Browsing by Author "Frogel, JA"

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    An infrared spectroscopic study of eight Galactic globular clusters
    (2004) Stephens, AW; Frogel, JA
    We have obtained medium-resolution infrared K-band spectra of 44 giants in seven heavily reddened clusters in the Galactic bulge, as well as 12 giants in omega Centauri. We measure the equivalent widths of the Na doublet, the Ca triplet, and the CO band head, and then apply the new technique of Frogel et al. to determine the metallicity of each star. Averaging these values, we estimate the metallicity for each cluster and compare our new [Fe/H] values with previous determinations from the literature. Our estimates for each cluster are NGC 6256 ( - 1.35), NGC 6539 (-0.79), HP 1 (-1.30), Liller 1 (-0.36), Palomar 6 (-0.52), Terzan 2 (-0.87), and Terzan 4 (-1.62). We briefly discuss differences between the various [Fe/H] scales on which it was possible to base our calibration, which is found to be the largest uncertainty in using this technique to determine metallicities.
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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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    The structure and stellar content of the central region of M33
    (2002) Stephens, AW; Frogel, JA
    Using Gemini QuickStart infrared observations of the central 22" of M33, we analyze the stellar populations in this controversial region. Based on the slope of the giant branch we estimate the mean metallicity to be -0.26+/-0.27, and from the luminosities of the most luminous stars, we estimate that there were two bursts of star formation, similar to2 and similar to0.5 Gyr ago. We show that the stellar luminosity function not has only a different bright end cutoff, but also a significantly different slope than that of the Galactic bulge, and we suggest that this difference is due to the young stellar component in M33. We combine our infrared Gemini data with optical Hubble Space Telescope Wide Field Planetary Camera 2 measurements revealing a color-magnitude diagram populated with young, intermediate-age, and old stellar populations. Using surface brightness profiles from 0".1 to 18', we perform simple decompositions and show that the data are best fitted by a three-component core plus bulge plus disk model. Finally, we find no evidence for radial variations of the stellar populations in the inner 3"-10" of M33, based on a spatial analysis of the color-magnitude diagrams and luminosity functions.