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

With three-dimensional hydrodynamical simulations we show that the size of the decretion disc and the structure of the accretion flow on to 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 maximizing the effects of tidal truncation. For low-disc aspect ratio, H/R ≲ 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 on to 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.