Accretion Disk Assembly During Common Envelope Evolution: Implications for Feedback and LIGO Binary Black Hole Formation

Abstract

During a common envelope episode in a binary system, the engulfed companion spirals to tighter orbital separations under the influence of drag from the surrounding envelope material. As this object sweeps through material with a steep radial gradient of density, net angular momentum is introduced into the flow, potentially leading to the formation of an accretion disk. The presence of a disk would have dramatic consequences for the outcome of the interaction because accretion might be accompanied by strong, polar outflows with enough energy to unbind the entire envelope. Without a detailed understanding of the necessary conditions for disk formation during common envelope, therefore, it is difficult to accurately predict the population of merging compact binaries. This paper examines the conditions for disk formation around objects embedded within common envelopes using the 'wind tunnel' formalism developed by MacLeod et al. (2017). We find that the formation of disks is highly dependent on the compressibility of the envelope material. Disks form only in the most compressible of stellar envelope gas, found in envelopes' outer layers in zones of partial ionization. These zones are largest in low-mass stellar envelopes, but comprise small portions of the envelope mass and radius in all cases. We conclude that disk formation and associated accretion feedback in common envelope is rare, and if it occurs, transitory. The implication for LIGO black hole binary assembly is that by avoiding strong accretion feedback, common envelope interactions should still result in the substantial orbital tightening needed to produce merging binaries.

The Evolution of Binaries in a Gaseous Medium: Three-dimensional Simulations of Binary Bondi–Hoyle–Lyttleton Accretion

“…In this paper, we use a very similar numerical approach to extend the efforts of Ruffert & Arnett (1994), Ruffert (1995Ruffert ( , 1996Ruffert ( , 1999, Blondin & Raymer (2012), MacLeod & Ramirez-Ruiz (2015, MacLeod et al (2017), andMurguia-Berthier et al (2017) to binary systems by simulating two sink particles orbiting within a supersonic BHL flow. While methodologically distinct, we note that a similar scenario has been simulated in 3D by Farris et al (2010) in the ultrarelativistic regime in which a pair of black holes decays rapidly to merger while traversing through a surrounding gaseous medium.…”

Formation of Double Neutron Star Systems

“…the B-field and the spin evolution of the NS. The spin-up, on the other hand, is expected to be most efficient during Case BB RLO where an accretion disk is always present (unlike the situation in a CE, Murguia-Berthier et al 2017), leading to a strong and stable accretion torque acting on the NS. Hence, the spin period of the recycled pulsar is primarily determined by the amount of accreted material during Case BB RLO.…”

Nucleosynthetic yields from neutron stars accreting in binary common envelopes

“…Calculations of Bondi-Hoyle accretion allow refinement of the value for λ BHL and determination of the accuracy of our solution to include the different velocity terms (Ruffert 1994a). In scenarios like our CE phase, there is both a velocity and density gradient across the Bondi radius, and these features drive instabilities in the accretion that can decrease the accretion rate (Ruffert & Arnett 1994;Ruffert 1994b;MacLeod & Ramirez-Ruiz 2014;MacLeod & Ramirez-Ruiz 2015;MacLeod et al 2017;Murguia-Berthier et al 2017). Bondi accretion also assumes that matter falling onto the neutron star accretes passively onto the neutron star.…”

“…If the material has enough angular momentum, it can form a disk that could ultimately drive a jet. This is believed to be rare in most CE scenarios (Murguia-Berthier et al 2017). For neutron star systems, even if the material does not have a sufficient angular momentum to form a disk, some of the accreting material will be reheated and ejected (Fryer et al 2006;Fryer 2009).…”