Speaker
Eric Keto
(Harvard-Smithsonian Center for Astrophysics)
Description
HII regions are the clearest sign of the formation of the most massive stars, and ionization feedback could be responsible for a number of other phenomena observed in the ISM. For example, the general field of turbulence is suspected to be driven, at least in part, by the expansion of HII regions. Ionization feedback may also be responsible, along with stellar winds, for the bubbles seen everywhere in the Spitzer mid-IR survey of the Galactic disk (Churchwell 2008, mid-IR GLIMPSE survey). Ionized outflows with observed velocities too low to be consistent with stellar winds (Jaffee and Martin-Pintado 1999) could be expanding HII regions (Keto et al 2008) which themselves drive large-scale molecular outflows (Peters et al 2012) whose size and morphology (Klaassen et al 2011) are inconsistent with scaled up versions of the bipolar outflows of lower mass stars. What do we know about the dynamics of ionization feedback and the evolution of HII regions? Recent observational (Keto and Wood 2006) and numerical studies (Peters et al 2010) suggest that the dynamics of HII regions cannot be understood in isolation, but only in the environment of the accretion flows that form the massive stars or clusters. To further investigate the dynamics of accretion with ionization feedback, I put together a theoretical, analytical model for the evolution of an HII region in the thick disk of an accretion flow of the type thought to form massive stars. This model expands upon the earlier studies of Hollenbach et al 1994, Lugo et al 2004 by using the jump conditions to calculate the dynamics of the boundary of the HII region within the accretion flow. The model shows that the thick disk confines the ionized flow into a cometary or bipolar HII region, the two most common morphological types (De Pree et al 2005) depending on whether the ionizing is on or off the mid plane of the disk. The pressure in the ionized flow is always less than in the molecular disk so that gas from the disk crosses the ionization front and replenishes the outflowing mass. The HII region can be in steady-state and long-lived as long as the star maintains the same level of ionization and the accretion flow provides the same molecular density. The outflow velocities are supersonic reaching Mach numbers of 4 or 5 on relevant length scales. The acceleration is provided by the density gradient maintained in the flow by the gravitational pull of the massive protostar and disk. These simple analytic calculations elucidate interesting details on the dynamics of the flow and the shock and ionization fronts that may not be obvious in complex numerical calculations.
Author
Eric Keto
(Harvard-Smithsonian Center for Astrophysics)