The White Dwarf Cooling Sequence of NGC 6397

By Brad M. S. Hansen, Jay Anderson, J. Brewer, Aaron Dotter, G. Fahlman, J. Hurley, J. Kalirai, Ivan R. King, David Reitzel, H. Richer and 3 more. 2005

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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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/cd5b4ba98e1f314981e31615f0bb0337b509fb63

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Citations in the corpus

NEW COOLING SEQUENCES FOR OLD WHITE DWARFS (2010, background): “…clocks to infer the age of a wide variety of stellar populations, such as the Galactic disk and halo (Winget et al. 1987; Garcı́a–Berro et al. 1988a, 1988b, 1999; Isern et al. 1998; Torres et al. 2002), and the system of globular and open clusters (Kalirai et al. 2001; Hansen et al. 2002, 2007).”
A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192 (2008, background): “(7) implies that DL ∝ 1/M , so even a white dwarf primary as old as the Galactic disk would be bright enough (Hansen et al. 2007) to detect in HST images.”
Co-formation of the disc and the stellar halo (2018, background): “For example, it has been determined that the halo’s globular clusters range in age from ∼10 to ∼13 Gyr depending on metallicity (see e.g. Hansen et al. 2002, 2007; VandenBerg et al. 2013).”
Spectroscopic analysis of DA white dwarfs with 3D model atmospheres (2013, methodology): “This is a fundamental technique in white dwarf research and astrophysics, such as in studies of galactic clusters and the halo (see, e.g., Hansen 1999; Fontaine et al. 2001; Dobbie et al. 2006; Hansen et al. 2007; Kalirai 2012).”
MEASUREMENT OF THE RADIUS OF NEUTRON STARS WITH HIGH SIGNAL-TO-NOISE QUIESCENT LOW-MASS X-RAY BINARIES IN GLOBULAR CLUSTERS (2012, methodology): “More recent photometric studies (CO WD sequence) have been performed, with d = 2.54±0.07 kpc (Hansen et al. 2007), or d = 2.34±0.13 kpc (Strickler et al. 2009), but since those results are model-dependent, they are not used in an effort to minimize unquantified systematics.”
The Initial-Final Mass Relation: Direct Constraints at the Low-Mass End (2007, methodology): “The relation used most recently in the study of NGC 6397 by Hansen et al. (2007) is shown as a dashed line in Figure 10 and is found to be in good agreement with our new low mass constraints (the disagreement at higher masses is not important since all of the observed white dwarfs in NGC 6397…”
Evolutionary and pulsational properties of white dwarf stars (2010, background): “…the disk (Winget et al. 1987; Garcı́a–Berro et al. 1988a,b; Hernanz et al. 1994; Torres et al. 2002) and globular and open clusters (Richer et al. 1997; Von Hippel & Gilmore 2000; Hansen et al. 2002; VonHippel et al. 2006; Hansen et al. 2007; Winget et al. 2009; Garcı́a–Berro et al. 2010).”