Program: GN-2006B-Q-83

Title:Towards a Nebular He II Diagnostic for Starbursts
PI:Sally Oey
Co-I(s): Daniel Schaerer, Paul Crowther

Abstract

Nebular \heii\ \lam4686 emission is seen in 1/3 of all Wolf-Rayet galaxies and offers a powerful diagnostic of the stellar population in distant starburst regions. Both predictions and observations show that its strength increases at low metallicity, and thus it promises to be an especially useful probe for relatively unevolved systems. WR stars are thought to be primarily responsible for the \heii\ emission, but stellar atmosphere model predictions vary greatly, and SNR contributions could be important. Owing to its high ionization potential at 54~eV, \heii\ is an especially sensitive diagnostic of both ionizing stellar energy distributions and young SNRs, as seen in starburst conditions. Our discovery of a dozen \heii\ nebula candidates in M33 at least doubles the number of known WR \heii\ nebulae, and offers a uniform sample of \heii-emitting SNRs. We request spectroscopic follow-up data of both the objects and associated stars. The data will strongly constrain the locus of stellar parameters that defines the boundary for He$^+$-ionizing conditions, and thereby provide greatly-needed empirical constraints for WR and O star model atmospheres. They will also quantify the SNR parameters that produce \heii\ emission, which could be shocks or X-ray photoionization. This will be the first systematic, empirical test of \heii-emitting conditions, to develop its use as a diagnostic for distant, low-metallicity and Pop III starbursts.

Publications using this program's data