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 metallicity, Coma Berenices open star cluster (Melotte 111). We find the optical to near-infrared energy distribution of WD 1216+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_eff = 15 739+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 ≳ 0.036 M⊙ (spatially unresolved) and M ≳ 0.034 M⊙ (spatially resolved and a ≲ 2500 au). Based on the predictions of CO core, thick H layer evolutionary models we determine the mass and cooling time of WD 1216+260 to be M_WD = 0.90 ± 0.04 M⊙ and τ_cool = 363+46−41 Myr, respectively. For an adopted cluster age of τ = 500 ± 100 Myr we infer the mass of its progenitor star to be M_init = 4.77+5.37−0.97 M⊙. We briefly discuss this result in the context of the form of the stellar initial mass–final mass relation.