Stratified Global MHD Models of Accretion Disks in Semi-Detached Binaries

TL;DR

This paper presents the first global MHD simulations of accretion disks in semi-detached binaries, revealing the complex interplay of spiral shocks, MRI turbulence, and accretion dynamics with implications for understanding binary accretion processes.

Contribution

It introduces global, stratified MHD models of Roche lobe overflow disks, highlighting features like spiral shocks and MRI turbulence that are not captured in local models.

Findings

Spiral shocks are inclined and vary rapidly, but their average position fits linear models.

MRI turbulence produces irregular butterfly diagrams, influenced by spiral structures.

Global models reveal features absent in local simulations, emphasizing their importance.

Abstract

We present results of the first global magnetohydrodynamic (MHD) simulations of accretion disks fed by Roche lobe overflow, including vertical stratification, in order to investigate the roles of spiral shocks, magnetorotational instability (MRI), and the accretion stream on disk structure and evolution. Our models include a simple treatment of gas thermodynamics, with orbital Mach numbers at the inner edge of the disk $M_{\rm in}$ of 5 and 10. We find mass accretion rates to vary considerably on all time scales, with only the Mach 5 model reaching a clear quasi-stationary state. For Mach 10, the model undergoes an outside-in magnetically-driven accretion event occurring on a time scale of $\sim10$ orbital periods of the binary. Both models exhibit spiral shocks inclined with respect to the binary plane, with their position and inclination changing rapidly. However, the time-averaged location of these shocks in the equatorial plane is well-fit by simple linear models. MRI turbulence in the disk generates toroidal magnetic field patterns (butterfly diagrams) that are in some cases irregular, perhaps due to interaction with spiral structure. While many of our results are in good agreement with local studies, we find some features (most notably those related to spiral shocks) can only be captured in global models such as studied here. Thus, while global studies remain computationally expensive -- even as idealized models -- they are essential (along with more sophisticated treatment of radiation transport and disk thermodynamics) for furthering our understanding of accretion in binary systems.