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Title:
The White Dwarf Cooling Sequence of NGC 6397
Authors:
Hansen, Brad M. S.; Anderson, Jay; Brewer, James; Dotter, Aaron; Fahlman, Greg. G.; Hurley, Jarrod; Kalirai, Jason; King, Ivan; Reitzel, David; Richer, Harvey B.; Rich, R. Michael; Shara, Michael M.; Stetson, Peter B.
Affiliation:
AA(Department of Astronomy, University of California Los Angeles, Los Angeles, CA 90095; , , .; Institute for Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, CA 90095.), AB(Department of Physics and Astronomy, Rice University, Houston, TX 77005; .), AC(Department of Physics and Astronomy, 6224 Agricultural Road, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada; .), AD(Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH; .), AE(Department of Physics and Astronomy, 6224 Agricultural Road, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada; .; Canada-France-Hawaii Telescope Corporation, P.O. Box 1597, Kamuela, HA, 96743; .), AF(Department of Mathematics and Statistics, Monash University, Clayton, Victoria 3800; Australia, .), AG(Hubble Fellow, Lick Observatory, University of California Santa Cruz, Santa Cruz, CA, 95064; .), AH(Department of Astronomy, University of Washington, Seattle, WA 98195; .), AI(Department of Astronomy, University of California Los Angeles, Los Angeles, CA 90095; , , .), AJ(Department of Physics and Astronomy, 6224 Agricultural Road, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada; .), AK(Department of Astronomy, University of California Los Angeles, Los Angeles, CA 90095; , , .), AL(Department of Astrophysics, Division of Physical Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY, 10024-5192 .), AM(Hertzberg Institute of Astrophysics, National Research Council, Victoria, BC, Canada; .)
Publication:
The Astrophysical Journal, Volume 671, Issue 1, pp. 380-401. (ApJ Homepage)
Publication Date:
12/2007
Origin:
UCP
ApJ Keywords:
Galaxy: Halo, Galaxy: Globular Clusters: Individual: NGC Number: NGC 6397, Stars: Luminosity Function, Mass Function, Stars: Population II, Stars: White Dwarfs
DOI:
10.1086/522567
Bibliographic Code:
2007ApJ...671..380H

Abstract

We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC 6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 mag in depth, with an apparent cutoff at magnitude F814W~27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in color and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity, and cooling model uncertainties. After marginalizing over these variables, we obtain values for the cluster distance modulus and age of μ0=12.02+/-0.06 and Tc=11.47+/-0.47 Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relations are in good agreement with other independent determinations, and the cluster age is consistent with, but more precise than, prior determinations made using the main-sequence turnoff method. In particular, within the context of the currently accepted ΛCDM cosmological model, this age places the formation of NGC 6397 at a redshift z~3, at a time when the cosmological star formation rate was approaching its peak.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal GO-10424.


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