Gemini GMOS spectroscopy of HeII nebulae in M33

C.Kehrig (1,2), M.S.Oey (2),P.A.Crowther (3), J.Fogel (2),E.Pellegrini (2), O.Schnurr (1,3), D. Schaerer (4), P. Massey (5) and K. Roth (6)

1 - Astrophysikalisches Institut Potsdam (AIP), innoFSPEC Potsdam, Germany
2 - Department of Astronomy, University of Michigan, Ann Arbor,MI 48109, USA
3 - Department of Physics and Astronomy, University of Sheffield, UK
4 - Geneva Observatory, University of Geneva, Switzerland
5 - Lowell Observatory, Flagstaff, AZ 86001, USA
6 - Gemini Observatory, Hilo, HI 96720, USA

We have carried out a narrow-band survey of the Local Group galaxy, M33, in the HeII4686 emission line, to identify HeII nebulae in this galaxy. With spectroscopic follow-up
observations, we confirm three of seven candidate objects, including identification of two new HeII nebulae, BCLMP651, HBW673. We also obtain spectra of associated ionizing stars for all the HII regions,identifying two new WN stars. We demonstrate that the ionizing source for the known HeII nebula, MA 1, is consistent with being the early-type WN star MC8 (M33-WR14), by carrying out a combined stellar and nebular analysis of MC8 and MA1. We were unable to identify the helium ionizing sources for HBW673 and BCLMP651, which do not appear to be Wolf-Rayet stars. According to the [OIII]5007/Hbeta vs [NII]6584/Halpha diagnostic diagram, excitation mechanisms apart from hot stellar continuum are needed to account for the nebular emission in HBW673, which appears to have no stellar source at all.

Reference: A&A, in press (arXiv:1011.2430)
Status: Manuscript has been accepted


Comments: The manuscript is accepted for publication in section 4. Extragalactic astronomy of Astronomy and Astrophysics.