The HE II Emitting Nebula N44C in the Large Magellanic Cloud: Optical/Ultraviolet Spectroscopy of the Nebula and Its Ionizing Star
Abstract
We present HST spectroscopy and imaging, along with new ground-based spectroscopy and ROSAT HRI imaging, of the He II emitting nebula N44C and its ionizing star. A GHRS spectrogram of the ionizing star yields a spectral type of about O7 for the star. The lack of P Cygni profiles for Si IV and C IV indicates that the star is not a supergiant. The nebular abundances in the ionized gas are consistent with average abundances for LMC H II regions, with the possible exception that nitrogen may be enhanced. Enrichment by a former evolved companion star is not evident. A long-slit echelle spectrogram in Hα+[N II] shows no evidence for high-velocity gas in N44C. This rules out high-velocity shocks as the source of the nebular He II emission. A 108 ks ROSAT HRI image of N44C shows no X-ray point source to a 3 σ upper limit LX<1034 ergs s-1 in the 0.1-2.0 keV band. Based on new measurements of the electron density in the He II emitting region, we derive recombination timescales of ~20 yr for He+2 and ~4 yr for Ne+4. If N44C is a fossil X-ray ionized nebula, this places severe constraints on when the putative X-ray source could have turned off. The presence of strong [Ne IV] emission in the nebula is puzzling if the ionizing source has turned off. It is possible the system is related to the Be X-ray binaries, although the O star in N44C does not show Be characteristics at the present time. Monitoring of X-rays and He II emission from the nebula, as well as a radial velocity study of the ionizing star, are needed to fully understand the emission line spectrum of N44C. Based in part on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2000
- DOI:
- 10.1086/317798
- arXiv:
- arXiv:astro-ph/0008038
- Bibcode:
- 2000ApJ...545..251G
- Keywords:
-
- galaxies: individual (Large Magellanic Cloud);
- ISM: H II Regions;
- ISM: individual (LMC N44C);
- Stars: Early-Type;
- Stars: Peculiar;
- Astrophysics
- E-Print:
- 37 pages, 7 figures (1 color .gif image)