Browsing by Author "Rich, RM"

Now showing 1 - 4 of 4
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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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    The metal content of the bulge globular cluster NGC 6528
    (2004) Zoccali, A; Barbuy, B; Hill, V; Ortolani, S; Renzini, A; Bica, E; Momany, Y; Pasquini, L; Minniti, D; Rich, RM
    High resolution spectra of five stars in the bulge globular cluster NGC 6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph. Out of the five stars, two of them showed evidence of binarity. The target stars belong to the horizontal and red giant branch stages, at 4000 < T-eff < 4800 K. Multiband V, I, J, H, K-s, photometry was used to derive initial effective temperatures and gravities. The main purpose of this study is the determination of metallicity and elemental ratios for this template bulge cluster, as a basis for the fundamental calibration of metal-rich populations. The present analysis provides a metallicity [Fe/H] = -0.1 +/- 0.2 and the alpha-elements O, Mg and Si, show [alpha/Fe] approximate to+0.1, whereas Ca and Ti are around the solar value or below, resulting in an overall metallicity Z approximate to Z(.).
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    The stellar content of the bulge of M31
    (2003) Stephens, AW; Frogel, JA; DePoy, DL; Freedman, W; Gallart, C; Jablonka, P; Renzini, A; Rich, RM; Davies, R
    We analyze the stellar populations present in M31 by using nine sets of adjacent Hubble Space Telescope NICMOS camera 1 and 2 fields with galactocentric distances ranging from 2' to 20'. These infrared observations provide some of the highest spatial resolution measurements of M31 to date; our data place tight constraints on the maximum luminosities of stars in the bulge of M31. The tip of the red giant branch is clearly visible at M-bol similar to -3.8, and the tip of the asymptotic giant branch (AGB) extends to M-bol similar to -5. This AGB peak luminosity is significantly fainter than previously claimed; through direct comparisons and simulations we show that previous measurements were affected by image blending. We do observe field-to-field variations in the luminosity functions, but simulations show that these differences can be produced by blending in the higher surface brightness fields. We conclude that the red giant branch of the bulge of M31 is not measurably different from that of the bulge of the Milky Way. We also find an unusually high number of bright bluish stars (7.3 arcmin(-2)), which appear to be Galactic foreground stars.
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    VLT-UVES analysis of two giants in the bulge metal-poor globular cluster HP-1 - Analysis of two giants in HP-1
    (2006) Barbuy, B; Zoccali, M; Ortolani, S; Momany, Y; Minniti, D; Hill, V; Renzini, A; Rich, RM; Bica, E; Pasquini, L; Yadav, RKS
    Context. Metal-poor globular clusters in the bulge are important tracers of early chemical evolution. HP-1 is among the six metal-poor clusters within 5. of the Galactic center, and could be the one closest to the center