Mass transfer in white dwarf-neutron star binaries

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Abstract

We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of M-WD, (crit) = 0.2 M-circle dot undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs.
Original languageEnglish
Pages (from-to)3556-3575
Number of pages20
JournalMonthly Notices of the Royal Astronomical Society
Volume467
Issue number3
DOIs
Publication statusPublished - 2017 Jun 1

Subject classification (UKÄ)

  • Astronomy, Astrophysics and Cosmology

Keywords

  • hydrodynamics
  • methods: numerical
  • binaries: close
  • stars: neutron
  • white dwarfs

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