Examinando por Autor "Cohen, R.E."

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  • Miniatura
    Ítem
    Near-infrared photometry of globular clusters towards the Galactic bulge: Observations and photometric metallicity indicators
    (OXFORD UNIV PRESS, 2016-09) Cohen, R.E.; Bidin, C.M.; Mauro, F.; Bonatto, C.; Geisler, D.
    We present wide-field JHKS photometry of 16 Galactic globular clusters located towards the Galactic bulge, calibrated on the Two Micron All-Sky Survey photometric system. Differential reddening corrections and statistical field star decontamination are employed for all of these clusters before fitting fiducial sequences to the cluster red giant branches (RGBs). Observed values and uncertainties are reported for several photometric features, including the magnitude of the RGB bump, tip, the horizontal branch (HB) and the slope of the upper RGB. The latest spectroscopically determined chemical abundances are used to build distance- and reddening-independent relations between observed photometric features and cluster metallicity, optimizing the sample size and metallicity baseline of these relations by supplementing our sample with results from the literature.We find that the magnitude difference between the HB and the RGB bump can be used to predict metallicities, in terms of both iron abundance [Fe/H] and global metallicity [M/H], with a precision of better than 0.1 dex in all three near-IR bandpasses for relatively metal-rich ([M/H] ≳ -1) clusters. Meanwhile, both the slope of the upper RGB and the magnitude difference between the RGB tip and bump are useful metallicity indicators over the entire sampled metallicity range (-2 ≲ [M/H] ≲ 0) with a precision of 0.2 dex or better, despite model predictions that the RGB slope may become unreliable at high (near-solar) metallicities. Our results agree with previous calibrations in light of the relevant uncertainties, and we discuss implications for clusters with controversial metallicities as well as directions for further investigation.
  • Miniatura
    Ítem
    New VVV Survey Globular Cluster Candidates in the Milky Way Bulge
    (Institute of Physics Publishing, 2017-11) Minniti, D.; Geisler, D.; Alonso-Garcia, J.; Palma, T.; Beamin, J.C.; Borissova, J.; Catelan, M.; Clariá, J.J.; Cohen, R.E.; Ramos, R.C.; Dias, B.; Fernández-Trincado, J.G.; Gómez, M.; Hempel, M.; Ivanov, V.D.; Kurtev, R.; Lucas, P.W.; Moni-Bidin, C.; Pullen, J.; Alegria, S.R.; Saito, R.K.; Valenti, E.
    It is likely that a number of Galactic globular clusters remain to be discovered, especially toward the Galactic bulge. High stellar density combined with high and differential interstellar reddening are the two major problems for finding globular clusters located toward the bulge. We use the deep near-IR photometry of the VISTA Variables in the Via Láctea (VVV) Survey to search for globular clusters projected toward the Galactic bulge, and hereby report the discovery of 22 new candidate globular clusters. These objects, detected as high density regions in our maps of bulge red giants, are confirmed as globular cluster candidates by their color-magnitude diagrams. We provide their coordinates as well as their near-IR color-magnitude diagrams, from which some basic parameters are derived, such as reddenings and heliocentric distances. The color-magnitude diagrams reveal well defined red giant branches in all cases, often including a prominent red clump. The new globular cluster candidates exhibit a variety of extinctions (0.06 < A Ks < 2.77) and distances (5.3 < D < 9.5 kpc). We also classify the globular cluster candidates into 10 metal-poor and 12 metal-rich clusters, based on the comparison of their color-magnitude diagrams with those of known globular clusters also observed by the VVV Survey. Finally, we argue that the census for Galactic globular clusters still remains incomplete, and that many more candidate globular clusters (particularly the low luminosity ones) await to be found and studied in detail in the central regions of the Milky Way.