Computer Science
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt.........4a&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF WISCONSIN - MADISON, 1996.Source: Dissertation Abstracts International, Volume: 57-06, Sectio
Computer Science
Scientific paper
A non-LTE analysis of radio recombination lines (RRL) from the cometary ultracompact (UC) H II region G29.96 -0.02 derives a mean electron temperature (T_ {e}) ~6500 K and a local density (ne) ~ 5 10^4 cm^ {-3} in the leading arc and n e ~2 10 ^4 cm^{-3} in the tail. Electron impact broadening is significant in H110alpha; the peak line amplitude is reduced by as much as 50% in the leading arc, where electron densities are highest. The morphology of the line emission and line profiles is consistent with a bow shock. The electron temperature and density have been determined for seventeen UC H II regions using multiple RRL's obtained with the VLA in scaled arrays. A galactocentric Te gradient was found of the form Te(K) (5537 +/- 387) + (320 +/- 64) DG (kpc). The gradient is about the same as that found by Shaver et al. (1983), but the temperatures are systematically higher by ~1500-2000 K at all D_{G } values. We attribute the higher average Te of our sources to the higher ne of our sample. Our photoionization-statistical equilibrium models determine that the our derived T_{e } gradient is primarily determined by metal abundances. We infer a galactocentric oxygen abundance gradient d (O/H) / d(DG) = -0.047 +/- 0.009 kpc^{ -1}.. In an attempt to calibrate the T_ {e}-DG gradient in terms of metal abundances with D_ {G}, we have measured IR lines of (O III), (S III), and (N III) toward 34 compact and UC H II regions using NASA's Kuiper Airborne Observatory (KAO). Detailed statistical-ionization equilibrium nebular models, combined with high resolution radio continuum images, were used to calculate the O/H, S/H, and N/H abundances in each nebula. We find the following abundance gradients: (S/H) = ( -4.45+/- 0.04) - (0.066 +/- 0.006) D_ {G} (kpc), (N/H) = (-3.58 +/- 0.04) - (0.074 +/- 0.006) D_ {G} (kpc), (O/H) = (-2.85 +/- 0.09) - (0.064 +/- 0.017) DG (kpc), and we derive Te = (4564 +/- 221) + (388 +/- 37) DG (kpc). Gradients in N^{++}/O ^{++} and O^ {++}/S^{++} are observed in the sense of increasing ionization with increasing DG, consistent with the decreased line blanketing implied by the derived abundance gradients. For all gradients, a constant slope is statistically superior to a multi-step fit.
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