Asymmetric Accretion Flows within a Common Envelope

TL;DR

This study investigates how density gradients within a common envelope affect accretion and drag on embedded objects, revealing that accretion rates are significantly reduced compared to traditional Hoyle-Lyttleton predictions.

Contribution

It provides the first detailed analysis of the impact of radial density gradients on accretion flows in common envelope phases, with new fitting formulae for rates.

Findings

Accretion rates drop by 1-2 orders of magnitude due to density gradients.

Drag rates are only mildly affected by the gradients.

Objects like black holes and neutron stars may grow less mass than previously estimated.

Abstract

This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). Flow in the vicinity of the embedded object is gravitationally focused leading to drag and potential to gas accretion. This process has been studied numerically and analytically in the context of Hoyle-Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of 3D HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by 1-2 orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically-derived rates of drag and accretion as a function of the density gradient. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.