A highly magnetized and rapidly rotating white dwarf as small as the Moon

Abstract

in binaries [1][2][3] . If the orbital period of the binary is short enough, energy losses from gravitational wave radiation can shrink the orbit until the two white dwarfs come into contact and merge 4 . Depending on the masses of the coalescing white dwarfs, the merger can lead to a supernova of type Ia, or it can give birth to a massive white dwarf 5 . In the latter case, the white dwarf remnant is expected to be highly magnetised 6,7 due to the strong dynamo that may arise during the merger, and rapidly rotating due to conservation of the orbital angular momentum of the binary 8 . Here we report the discovery of a white dwarf, ZTF J190132.9+145808.7, which presents all these properties, but to an extreme: a rotation period of 6.94 minutes, one of the shortest measured for an isolated white dwarf 9, 10 , a magnetic field ranging between 600 MG and 900 MG over its surface, one of the highest fields ever detected on a white dwarf 11 , and a stellar radius of 1810 km, slightly larger than the radius of the Moon. Such a small radius implies the star's mass is the closest ever detected to the white dwarf maximum mass, or Chandrasekhar mass 12 . In fact, as the white dwarf cools and its composition stratifies, it may become unstable and collapse due to electron capture, exploding into a thermonuclear supernova or collapsing into a neutron star. Neutron stars born in this fashion could account for ∼10% of their total population.The Zwicky Transient Facility [13][14][15] (ZTF) is a synoptic optical survey that uses the 48-inch Samuel Oschin Telescope of the Palomar Observatory to image the Northern sky on a regular basis.We discovered ZTF J190132.9+145808.7 (hereafter ZTF J1901+1458) during a search for fast variability in massive white dwarfs. To this end, we selected objects in a white dwarf catalogue 16 that lie below the main white dwarf cooling sequence in the Gaia 17 color-magnitude diagram (CMD,

Cooling Models for the Most Massive White Dwarfs

“…The downward triangles mark when 10 % of the mass is solid while the upward triangles mark when 90% of the mass is solid. The large star shows ZTF J190132.9+145808.7 (Caiazzo et al 2021) and the small star shows WD J183202.83+085636.24 (Pshirkov et al 2020).…”

“…In addition to controlling when the latent heat of the phase transition is released, once material becomes solid, the gravitational potential energy stored in neutron-rich isotopes can no longer be released through their preferential transport towards the center. Realizing a scenario where a massive WD is meta-stable and will exceed its effective Chandrasekhar mass due to sedimentation, such as is speculated by Caiazzo et al (2021) for ZTF J190132.9+145808.7, requires rapid and near-complete sedimentation.…”

“…As an example, we apply our models to ZTF J190132.9+145808.7 (Caiazzo et al 2021) 5, the best fit parameters place ZTF J190132.9+145808.7 in a region where neutrinos produced through the Urca process have accelerated the cooling of the WD.…”

“…As our sample of nearby WDs becomes increasing complete, thanks in large part to Gaia (Gaia Collaboration et al 2018;Gentile Fusillo et al 2019), a significant number of white dwarfs with masses 1.3 M are being revealed and better described. These include longstudied objects like GD 50 (Bergeron et al 1991;Gagné et al 2018) and RE J0317-853 (Barstow et al 1995;Ferrario et al 1997;Külebi et al 2010) as well more recently characterized objects like WD J183202.83+085636.24 (Pshirkov et al 2020) and ZTF J190132.9+145808.7 (Caiazzo et al 2021). In their analysis of the 100 pc sample from the Montreal White Dwarf Database, Kilic et al (2021) identify 25 WDs with masses > 1.3 M (assuming H atmospheres and C/O cores).…”

The evolution of ultra-massive carbon oxygen white dwarfs

“…These stars play a key role in ★ The cooling sequences are publicly available for download at http://evolgroup.fcaglp.unlp.edu.ar/TRACKS/tracks.html † E-mail: maria.camisassa@colorado.edu our understanding of type Ia Supernova explosions, the occurrence of physical processes in the asymptotic giant-branch (AGB) phase, the existence of high-field magnetic white dwarfs, and the occurrence of double white dwarf mergers (Dunlap & Clemens 2015;Reindl et al 2020). The observation of ultra-massive white dwarfs has been reported in several studies in the literature (Mukadam et al 2004;Nitta et al 2016;Gianninas et al 2011;Kleinman et al 2013;Bours et al 2015;Kepler et al 2016;Curd et al 2017;Kilic et al 2021;Hollands et al 2020;Caiazzo et al 2021;Miller et al 2021).…”