Merger of a white dwarf–neutron star binary to 10²⁹carat diamonds: origin of the pulsar planets

Abstract

We show that the merger and tidal disruption of a C/O white dwarf (WD) by a neutron star (NS) binary companion provides a natural formation scenario for the PSR B1257+12 planetary system. Starting with initial conditions for the debris disk produced of the disrupted WD, we model its long term viscous evolution, including for the first time the effects of mass and angular momentum loss during the early radiatively inefficient accretion flow (RIAF) phase and accounting for the unusual C/O composition on the disk opacity. For plausible values of the disk viscosity α ∼ 10 −3 − 10 −2 and the RIAF mass loss efficiency, we find that the disk mass remaining near the planet formation radius at the time of solid condensation is sufficient to explain the pulsar planets. Rapid rocky planet formation via gravitational instability of the solid carbon-dominated disk is facilitated by the suppression of vertical shear instabilities due to the high solid-to-gas ratio. Additional evidence supporting a WD-NS merger scenario includes (1) the low observed occurrence rate of pulsar planets ( 1% of NS birth), comparable to the expected WD-NS merger rate; (2) accretion by the NS during the RIAF phase is sufficient to spin PSR B1257+12 up to its observed 6 ms period; (3) similar models of 'low angular momentum' disks, such as those produced from supernova fallback, find insufficient mass reaching the planet formation radius. The unusually high space velocity of PSR B1257+12 of 326 km s −1 suggests a possible connection to the Calcium-rich transients, dim supernovae which occur in the outskirts of their host galaxies and were proposed to result from mergers of WD-NS binaries receiving SN kicks. The C/O disk composition implied by our model likely results in carbon-rich planets with diamond interiors.

Two New Calcium-rich Gap Transients in Group and Cluster Environments

“…The general lack of underlying/faint host systems is consistent with previous findings (Lyman et al 2014(Lyman et al , 2016, and has been used to argue that Ca-rich gap transients do not form "in situ" at these remote locations, but have traveled to their explosion sites. One attractive property of the NS-WD merger scenario (Metzger 2012;Sell et al 2015;Margalit & Metzger 2017) is that the binary would have received a kick in the SN explosion that creates the neutron star; such SN kicks are indeed also evoked to explain the offset distribution of sGRBs, which is consistent with theoretical predictions for NS-NS mergers . In this context, it is interesting to note that the offset distribution for Ca-rich gap transients is significantly more extreme than that of sGRBs (Section 4.2; Figure 11).…”

Exoplanet recycling in massive white-dwarf debris discs

“…( 46)), indicating that the recycling mechanism as presented here is not responsible for them. We note, however, that one of the suggested formation scenarios for the pulsar planets involves the tidal disruption of a companion star (e.g., Phinney & Hansen 1993;Currie & Hansen 2007;Margalit & Metzger 2017), bearing some resemblance to the recycling scenario presented in this paper.…”

Nuclear-dominated accretion flows in two dimensions – II. Ejecta dynamics and nucleosynthesis for CO and ONe white dwarfs

“…Outflow velocities were found to be ∼ 10 4 km s −1 , with ∼ 10 −3 M of radioactive 56 Ni produced. At very late times, these disks could in principle be a formation site of planets around the NS (Margalit & Metzger 2017).…”