On the Accretion-Fed Growth of Neutron Stars During Common Envelope

TL;DR

This paper models neutron star growth during common envelope phases, showing that density gradients limit accretion and allow NS survival, impacting binary evolution and gravitational wave event predictions.

Contribution

It revisits neutron star accretion during common envelope episodes, incorporating density gradient effects to demonstrate NS survival with modest mass gain.

Findings

Neutron stars accrete less than 0.1 solar masses during CE.

Density gradients impose angular momentum barriers, limiting accretion.

NSs are likely to survive CE events, affecting binary merger rates.

Abstract

This paper models the orbital inspiral of a neutron star (NS) through the envelope of its giant-branch companion during a common envelope (CE) episode. These CE episodes are necessary to produce close pairs of NSs that can inspiral and merge due to gravitational wave losses in less than a Hubble time. Because cooling by neutrinos can be very efficient, NSs have been predicted to accumulate significant mass during CE events, perhaps enough to lead them to collapse to black holes. We revisit this conclusion with the additional consideration of CE structure, particularly density gradients across the embedded NS's accretion radius. This work is informed by our recent numerical simulations that find that the presence of a density gradient strongly limits accretion by imposing a net angular momentum to the flow around the NS. Our calculations suggest that NSs should survive CE encounters. They accrete only modest amounts of envelope material, $\lesssim 0.1M_\odot$, which is broadly consistent with mass determinations of double NS binaries. With less mass gain, NSs must spiral deeper to eject their CE, leading to a potential increase in mergers. The survival of NSs in CE events has implications for the formation mechanism of observed double NS binaries, as well as for predicted rates of NS binary gravitational wave inspirals and their electromagnetic counterparts.