A comparative analysis of the observed white dwarf cooling sequence from globular clusters

Abstract

We report our study of features at the observed red end of the white dwarf cooling sequences for three Galactic globular clusters: NGC 6397, 47 Tucanae and M 4. We use deep colour-magnitude diagrams constructed from archival Hubble Space Telescope (ACS) to systematically investigate the blue turn at faint magnitudes and the age determinations for each cluster. We find that the age difference between NGC 6397 and 47 Tuc is 1.98 +0.44 −0.26 Gyr, consistent with the picture that metal-rich halo clusters were formed later than metal-poor halo clusters. We self-consistently include the effect of metallicity on the progenitor age and the initial-to-final mass relation. In contrast with previous investigations that invoked a single white dwarf mass for each cluster, the data shows a spread of white dwarf masses that better reproduce the shape and location of the blue turn. This effect alone, however, does not completely reproduce the observational data -the blue turn retains some mystery. In this context, we discuss several other potential problems in the models. These include possible partial mixing of H and He in the atmosphere of white dwarf stars, the lack of a good physical description of the collision-induced absorption process and uncertainties in the opacities at low temperatures. The latter are already known to be significant in the description of the cool main sequence. Additionally, we find that the present day local mass function of NGC 6397 is consistent with a top-heavy type, while 47 Tuc presents a bottom-heavy profile.

Pulsating white dwarfs: new insights

“…As mentioned, despite the fact that the occurrence of the TP-AGB phase is expected for most of single WD progenitors, it is not discarded that some WDs could have evolved from progenitor stars that avoided this phase. In fact, it is well known that low-mass He-burning stars located at the extreme horizontal branch, and thus characterized by extremely thin H envelopes (Faulkner, 1972), evolve directly to the WD stage, avoiding the AGB (the AGB-Manqué and post early AGB stars; see Caloi, 1989;Brocato et al, 1990;Greggio and Renzini, 1990). In line with this, recent evidence suggests that most He rich stars of the globular cluster NGC2808 do not reach the AGB phase, evolving directly to the WD state after the end of the He core burning (Marino et al, 2017).…”

PopSyCLE: A New Population Synthesis Code for Compact Object Microlensing Events

“…This is justified as these old WDs have absolute magnitudes of M∼18 in redoptical filters and they contribute negligible flux at typical distances for lenses and sources (e.g. Campos et al 2016). Thus a PopSyCLE population will contain both luminous WDs from the MIST models and dark WDs from the K08 IFMR.…”

Evidence of spectral evolution on the white dwarf sample from the Gaia mission

“…So far several works used population synthesis approaches to study properties from several white dwarf populations (e.g. Wood & Oswalt 1998;García-Berro et al 1999;Torres et al 2001Torres et al , 2002García-Berro et al 2004;Chen & Hansen 2012;Torres et al 2013;Campos et al 2016;García-Berro & Oswalt 2016;Tremblay et al 2016). In our work, we perform Monte Carlo population synthesis using as input the white dwarf evolutionary sequences, the SFH and the final mass function (FMF ).…”