Further investigation of white dwarfs in the open clusters NGC 2287 and NGC 3532

Abstract

We report the results of a CCD imaging survey, complemented by astrometric and spectroscopic follow‐up studies, that aims to probe the fate of heavy‐weight intermediate‐mass stars by unearthing new, faint, white dwarf members of the rich, nearby, intermediate‐age open clusters NGC 3532 and NGC 2287. We identify a total of four white dwarfs with distances, proper motions and cooling times which can be reconciled with membership of these populations. We find that WD J0643−203 in NGC 2287, with an estimated mass of M= 1.02–1.16 M⊙, is potentially the most massive white dwarf so far identified within an open cluster. Guided by the predictions of modern theoretical models of the late‐stage evolution of heavy‐weight intermediate‐mass stars, we conclude that there is a distinct possibility that it has a core composed of O and Ne. We also determine that despite the cooling times of the three new white dwarfs in NGC 3532 and the previously known degenerate member NGC 3532−10 spanning ∼90 Myr, they all have remarkably similar masses (M∼ 0.9–1 M⊙). This is fully consistent with the results from our previous work on a heterogeneous sample of ∼50 white dwarfs from 12 stellar populations, on the basis of which we argued that the stellar initial–final mass relation is less steep at Minit > 4 M⊙ than in the adjacent lower initial mass regime. This change in the gradient of the relation could account for the secondary peak observed in the mass distribution of the field white dwarf population and mitigate the need to invoke close binary evolution to explain its existence. Spectroscopic investigation of numerous additional candidate white dwarf members of NGC 3532 unearthed by a recent independent study would be useful to confirm (or otherwise) these conclusions.

A search for white dwarfs in the Galactic plane: the field and the open cluster population

“…This is the only cluster in our list with known white dwarf members (seven; Reimers & Koester 1989;Koester & Reimers 1993;Dobbie et al 2009Dobbie et al , 2012. In the most recent photometric study of the cluster, Clem et al (2011) confirmed a distance of 492 ± 12 pc and an age of 300 ± 100 Myr.…”

“…Table 1). Red squares mark the positions of the white dwarf candidates, blue circles show the confirmed white dwarfs in NGC 3532 (Dobbie et al 2012). …”

“…The fluxes of model spectra On the left, we represent the colour cuts we applied to select white dwarf candidates (black crosses) as a grey shaded area. The stars we followed up are overplotted as red squares with error bars, and the known cluster white dwarfs (Dobbie et al 2012) with VPHAS+ DR2 photometry are represented by blue squares with error bars. The main sequence (Drew et al 2014) is shown as a black curve, which is displaced by three different magnitudes of interstellar reddening, i.e.…”

“…The two panels of Fig. 10 may not be directly comparable, as the cluster ages that we used are typically determined from evolutionary (Dobbie et al , 2012 and the highest-mass white dwarf, VPHAS J1103−5837, are plotted. The initial-to-final mass relations from the literature are depicted as in Fig.…”

“…There are presumably only three open clusters approaching a fully retrieved white dwarf cooling sequence: the Pleiades (Wegner, Reid & McMahan 1991;Dobbie et al 2006), the Hyades (Schilbach & Röser 2012) and Praesepe ), which are all three nearby (d < 200 pc) and above the densest regions of the Galactic plane (|b| > 10 deg). While some new cluster members were discovered recently (Dobbie et al 2012;Cummings et al 2015, in NGC 3532 and M 37, respectively), several observational factors have worked against the identification of complete white dwarf populations. First, most clusters are in crowded, reddened areas of the Galactic plane.…”