Atypical White Dwarfs in Open Clusters

Abstract

Open clusters are powerful laboratories for investigating the formation and evolution of white dwarfs (WDs), as their well-constrained ages link WDs to their progenitor stars. This is particularly valuable for atypical WDs, including magnetic WDs and those with non-hydrogen-dominated spectra, whose origins can be difficult to disentangle in the field. We assemble the largest sample to date of atypical WDs in open clusters: eight non-magnetic WDs with helium-dominated spectra, five with featureless spectra, and five magnetic WDs showing hydrogen or helium lines. We find no significant difference between the initial--final mass relations of hydrogen- and helium-atmosphere WDs, nor robust spatial or kinematic differences. The strongest difference from the field is a deficit of non-DAs between $0.8$ and $1.0\,\mathrm{M}_\odot$, where the field predicts $6.0^{+0.8}_{-0.7}$ WDs, yet none are observed, with a probability of only $0.37\%$. Massive magnetic WDs show a similar deficit: only one of 20 cluster WDs with $M_{\mathrm{WD}}\geq1.0\,\mathrm{M}_\odot$ and $T_{\mathrm{eff}}\geq20{,}000\,{\rm K}$ is magnetic, compared with 10 of 16 in the field. Together, these deficits are consistent with an additional field population of massive merger remnants, particularly high-mass magnetic WDs and warm/hot DQs. Such objects may be underrepresented in clusters because young systems provide limited time for certain classes of delayed mergers, while older clusters lose members or dissolve; the field accumulates binary evolution products over Galactic timescales. Magnetic cluster WDs consistent with single-star evolution further suggest mass-dependent magnetic-field emergence, with fields appearing early in some massive WDs while young lower-mass WDs remain consistent with delayed emergence.