Browsing by Author "Cassisi, S."

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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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    Helium enhanced stars and multiple populations along the horizontal branch of NGC 2808 : direct spectroscopic measurements
    (2014) Marino, F.; Milone, P.; Przybilla, N.; Bergemann, M.; Lind, K.; Asplund, M.; Cassisi, S.; Catelan, Márcio; Zoccali, Manuela; Angeloni, Rodolfo
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    Iron and s-elements abundance variations in NGC 5286: comparison with 'anomalous' globular clusters and Milky Way satellites
    (2015) Marino, A. F.; Milone, A. P.; Karakas, A. I.; Casagrande, L.; Yong, D.; Shingles, L.; Da Costa, G.; Norris, J. E.; Stetson, P. B.; Lind, K.; Asplund, M.; Collet, R.; Jerjen, H.; Sbordone, L.; Aparicio, A.; Cassisi, S.
    We present a high-resolution spectroscopic analysis of 62 red giants in the Milky Way globular cluster (GC) NGC 5286. We have determined abundances of representative light proton-capture, a, Fe-peak and neutron-capture element groups, and combined them with photometry of multiple sequences observed along the colour-magnitude diagram. Our principal results are: (i) a broad, bimodal distribution in s-process element abundance ratios, with two main groups, the s-poor and s-rich groups; (ii) substantial star-to-star Fe variations, with the s-rich stars having higher Fe, e.g. <[Fe/H]>(s-rich) - <[Fe/H]>(s-poor) similar to 0.2 dex; and (iii) the presence of O-Na-Al (anti) correlations in both stellar groups. We have defined a new photometric index, c(BVI) = (B - V) -(V - I), to maximize the separation in the colour-magnitude diagram between the two stellar groups with different Fe and s-element content, and this index is not significantly affected by variations in light elements (such as the O-Na anticorrelation). The variations in the overall metallicity present in NGC 5286 add this object to the class of anomalous GCs. Furthermore, the chemical abundance pattern of NGC 5286 resembles that observed in some of the anomalous GCs, e.g. M 22, NGC 1851, M 2, and the more extreme omega Centauri, that also show internal variations in s-elements, and in light elements within stars with different Fe and s-elements content. In view of the common variations in s-elements, we propose the term s-Fe-anomalous GCs to describe this sub-class of objects. The similarities in chemical abundance ratios between these objects strongly suggest similar formation and evolution histories, possibly associated with an origin in tidally disrupted dwarf satellites.
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    Mapping the stellar age of the Milky Way bulge with the VVV I. The method
    (2019) Surot, F.; Valenti, E.; Hidalgo, S.L.; Zoccali, Manuela; Sokmen, E.; Rejkuba, M.; Minniti, D.; Gonzalez, O.A.; Cassisi, S.; Renzini, O.; Weiss, A.
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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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    The halo plus cluster system of the Galactic globular cluster NGC 1851(a similar to...)
    (2014) Marino, A.F.; Milone, A.P.; Yong, D.; Dotter, A.; Da Costa, G.; Asplund, M.; Jerjen, H.; Cassisi, S.; Sbordone, L.; Angeloni, Rodolfo
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    The RR Lyrae Instability Strip in the Split Horizontal Branch Globular Cluster NGC 6569
    (2015) Kunder, A.; Stetson, P. B.; Catelan, Marcio; Walker, A.; Cassisi, S.; Johnson, C.; Soto, M.
    The first calibrated broadband BVI time-series photometry is obtained for the RR Lyrae variable stars in the split horizontal branch (HB) cluster NGC 6569. As a result, a sizeable population of 27 RR Lyrae variables has been identified. An eclipsing binary in the RR Lyrae instability strip was also found, which is most likely a field star. The 12 fundamental-mode RR Lyrae variables (RR0) have an average period of [P]RR0= 0.57±0.02 d and the 15 first-overtone variables (RR1) have an average period of [P]RR1=0.30±0.01 d. The mean periods of the RR Lyrae stars are consistent with an Oosterhoff I designation, and the number ratio of RR1- to RR0-type variables, N1/NRR=0.56, is more consistent with an Oosterhoff II designation. Compared to the M3 variables, the NGC 6569 RR Lyrae variables are not shifted to a longer periods at a given amplitude, making it unlikely that these stars are He-enhanced. The similarity in the periods and amplitudes between the RR Lyrae stars in NGC 6569 and the bulge field RR Lyrae population may indicate similar formation conditions.
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    The rr lyrae variables and horizontal branch of NGC 6656 (m22)
    (2013) Kunder, A.; Stetson, P.; Cassisi, S.; Layden, A.; Bono, G.; Catelan, Márcio; Walker, A.; Paredes Alvarez, L.; Clem, J.; Matsunaga, N.; Salaris, M.; Lee, J.; Chaboyer, B.
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    Theoretical modeling of the RR Lyrae variables in NGC 1851
    (2015) Kunder, A.; Salaris, M.; Cassisi, S.; de Propris, R.; Walker, A.; Stetson, P. B.; Catelan, Marcio; Amigo, P.
    The RR Lyrae instability strip (IS) in NGC 1851 is investigated, and a model is presented which reproduces the pulsational properties of the RR Lyrae population. In our model, a stellar component within the IS that displays minor helium variations (Y∼0.248-0.270) is able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae variables. The RR Lyrae variables therefore may belong to an He-enriched second generation of stars. The RR Lyrae variables with a slightly enhanced helium (Y∼0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y∼0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables. %.
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