Browsing by Author "Geisler, Douglas"

Now showing 1 - 4 of 4
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    APOGEE chemical abundances of globular cluster giants in the inner Galaxy
    (2017) Schiavon, Ricardo P.; Johnson, Jennifer A.; Frinchaboy, Peter M.; Zasowski, Gail; Meszaros, Szabolcs; Garcia-Hernandez, D. A.; Cohen, Roger E.; Tang, Baitian; Villanova, Sandro; Geisler, Douglas; Beers, Timothy C.; Fernandez-Trincado, J. G.; Garcia Perez, Ana E.; Lucatello, Sara; Majewski, Steven R.; Martell, Sarah L.; O'Connell, Robert W.; Allende Prieto, Carlos; Bizyaev, Dmitry; Carrera, Ricardo; Lane, Richard R.; Malanushenko, Elena; Malanushenko, Viktor; Munoz, Ricardo R.; Nitschelm, Christian; Oravetz, Daniel; Pan, Kaike; Roman-Lopes, Alexandre; Schultheis, Matthias; Simmons, Audrey
    We report chemical abundances obtained by Sloan Digital Sky Survey (SDSS)-III/Apache Point Observatory Galactic Evolution Experiment for giant stars in five globular clusters located within 2.2 kpc of the Galactic Centre. We detect the presence of multiple stellar populations in four of those clusters (NGC 6553, NGC 6528, Terzan 5 and Palomar 6) and find strong evidence for their presence in NGC 6522. All clusters with a large enough sample present a significant spread in the abundances of N, C, Na and Al, with the usual correlations and anticorrelations between various abundances seen in other globular clusters. Our results provide important quantitative constraints on theoretical models for self-enrichment of globular clusters, by testing their predictions for the dependence of yields of elements such as Na, N, C and Al on metallicity. They also confirm that, under the assumption that field N-rich stars originate from globular cluster destruction, they can be used as tracers of their parental systems in the high-metallicity regime.
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    Deep Hubble Space Telescope Imaging of Globular Clusters toward the Galactic Bulge: Observations, Data Reduction, and Color-magnitude Diagrams
    (2018) Cohen, Roger E.; Mauro, Francesco; Alonso-Garcia, Javier; Hempel, Maren; Sarajedini, Ata; Ordonez, Antonio J.; Geisler, Douglas; Kalirai, Jason S.
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    RGB Properties of the LMC/SMC Clusters in the Infrared
    (2009) Kurtev, Radostin; Ivanov, Valentin; Borissova, Jura; Catelan , Marcio; Geisler, Douglas
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    WIDE FIELD NEAR-INFRARED PHOTOMETRY OF 12 GALACTIC GLOBULAR CLUSTERS: OBSERVATIONS VERSUS MODELS ON THE RED GIANT BRANCH
    (2015) Cohen, Roger E.; Hempel, Maren; Mauro, Francesco; Geisler, Douglas; Alonso-Garcia, Javier; Kinemuchi, Karen
    We present wide field near-infrared (near-IR) photometry of 12 Galactic globular clusters, typically extending from the tip of the cluster red giant branch (RGB) to the main sequence turnoff. Using recent homogenous values of cluster distance, reddening and metallicity, the resulting photometry is directly compared to the predictions of several recent libraries of stellar evolutionary models. Of the sets of models investigated, Dartmouth and Victoria-Regina models best reproduce the observed RGB morphology, albeit with offsets in J-K-S color which vary in their significance in light of all sources of observational uncertainty. Therefore, we also present newly recalibrated relations between near-IR photometric indices describing the upper RGB versus cluster iron abundance as well as global metallicity. The influence of enhancements in alpha elements and helium are analyzed, and we find that the former affect the morphology of the upper RGB in accord with model predictions. Meanwhile, the empirical relations we derive are in good agreement with previous results, and minor discrepancies can likely be attributed to differences in the assumed cluster distances and reddenings. In addition, we present measurements of the horizontal branch (HB) and RGB bump magnitudes, finding a non-negligible dependence of the near-IR HB magnitude on cluster metallicity. Lastly, we discuss the influence of assumed cluster distances, reddenings and metallicities on our results, finding that our empirical relations are generally insensitive to these factors to within their uncertainties.