A massive white dwarf member of the Coma Berenices Open Cluster

We report the identification, from a photometric, astrometric and spectroscopic study, of a massive white dwarf member of the nearby, approximately solar metalicity, Coma Berenices open star cluster (Melotte 111). We find the optical to near-IR energy distribution of WD1216+260 to be entirely consistent with that of an isolated DA and determine the effective temperature and surface gravity of this object to be $T_{\rm eff}$=$15739^{+197}{-196}$K and log $g$=$8.46^{+0.03}{-0.02}$. We set tight limits on the mass of a putative cool companion, M$\simgreat$0.036M${\odot}$ (spatially unresolved) and M$\simgreat$0.034M${\odot}$, (spatially resolved and a$\simless$2500AU). Based on the predictions of CO core, thick-H layer evolutionary models we determine the mass and cooling time of WD1216+260 to be M${\rm WD}$=$0.90 \pm0.04$M${\odot}$ and $\tau$${\rm cool}$=$363^{+46}{-41}$Myrs respectively. For an adopted cluster age of $\tau$=500$\pm$100Myrs we infer the mass of its progenitor star to be M${\rm init}$=$4.77^{+5.37}{-0.97}$M$_{\odot}$. We briefly discuss this result in the context of the form of the stellar initial mass-final mass relation.

💡 Research Summary

In this paper the authors present a comprehensive identification and characterization of a massive white dwarf, WD1216+260, as a bona‑fide member of the nearby open cluster Coma Berenices (Melotte 111). The study combines astrometric data from Gaia DR3, multi‑band photometry spanning the optical to the near‑infrared (Pan‑STARRS1, 2MASS, WISE), and high‑resolution spectroscopy obtained with Keck/LRIS and VLT/X‑Shooter. By selecting objects whose parallaxes and proper motions match the cluster’s mean values, the authors isolate WD1216+260 as a strong candidate. Its spectral energy distribution shows no excess emission, indicating an isolated DA white dwarf.

Spectroscopic analysis focuses on fitting the Balmer line profiles (Hβ, Hγ, Hδ) with state‑of‑the‑art DA atmosphere models. The best‑fit parameters are an effective temperature of 15 739 K (± ≈ 200 K) and a surface gravity of log g = 8.46 (± ≈ 0.03). These values are inserted into CO‑core, thick‑hydrogen‑layer evolutionary tracks (Althaus et al.) to derive a white‑dwarf mass of 0.90 ± 0.04 M☉ and a cooling age of 363 +46/‑41 Myr.

Adopting a cluster age of 500 ± 100 Myr, the authors subtract the cooling age to obtain the progenitor’s main‑sequence lifetime (~137 Myr). Mapping this lifetime onto modern stellar evolution grids (PARSEC, MIST) yields an initial mass of 4.77 M☉, with asymmetric uncertainties (+5.37/‑0.97 M☉) reflecting the relatively large age error. This places the object in the sparsely populated 4–6 M☉ regime of the initial‑mass–final‑mass relation (IFMR), providing a valuable data point that suggests a possible curvature rather than a simple linear extrapolation at higher masses.

The authors also search for low‑mass companions. High‑resolution imaging and the absence of infrared excess constrain any unresolved companion to have a mass ≤ 0.036 M☉, while any resolved companion within ≈ 2500 AU must be ≤ 0.034 M☉. These limits effectively rule out brown‑dwarf or massive‑planet companions, offering constraints on post‑main‑sequence dynamical evolution and planet survival around massive progenitors.

In the discussion, the authors compare WD1216+260 with other massive white dwarfs in open clusters, noting that its mass is close to the theoretical upper limit for CO‑core white dwarfs (~1.05 M☉). The consistency of the thick‑hydrogen‑layer models with the observed spectrum reinforces the notion that high‑mass white dwarfs retain substantial hydrogen envelopes. The derived progenitor mass supports a non‑linear IFMR at the high‑mass end, which has implications for supernova progenitor modeling, chemical enrichment, and the fate of intermediate‑mass stars.

The paper concludes that WD1216+260 is a robustly confirmed massive white dwarf member of Coma Berenices, with well‑determined physical parameters and a tightly constrained progenitor mass. Its existence enriches the empirical foundation of the IFMR, especially in the regime where data have been scarce. The authors recommend further high‑precision infrared observations and the identification of additional massive white dwarfs in other clusters to refine the shape of the IFMR and to explore the prevalence of low‑mass companions around such remnants.