Astronomy and Astrophysics – Astrophysics
Scientific paper
2000-08-01
Astronomy and Astrophysics
Astrophysics
37 pages, 7 figures (1 color .gif image); accepted for publication in the 10 Dec 2000 Astrophysical Journal. Complete PostScri
Scientific paper
10.1086/317798
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 H-alpha + [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-sigma upper limit L(X) < 10^34 erg 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 approximately 20 yrs for He^+2 and approximately 4 yrs 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.
Chu Y.-H. Y.-H.
Galarza Vanessa C.
Garnett Donald R.
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