NSF Award
1515763
This study will explore a tantalizing new mechanism for explaining activity within the region between an inward flow of material and the central object onto which it is falling. Detailed numerical work including for the first time all of the relevant physical ingredients will predict what should be observed. The project promises finally to make progress after a long time spent on ideas that simply did not work, and it will train students in this cutting-edge research topic.<br/><br/>Astrophysical accretion often proceeds onto a central object with a surface, in which case a boundary layer forms at the inner edge of the disk. Energy release in this layer dramatically affects the spectra and variability of accreting objects, which requires a good working model of the boundary. This project starts with a recently discovered important new mechanism for angular momentum and mass transport across this region. The transport mechanism comes from supersonic shear flows that give rise to global acoustic waves that dissipate in weak shocks. It appears to be very efficient and robust, and needs to be further studied. This work will identify and explore the properties of acoustic modes in fully three-dimensional, magneto-hydrodynamic (MHD) boundary layers, in which the effects of radiation transfer and/or radiation pressure are important. The study involves detailed simulations using a three-dimensional (3D) MHD code and careful analysis of the results, trying to find simple physically motivated models of transport driven by acoustic modes. The culmination of the work is a set of simulations combining these previously explored physical ingredients. This parameter space exploration to understand the global geometry and characteristics of the boundary layer has never been done before, and will lead to direct prediction of observable quantities. These calculations will be the first self-consistent model of the structure and evolution of boundary layers not relying on ad-hoc angular momentum transport prescriptions.<br/><br/>The project will make special efforts to recruit students from minority and underrepresented groups, training personnel highly qualified for future success. The simulations will result in visualizations appropriate for the general public, for public talks and general outreach purposes.
