An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant

Abstract

White dwarfs are dense, cooling stellar embers consisting mostly of carbon and oxygen 1 , or oxygen and neon (with a few percent carbon) at higher initial stellar masses 2 . These stellar cores are enveloped by a shell of helium which in turn is usually surrounded by a layer of hydrogen, generally prohibiting direct observation of the interior composition. However, 1 arXiv:2003.00028v1 [astro-ph.SR] 28 Feb 2020 carbon is observed at the surface of a sizeable fraction of white dwarfs 3, 4 , sometimes with traces of oxygen, and it is thought to be dredged-up from the core by a deep helium convection zone 5, 6 . In these objects only traces of hydrogen are found 7, 8 as large masses of hydrogen are predicted to inhibit hydrogen/helium convective mixing within the envelope 9 . We report the identification of WD J055134.612+413531.09, an ultra-massive (1.14 M ) white dwarf with a unique hydrogen/carbon mixed atmosphere (C/H = 0.15 in number ratio). Our analysis of the envelope and interior indicates that the total hydrogen and helium mass fractions must be several orders of magnitude lower than predictions of single star evolution 10 : less than 10 −9.5 and 10 −7.0 , respectively. Due to the fast kinematics (129 ± 5 km s −1 relative to the local standard of rest), large mass, and peculiar envelope composition, we argue that WD J0551+4135 is consistent with formation from the merger of two white dwarfs in a tight binary system [11][12][13][14] .WD J0551+4135 was identified as a candidate high-mass white dwarf 15 from its location in the Gaia Hertzsprung-Russell diagram 16 (Fig. 1). The Gaia parallax places WD J0551+4135 at 46.45 ± 0.15 pc away from the Sun. Compared to most other white dwarfs of the same G BP − G RP colour, WD J0551+4135 is fainter in absolute magnitude, signifying a relatively small radius, and thus a large mass given the mass-radius relation of white dwarfs whose structure is dominated by electron degeneracy 17 .− log(1 − m r /M wd ) = 4 (where m r is the mass enclosed within a sphere of radius r), the models are almost indistinguishable. Therefore the degree of oxygen dredge-up is not found to depend upon the core-composition, and so the atmospheric non-detection of oxygen cannot be used to test the evolutionary history. 6

A catalogue of white dwarfs in Gaia EDR3

“…More recently, Blouin, Daligault & Saumon (2021) reconciled these results showing that a distillation process during 22 Ne phase separation in crystallizing white dwarfs could explain both the cooling delay of standard white dwarfs and the extra delay experienced by high-mass double white dwarf mergers (see also Bauer et al 2020;Camisassa et al 2021). A number of additional studies have focused on the spectral properties of ultramassive white dwarfs, consolidating the idea that many of these systems are the result of double white dwarf mergers (Hollands et al 2020;Kawka, Vennes & Ferrario 2020;Kilic et al 2021).…”

Forever young white dwarfs: when stellar ageing stops

“…ONe core white dwarfs are unable to predict these delays. Moreover, the high percentage of observed carbon-rich atmosphere stars (DQ white dwarfs) on the Q branch 20 supports the hypothesis that a large fraction of the white dwarfs on the Q branch would certainly have been formed through merger events 35 .…”

White Dwarf Subsystems in Core-Collapsed Globular Clusters

“…However, we note that WD J0551+4135 (Gentile Fusillo et al 2019), identified as a potential high-mass, ≈1.15 M e , WD candidate based on its position in the Gaia H-R diagram (Gaia Collaboration et al 2018), occupies a similar location in the H-R diagram to our proposed WD merger remnants (i.e., to the upper left of the standard WD cooling sequence). Hollands et al (2020) argue that the properties of this object are consistent with it having formed from an earlier WD merger. Although WD J0551+4135 is not itself identified in a star cluster, its observed features may provide a clue to the properties of WD merger remnants that may potentially be observed in clusters.…”

“…Although WD J0551+4135 is not itself identified in a star cluster, its observed features may provide a clue to the properties of WD merger remnants that may potentially be observed in clusters. For reference, we show the properties of WD J0551+4135 derived from Hollands et al (2020) as a red star in Figure 10. If indeed a fraction of WD mergers result in young, massive WD remnants, we predict core-collapsed clusters like NGC 6397 may host up to  10 20…”