A Massive Magnetic Helium Atmosphere White Dwarf Binary in a Young Star Cluster

Abstract

We have searched the Gaia DR2 catalogue for previously unknown hot white dwarfs in the direction of young open star clusters. The aim of this experiment was to try and extend the initial-final mass relation (IFMR) to somewhat higher masses, potentially providing a tension with the Chandrasekhar limit currently thought to be around 1.38 M . We discovered a particularly interesting white dwarf in the direction of the young ∼150 Myr old cluster Messier 47 (NGC 2422). All Gaia indicators (proper motion, parallax, location in the Gaia colour-magnitude diagram) suggest that it is a cluster member. Its spectrum, obtained from Gemini South, yields a number of anomalies: it is a DB (helium-rich atmosphere) white dwarf, it has a large magnetic field (2.5 MG), is of high mass (∼1.06 M ) and its colours are very peculiar -particularly the redder ones (r, i, z and y), which suggest that it has a late-type companion. This is the only magnetized, detached binary white dwarf with a non-degenerate companion of any spectral type known in or out of a star cluster. If the white dwarf is a cluster member, as all indicators suggest, its progenitor had a mass just over 6 M . It may, however, be telling an even more interesting story than the one related to the IFMR, one about the origin of stellar magnetic fields, Type I supernovae and gravitational waves from low mass stellar systems.

White dwarf-open cluster associations based on Gaia DR2

“…Our analysis only recovered the WD found by Richer et al (2019), as the other one is not included in the catalog by Gentile Fusillo et al (2019). However, taking advantage of GDR2 astrometry, it can clearly be seen that the WD member candidate of Koester & Reimers (1981) is most certainly not a cluster member and that it lies in the foreground.…”

“…NGC 2422 is a rather young (∼ 150 Myr) OC with a current turnoff age of about 5.4 M (Richer et al 2019). The potential WD content of the cluster was first investigated by Koester & Reimers (1981), who found a potential WD candidate (GDR2 3030026344167186304) that may also be a cluster member.…”

“…Since the advent of Gaia, the knowledge and census of Galactic OCs have also been substantially furthered (e.g., Gaia Collaboration et al 2017;Cantat-Gaudin et al 2018a,b). Furthermore, a large number of new WDs have been discovered and characterized (Gentile Fusillo et al 2019), including WDs in OCs (e.g., Salaris & Bedin 2018Richer et al 2019).…”

Gaia white dwarfs within 40 pc – I. Spectroscopic observations of new candidates

“…The former is cool enough that the atmosphere could be pure-hydrogen, while WD J2305+3922 is helium dominated with a relatively large upper limit on its hydrogen content (log(H/He) < −2.5). The progenitor of WD J2305+3922 could be a massive helium-rich DB white dwarf, which are rare (Tremblay et al 2019a) but occasionally found (Richer et al 2019). Alternatively, it could also have experienced convective mixing earlier in its evolution (Cunningham et al 2020) or could be a cooled-down example of warm DQ white dwarfs (Coutu et al 2019;Koester, Kepler & Irwin 2020) where carbon has diffused downward, for instance, from the shrinking of the convection zone or upwards diffusion of helium.…”

Discovery of a Sirius-like binary system with a very strongly magnetic white dwarf

“…A final entry in Table 3 is the DBA MWD-dM binary recently discovered in the open cluster NGC 2422 by Richer et al (2019). This system is explicitly discussed by the authors as a possible counter-example to the assertion by Liebert et al (2015), although at a distance of 500 pc, it is not completely certain that the system is actually a physical binary, and we will not know if the NGC 2422 binary could be a precursor to a CV system until a good series of radial velocity measurements of the WD is available.…”