PI: Roman Rafikov

Project dp326, which started in April of 2024, 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. Our study 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 characteristic pattern of evolution 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.

In the present study we focused on studying the effect of the inner structure of the accretor (the star) on the particular mix of the modes excited in the boundary layer and on the efficiency of mass and angular momentum transport. Almost all existing studies of acoustic modes in the boundary layer assume isothermal equation of state, which results in stellar density steeply rising with depth inside the star. In our present study we relaxed this assumption and considered more realistic assumptions about the stellar structure, namely a polytropic equation of state with varying polytropic index. To carry out our investigation, we ran a suite of two- and three-dimensional high-resolution hydrodynamic simulations with the Godunov-type code Athena++ on Durham super-computing facility. By analyzing the results of our simulations, we found that stars with stiffer equation of state have mix of modes which is generally quite different from the modes excited when an accretor has a softer equation of state. Moreover, we observed a clear trend of the mass and angular momentum transport being more effective in boundary layers with softer equation of state, which have higher mass inside the accretor in our simulation domain. The results of this study are being prepared for publication.