Browsing by Author "Catelan, M"

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    Age as the second parameter in NGC 288/NGC 362? II. The horizontal branch revisited
    (2001) Catelan, M; Bellazzini, M; Landsman, WB; Ferraro, FR; Pecci, FF; Galleti, S
    We revisit the "second-parameter" pair of globular clusters NGC 288/362 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" in Paper I, where it was found that NGC 288 is 2 +/- 1 Gyr older than NGC 362. We find that adoption of a higher metallicity ([Fe/H] approximate to -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] approximate to -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] approximate to -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-another major second-parameter candidate. We argue that, if confirmed, this central density-HB mass dispersion correlation will have to be taken into account in order to conclusively determine whether age may be considered the (sole) second parameter of HB morphology for this (and other) second-parameter pair(s).
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    Hubble Space Telescope snapshot study of variable stars in globular clusters
    (2003) Pritzl, BJ; Smith, HA; Stetson, PB; Catelan, M; Sweigart, AV; Layden, AC; Rich, RM
    We present the results of a Hubble Space Telescope snapshot program to survey the inner region of the metal-rich globular cluster NGC 6441 for its variable stars. A total of 57 variable stars were found, including 38 RR Lyrae stars, six Population II Cepheids, and 12 long-period variables. Twenty-four of the RR Lyrae stars and all of the Population II Cepheids were previously undiscovered in ground-based surveys. Of the RR Lyrae stars observed in this survey, 26 are pulsating in the fundamental mode with a mean period of 0.753 days and 12 are first-overtone-mode pulsators with a mean period of 0.365 days. These values match up very well with those found in ground- based surveys. Combining all the available data for NGC 6441, we find mean periods of 0.759 and 0.375 days for the RRab and RRc stars, respectively. We also find that the RR Lyrae stars in this survey are located in the same regions of a period-amplitude diagram as those found in ground- based surveys. The overall ratio of RRc to total RR Lyrae stars is 0.33. Although NGC 6441 is a metal-rich globular cluster and would, on that ground, be expected either to have few RR Lyrae stars or to be an Oosterho. type I system, its RR Lyrae stars more closely resemble those in Oosterho. type II globular clusters. However, even compared with typical Oosterho. type II systems, the mean period of its RRab stars is unusually long. We also derived I-band period-luminosity relations for the RR Lyrae stars. Of the six Population II Cepheids, five are of W Virginis type and one is a BL Herculis variable star. This makes NGC 6441, along with NGC 6388, the most metal-rich globular cluster known to contain these types of variable stars. Another variable, V118, may also be a Population II Cepheid, given its long period and its separation in magnitude from the RR Lyrae stars. We examine the period-luminosity relation for these Population II Cepheids and compare it with those in other globular clusters and in the Large Magellanic Cloud. We argue that there does not appear to be a change in the period-luminosity relation slope between the BL Herculis and W Virginis stars, but that a change of slope does occur when the RV Tauri stars are added to the period-luminosity relation.
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    M75, a globular cluster with a trimodal horizontal branch.: II.: BV photometry of the RR lyrae variables
    (2003) Corwin, TM; Catelan, M; Smith, HA; Borissova, J; Ferraro, FR; Raburn, WS
    We present new BV CCD photometry, light curves, and ephemerides for nine previously known, 29 newly detected RR Lyrae variables, and one newly detected variable of an unknown type in the globular cluster M75. The photometry used for the detection of the additional variables was obtained with the image subtraction package ISIS. The data were acquired on an observing run in 1999 July and range over seven observing nights. Estimates of fundamental photometric parameters are presented including intensity- and magnitude-averaged B and V magnitudes, magnitude-averaged colors, pulsation periods, and pulsation amplitudes. The mean period of the RRab variables, [P-ab] = 0.5868 days, and the number fraction of RRc stars, N-c/N-RR = 0.342, are both large for an Oosterho. type I (OoI) globular cluster, suggesting that M75 may be Oosterhoff-intermediate. Possible conflicts between Oosterhoff-type determination based on the A(V)-log P and A(B)-log P diagrams are discussed. The physical parameters of the RRc and RRab variables, as obtained from Fourier decomposition of their light curves, do not show any clear deviation from normal OoI behavior.
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    New metallicities of RR Lyrae stars in ω Centauri
    (2006) Sollima, A; Borissova, J; Catelan, M; Smith, HA; Minniti, D; Cacciari, C; Ferraro, FR
    We present new spectroscopic metal abundances for 74 RR Lyrae (RRL) stars in omega Cen obtained with FLAMES. The well-known metallicity spread is visible among the RRL variables. The metal-intermediate (MI) RRL stars ([Fe/H] similar to -1.7) are fainter than the bulk of the dominant metal-poor population ([Fe/H] similar to -1.7), in good agreement with the corresponding zero-age horizontal-branch models with cosmological helium abundance. This result Y = 0.246 conflicts with the hypothesis that the progenitors of the MI RRL stars correspond to the anomalous blue main-sequence stars, which share a similar metallicity but whose properties are currently explained by assuming for them a large helium enhancement. Therefore, in this scenario, the coexistence within the cluster of two different populations with similar metallicities ([Fe/H] similar to -1.7) and different helium abundances has to be considered.