Physics Colloquium: Too Much of a Good Thing: the Curious Case of Lyman Excess HII Regions

This talk will describe the struggle to understand the nature of recently discovered “Lyman Excess” HII regions. These intriguing regions of massive star formation apparently have an additional source of ionizing radiation beyond what would be expected from current models of massive stars. Prosaic origins for the effect, such as errors in distance or luminosity, were explored, but a sizable number of regions could not be explained away in this fashion. Various astrophysical phenomena, such as accretion shocks, outflows, and Balmer continuum ionization, were then examined. Surprisingly, none of these processes were able to explain the excess emission associated with the HII regions. It appears that some adjustment of massive star model atmospheres in the extreme ultraviolet region is necessary—a result only previously hinted at in the few direct observations of massive stars in this wavelength regime.