The rate of WD-WD head-on collisions may be as high as the SNe Ia rate

TL;DR

This study shows that WD-WD binaries with distant stellar companions have a few percent chance of colliding within 5 billion years, potentially explaining the rate of Type Ia supernovae and predicting unique gravitational wave signatures.

Contribution

It demonstrates that WD-WD collisions induced by triple systems can occur at rates comparable to SNe Ia, offering a new possible origin for these supernovae.

Findings

A few percent of WD-WD binaries with outer perturbers collide within 5 Gyr.

Collisions occur with high velocities >3000 km/s, likely causing detonations.

Collision rates could match the observed SNe Ia rate if such triples are common.

Abstract

We show that a White Dwarf-White Dwarf (WD-WD) binary with semi-major axis a=1-300 AU, which is orbited by a stellar mass outer perturber with a moderate pericenter r_{p, out} \sim 3-10 x a, has a few percent chance of experiencing a head-on collision within ~5 Gyr. Such a perturber is sufficiently distant to allow the triple system to remain intact for millions of orbits while efficiently exchanging angular momentum with the WD-WD binary. In ~ 5% of the initial orientations, the inner orbit efficiently scans the (equal energy) phase space in the region of zero angular momentum. In these systems, the binary experiences increasingly closer, stochastic, pericenter approaches r_p ~ a/2N with the increasing number (N) of orbits elapsed. Within N~10^5(a/30AU) orbits, a collision is likely to occur. This is shown by performing \simten thousand 3-body integrations and is explained by simple analytic arguments. The collisions are conservatively restricted to "clean" collisions in which all passages prior to the collision are greater than 4R_WD=4x10^9cm. In particular, within the last single orbit, the pericenter changes from r_p>4R_WD to a collision value of r_p<2R_WD. The effects of tidal deformations and General Relativistic (GR) corrections are negligible in these scenarios. The WDs approach each other with a high velocity >3000 km/s and the collision is likely to detonate the WDs leading to a type Ia SNe. If a significant fraction of WDs reside in such triples, the rate of such collisions is as high as the SNe Ia rate, and it is possible that some or all type Ia SNe occur in this way. Such SNe have a unique gravitational wave signature, which will allow a decisive identification in the future.