-MInt). Accurate iron, copper, and ?-element (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, = -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 ? Cen. In addition, we have found a significant difference in the ?-element enhancement of the two populations. The three RGB-MInt stars have the expected overabundance, typical of halo and globular cluster stars: = 0.29 ? 0.01. The three RGB-a stars show, instead, a significantly lower ?-enhancement: = 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. 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 ? Cen are briefly discussed.

The Draco and Ursa Minor Dwarf Spheroidal Galaxies: 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.

A Near-Infrared Spectroscopic Screening of the Red Giant Populations in ω Centauri

Near-infrared spectra of 21 giants in ω Cen 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 the infrared, aimed at studying the complex stellar population of ω Cen 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 and -1.2 and [Fe/H] ≤ -0.5 has been confirmed. The most metal-rich stars in our sample show a lower (if any) α enhancement when compared with 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 12C/13C 4, which clearly indicates that extra-mixing processes occurred in the stellar interiors during the ascent on the red giant branch.

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.

First Detection of the Red Giant Branch Bump in the Sagittarius Dwarf Spheroidal Galaxy

We present V, I photometry of the Sagittarius Dwarf Spheroidal galaxy (Sgr) for a region of ∼ 1 • × 1 • , 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 1 Based on observations made with the European Southern Observatory telescopes, using the Wide Field Imager, as part of the observing program 65.L-0463. Also based on data obtained from the ESO/ST-ECF Science Archive Facility. 2 This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation – 2 – 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.We present V, I photometry of the Sagittarius (Sgr) dwarf spheroidal galaxy for a region of ~1? ? 1?, centered on the globular cluster M54. 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 red giant branch (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.

Discovery of an Accreted Stellar System within the Globular Cluster ω 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.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 ω 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 ω 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 subgalactic scale.