Decretion disc size in Be/X-ray binaries depends upon the disc aspect ratio

TL;DR

This study uses 3D hydrodynamical simulations to demonstrate that the size and structure of decretion discs in Be/X-ray binaries are strongly influenced by the disc aspect ratio, affecting tidal truncation and accretion flow.

Contribution

It provides new insights into how the disc aspect ratio determines disc truncation, outflow, and accretion processes in Be/X-ray binaries through detailed simulations.

Findings

Low $H/R$ leads to efficient tidal truncation and accretion via streams.

Higher $H/R$ results in larger, less truncated discs that extend to the orbit.

Outflow rate increases with $H/R$, but accretion fraction decreases.

Abstract

With three-dimensional hydrodynamical simulations we show that the size of the decretion disc and the structure of the accretion flow onto the neutron star in a Be/X-ray binary strongly depends upon the disc aspect ratio, $H/R$. We simulate a Be star disc that is coplanar to the orbit of a circularly or moderately eccentric neutron star companion, thereby maximising the effects of tidal truncation. For low disc aspect ratio, $H/R\lesssim 0.1$, the disc is efficiently tidally truncated by the neutron star. Most material that escapes the Roche lobe of the Be star is accreted by the neutron star through tidal streams. For larger disc aspect ratio, the outflow rate through the Be star disc is higher, tidal truncation becomes inefficient, the disc fills the Roche lobe and extends to the orbit of the companion. Some material escapes the binary as a gas stream that begins near the L2 point. While the accretion rate onto the neutron star is higher, the fraction of the outflow that is accreted by the neutron star is smaller. Low density Be star discs are expected to be approximately isothermal, such that $H/R$ increases with radius. Tidal truncation is therefore weaker for larger separation binaries, and lower mass primaries.