PI: Roman Rafikov

A current stage of the Project dp326, which started in April of 2025, is devoted to exploring the properties of the boundary layers – narrow interface regions at the inner edges of accretion discs, where the rapidly rotating gas gets slowed down before joining the star. As before, our numerical investigation focuses on exploring a particular mechanism of angular momentum transport in the boundary layer that is responsible for slowing down the incoming matter – excitation of acoustic modes in the supersonic shear layer between the disc and the star (see figure for several characteristic spatial patterns of these waves). This mechanism has been recently shown to efficiently transport angular momentum in the inner pars of the disc even in the absence of any other transport mechanisms. This time, our use of DiRAC computing time was focused on studying the effect of the internal structure of the accreting objects on the properties of the acoustic modes excited in the boundary layer.

We ran and analyzed a suite of 2D and 3D simulations of the acoustic mode-mediated boundary layers with varied internal structures of the accretor using a modern finite-volume hydrodynamic code Athena++. We found certain differences in the mix of the modes excited in the boundary layer (different modes are illustrated in the figure) depending on the accretor’s density distribution. A clear trend of the mass accretion rate and the angular momentum transport efficiency across the boundary layer with the softness of the equation of state determining the inner structure of the accretor has been established in this study. The work detailing these finding and acknowledging the DiRAC support has been published in the Monthly Notices of the Royal Astronomical Society in 2025.