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Global MHD Simulations of Accretion Disks in Cataclysmic Variables (CVs): I. The Importance of Spiral Shocks

We present results from the first global 3D MHD simulations of accretion disks in Cataclysmic Variable (CV) systems in order to investigate the relative importance of angular momentum transport via turbulence driven by the magnetorotational instability (MRI) compared to that driven by spiral shock waves. Remarkably, we find that even with vigorous MRI turbulence, spiral shocks are an important component to the overall angular momentum budget, at least when temperatures in the disk are high (so that Mach numbers are low). In order to understand the excitation, propagation, and damping of spiral density waves in our simulations more carefully, we perform a series of 2D global hydrodynamical simulations with various equation of states and both with and without mass inflow via the Lagrangian point (L1). Compared with previous similar studies, we find the following new results. 1) Linear wave dispersion relation fits the pitch angles of spiral density waves very well. 2) We demonstrate explicitly that mass accretion is driven by the deposition of negative angular momentum carried by the waves when they dissipate in shocks. 3) Using Reynolds stress scaled by gas pressure to represent the effective angular momentum transport rate alpha_{eff} is not accurate when mass accretion is driven by non-axisymmetric shocks. 4) Using the mass accretion rate measured in our simulations to directly measure alpha defined in standard thin-disk theory, we find 0.02 < alpha_{eff} < 0.05 for CV disks, consistent with observed values in quiescent states of dwarf novae (DNe). In this regime the disk may be too cool and neutral for the MRI to operate and spiral shocks are a possible accretion mechanism. However, we caution that our simulations use unrealistically low Mach numbers in this regime, and therefore future models with more realistic thermodynamics and non-ideal MHD are warranted.