Date:
Thursday, March 8, 2018, 11:00am to 12:00pm
Location:
Pratt
"Accretion physics in high mass star formation"
The accretion flows that form O and B type share similarities with other accretion flows in astrophysics but have some unique characteristics. Accretion flows around type B stars resemble scaled-up flows around lower mass stars although the disks around B stars are massive enough to be self-gravitating. Nonetheless, observations suggest these disks are stable under Toomre's criterion because of their high temperatures of a few thousand degrees. Type O stars are hot enough to significantly ionize their surroundings including their own accretion flows. These stars often form in groups of several O stars close enough together that their combined mass is effectively a point-source with respect to the larger-scale accretion flows of several thousand AU. The combination of the strong inward gravitational and outward pressure forces creates some interesting and perhaps unexpected dynamics. For example, the ultra-compact HII regions that we observe around clusters of O stars are not simply expanding bubbles of ionized gas but an ionized accretion-outflow system. The accretion flows transition inward from molecular to ionized passing supersonically through an ionization front which slows and compresses the flow such that the ionized region is higher density than the molecular. The magnetically-driven bipolar outflows that are found around low-mass stars are not seen, replaced by pressure-driven outflows of ionized gas. Recent ALMA observations suggest new, interesting, and well-defined problems in theoretical astrophysics.