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The Astrophysical Journal Volume 671 Number 1 Create an alert RSS this journal

Brad M. S. Hansen et al. 2007 ApJ 671 380 doi:10.1086/522567

The White Dwarf Cooling Sequence of NGC 6397*

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Brad M. S. Hansen1,2, Jay Anderson3, James Brewer4, Aaron Dotter5, Greg. G. Fahlman4,6, Jarrod Hurley7, Jason Kalirai8, Ivan King9, David Reitzel1, Harvey B. Richer4, R. Michael Rich1, Michael M. Shara10 and Peter B. Stetson11

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Abstract References Cited By

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.


Footnote
*  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.
Subject headings

Galaxy: halo; globular clusters: individual (NGC 6397); stars: luminosity function, mass function; stars: Population II; white dwarfs


Dates

Issue 1 (2007 December 10)

Received 2007 February 20, accepted for publication 2007 July 2



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