Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-04-20
Astrophys.J. 611 (2004) 338-352
Astronomy and Astrophysics
Astrophysics
29 pages, 6 figures, ApJ in press
Scientific paper
10.1086/422028
Although variations in elemental abundance ratios in the Milky Way Galaxy certainly exist, details remain uncertain, particularly in the inner Galaxy, where stars and H II regions in the Galactic plane are obscured optically. In this paper we revisit two previously studied, inner Galaxy H II regions: G333.6-0.2 and W43. We observed three new positions in G333.6-0.2 with the Kuiper Airborne Observatory and reobserved the central position with the Infrared Space Observatory's Long Wavelength Spectrometer in far-infrared lines of S++, N++, N+, and O++. We also added the N+ lines at 122 and 205 micron to the suite of lines measured in W43 by Simpson et al. (1995). The measured electron densities range from ~40 to over 4000 cm-3 in a single H II region, indicating that abundance analyses must consider density variations, since the critical densities of the observed lines range from 40 to 9000 cm-3. We propose a method to handle density variations and make new estimates of the S/H and N/H abundance ratios. We find that our sulfur abundance estimates for G333.6-0.2 and W43 agree with the S/H abundance ratios expected for the S/H abundance gradient previously reported by Simpson et al. The estimated N/H, S/H, and N/S ratios are the most reliable because of their small corrections for unseen ionization states (< 10%). We compute models of the two H II regions to estimate corrections for the other unseen ionization states. We discuss these predictions and conclude that only a few of the latest models adequately reproduce H II region observations, including the well-known, relatively-large observed Ne++/O++ ratios in low- and moderate-excitation H II regions.
Colgan Sean W. J.
Erickson Edwin F.
Haas Michael R.
Rubin Robert H.
Simpson Janet P.
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