UNC-CH Physics and Astronomy Senior Honors Thesis Defense

Maggie Hilderbran

“Numerical Simulations of the Ram Pressure-Driven Rayleigh-Taylor Instability”

Star formation in the Galactic disk is fueled by continuous accretion of gas from the Galactic halo. This infalling gas is usually identified in the form of high-velocity clouds (HVCs), so named because their velocities do not match a standard Galactic rotation pattern. As they pass through the halo, HVCs are disrupted by a series of fluid instabilities. The Rayleigh-Taylor instability (RTI), driven by ram pressure, occurs at the interface between the cold, dense cloud and the hot, diffuse halo. I perform a systematic investigation of the RTI under conditions relevant to HVCs in the halo, exploring how the instability contributes to the breakup of these clouds. I use the grid-based fluid dynamics code Athena (Stone et al. 2008) to solve the equations of ideal magnetohydrodynamics, simulating the instability at the halo-cloud interface. Step-by-step addition of radiative processes and magnetic fields provides an understanding of their effects on the evolution of the RTI. Cooling causes rapid fragmentation of the interface and dampens the instability, in line with Vietri et al. (1997). The addition of a uniform magnetic field suppresses the instability in two dimensions but not in three dimensions, consistent with Stone & Gardiner (2007). In contrast, tangled magnetic fields, which may be a more realistic model, produce results more like the hydrodynamical case. Because the plane-parallel geometry I assume does not take into account mass flow around (and away) from the interface, the instability growth found in my models is likely an upper limit compared to more realistic situations.