Browsing by Author "Di Cecco, A."

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    Constraints on the formation of the globular cluster IC 4499 from multiwavelength photometry
    (2011) Walker, A. R.; Kunder, A. M.; Andreuzzi, G.; Di Cecco, A.; Stetson, P. B.; Monelli, M.; Cassisi, S.; Bono, G.; De Propris, R.; Dall'Ora, M.; Nemec, J. M.; Zoccali, M.
    We present new multiband photometry for the Galactic globular cluster IC 4499 extending well past the main-sequence turn-off in the U, B, V, R, I and DDO51 bands. This photometry is used to determine that IC 4499 has an age of 12 +/- 1 Gyr and a cluster reddening of E(B - V) = 0.22 +/- 0.02. Hence, IC 4499 is coeval with the majority of Galactic globular clusters, in contrast to suggestions of a younger age. The density profile of the cluster is observed to not flatten out to at least r similar to 800 arcsec, implying that either the tidal radius of this cluster is larger than previously estimated, or that IC 4499 is surrounded by a halo. Unlike the situation in some other, more massive, globular clusters, no anomalous colour spreads in the ultraviolet are detected among the red giant branch stars. The small uncertainties in our photometry should allow the detection of such signatures apparently associated with variations of light elements within the cluster, suggesting that IC 4499 consists of a single stellar population.
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    ON THE RADIAL DISTRIBUTION OF HORIZONTAL BRANCH STARS IN NGC 2808
    (2009) Iannicola, G.; Monelli, M.; Bono, G.; Stetson, P. B.; Buonanno, R.; Calamida, A.; Zoccali, M.; Caputo, F.; Castellani, M.; Corsi, C. E.; Dall'Ora, M.; Di Cecco, A.; Degl'Innocenti, S.; Ferraro, I.; Nonino, M.; Pietrinferni, A.; Pulone, L.; Moroni, P. G. Prada; Romaniello, M.; Sanna, N.; Walker, A. R.
    We present accurate new ultraviolet and optical BVI photometry for the Galactic globular cluster NGC 2808 based on both ground-based and archival Hubble Space telescope imagery. From this we have selected a sample of similar to 2000 Horizontal Branch (HB) stars; given the extensive wavelength range considered and the combination of both high angular resolution and wide-field photometric coverage, our sample should be minimally biased. We divide the HB stars into three radial bins and find that the relative fractions of cool, hot, and extreme HB stars do not change radically when moving from the center to the outskirts of the cluster: the difference is typically smaller than similar to 2 sigma. These results argue against the presence of strong radial differentiation among any stellar subpopulations having distinctly different helium abundances. The ratio between HB and red giant (RG) stars brighter than the ZAHB steadily increases when moving from the innermost to the outermost cluster regions. The difference is larger than similar to 4 sigma and indicates a deficiency of bright RGs in the outskirts of the cluster.
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    ON THE ΔVHBbump PARAMETER IN GLOBULAR CLUSTERS
    (2010) Di Cecco, A.; Bono, G.; Stetson, P. B.; Pietrinferni, A.; Becucci, R.; Cassisi, S.; Degl'Innocenti, S.; Iannicola, G.; Moroni, P. G. Prada; Buonanno, R.; Calamida, A.; Caputo, F.; Castellani, M.; Corsi, C. E.; Ferraro, I.; Dall'Ora, M.; Monelli, M.; Nonino, M.; Piersimoni, A. M.; Pulone, L.; Romaniello, M.; Salaris, M.; Walker, A. R.; Zoccali, M.
    We present new empirical estimates of the del V-HB(bump) parameter for 15 Galactic globular clusters (GGCs) using accurate and homogeneous ground-based optical data. Together with similar evaluations available in the literature, we ended up with a sample of 62 GGCs covering a very broad range in metal content (-2.16 dex <= [M/H] <= -0.58 dex). Adopting the homogeneous metallicity scale provided either by Kraft & Ivans or by Carretta et al., we found that the observed del V-HB(bump) parameters are larger than predicted. In the metal-poor regime ([M/H] less than or similar to -1.7, -1.6 dex) 40% of GCs show discrepancies of 2s (approximate to 0.40 mag) or more. Evolutionary models that account either for alpha- and CNO-enhancement or for helium enhancement do not alleviate the discrepancy between theory and observations. The outcome is the same if we use the new solar heavy-element mixture. The comparison between alpha- and CNO-enhanced evolutionary models and observations in the Carretta et al. metallicity scale also indicates that observed del V-HB(bump) parameters, in themetal- rich regime ([M/H] >= 0), might be systematically smaller than predicted.
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    On the ΔV bump HB Parameter in Globular Clusters
    (2010) Di Cecco, A.; Zoccali, Manuela