PI: Sally Oey, Lowell Observatory, firstname.lastname@example.org
Address: 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA
CoI: Daniel Schaerer, Obs. de Geneve
Title: Search for He II emission nebulae from WR and O stars
Abstract: Nebular \heii emission is seen in 1/3 of all WR galaxies and offers a powerful diagnostic of the stellar population in distant star-forming regions. Numerous modeling studies show that its strength increases in low-metallicity systems, and thus it promises to be an especially useful probe for high-redshift and unevolved systems. However, although stellar atmosphere models predict nebular \heii emission to be associated with almost all early-type WR stars, and some O stars, it is perplexing that only 6 \heii nebulae have been identified to date. Owing to its high ionization potential at 54 eV, \heii is an especially sensitive diagnostic of the ionizing stellar energy distribution. We propose to conduct a straightforward imaging search for \heii\ (lambda)4686 emission in the Local Group galaxies M33 and NGC 6822. These 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 take a major step in calibrating \heii as a nebular diagnostic of distant star-forming regions, and allow future follow-up with spectroscopic observations.
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