The White Dwarf Cooling Sequence of NGC 6397

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 magnitudes in depth, with an apparent cutoff at magnitude F 814W ∼ 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 colour and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model uncertainties. After marginalising over these variables, we obtain values for the cluster distance modulus and age of µ 0 = 12.02 ± 0.06 and T c = 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

Massive star clusters in galaxies

“…Published work (e.g. Gratton et al 2003;De Angeli et al 2005;Hansen et al 2007;Marín-Franch et al 2009; among many others) indicates that the absolute age of the metal-poor clusters is near 13 Gyr and shows a cluster-to-cluster dispersion that may be as low as ±0.5 Gyr, confirming their classic status as among the first stellar structures to have formed in the galaxy. The mean age for the metal-richer subsystem is perhaps 2 Gyr younger than the extremely old, metal-poor subsystem, and exhibits a higher cluster-to-cluster age scatter, near ±2 Gyr rms.…”

New evolutionary sequences for extremely low-mass white dwarfs

“…White dwarf stars are routinely used to constrain the age and past history of the Galactic populations, including the solar neighborhood and open and globular clusters (Von Hippel & Gilmore 2000;Hansen et al 2007;Winget et al 2009;García-Berro et al 2010;Bono et al 2013, and references therein). In addition, they are used to place constraints on properties of elementary particles, such as axions (Isern et al 1992(Isern et al , 2008Córsico et al 2012a,b) and neutrinos (Winget et al 2004), or on alternative theories of gravitation (García-Berro et al 1995.…”

Hubble Space Telescope observations of the Kepler-field cluster NGC 6819 – I. The bottom of the white dwarf cooling sequence★

“…During the last few decades both observations and theory have improved to a level that has made it possible to employ white dwarf (WD) stars for estimating ages of stellar populations in the solar neighbourhood (i.e., Winget et al 1987;García-Berro et al 1988;Oswalt et al 1996), open (i.e., Richer et al 1998;von Hippel 2005;Bedin et al 2008aBedin et al , 2010 and globular (i.e., Hansen et al 2004Hansen et al , 2007Bedin et al 2009) clusters.…”