Hydrodynamic 3D Simulation of Roche Lobe Overflow in High-mass X-Ray Binaries

TL;DR

This paper introduces a high-resolution hydrodynamic simulation method for Roche lobe overflow in high-mass X-ray binaries, revealing new insights into mass transfer processes and their implications for binary evolution.

Contribution

It presents a novel simulation approach that models RLO systems across multiple regimes without relying on universal assumptions, applied to M33 X-7 to compare with observations.

Findings

Extreme overflow (f=1.1) is conservative in mass and angular momentum transfer.

Nonconservative stable mass transfer occurs at f=1.01, with distinct stream and disk characteristics.

Existing assumptions about RLO binaries may misrepresent their role as progenitors of BBHs.

Abstract

While binary merger events have been an active area of study in both simulations and observational work, the formation channels by which a high-mass star extends from Roche lobe overflow (RLO) in a decaying orbit of a black-hole (BH) companion to a binary black-hole (BBH) system merits further investigation. Variable length-scales must be employed to accurately represent the dynamical fluid transfer and morphological development of the primary star as it conforms to a diminishing Roche lobe under the runaway influence of the proximal BH. We have simulated and evolved binary mass flow under these conditions to better identify the key transitional processes from RLO to BBHs. We demonstrate a new methodology to model RLO systems to unprecedented resolution simultaneously across the envelope, donor wind, tidal stream, and accretion disk regimes without reliance upon previously universal symmetry, mass flux, and angular momentum flux assumptions. We have applied this method to the semidetached high-mass X-ray binary M33 X-7 in order to provide a direct comparison to recent observations of an RLO candidate system at two overflow states of overfilling factors f = 1.01 and f = 1.1. We found extreme overflow (f = 1.1) to be entirely conservative in both mass and angular momentum transport, forming a conical L1 tidal stream of density and deflected angle comparable to existing predictions. This case lies within the unstable mass transfer (MT) regime as recently proposed of M33 X-7. The f = 1.01 case differed in stream geometry, accretion disk size, and efficiency, demonstrating nonconservative stable MT through a ballistic uniform-width stream. The nonconservative and stable nature of the f = 1.01 case MT also suggests that existing assumptions of semidetached binaries undergoing RLO may mischaracterize their role and distribution as progenitors of BBHs and common envelopes.