Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Heavy elements in Galactic and Magellanic Cloud HII regions:
  recombination-line versus forbidden-line abundances   We have obtained deep optical, long-slit spectrophotometry of the Galactic
HII regions M 17, NGC 3576 and of the Magellanic Cloud HII regions 30 Doradus,
LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of CII,
NII and OII. Temperature-insensitive ORL C2+/O2+ and N2+/O2 ratios are obtained
for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable
agreement within each galactic system, while also being in agreement with the
corresponding CEL ratios. For all five nebulae, the O2+/H+ abundance derived
from multiple OII ORLs is found to be higher than the corresponding value
derived from the strong [OIII] 4959, 5007A CELs, by factors of 1.8--2.7 for
four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy
factor for the O2+ ion, ~5. Thus these HII regions exhibit ORL/CEL abundance
discrepancy factors that are similar to those previously encountered amongst
planetary nebulae.
  Our optical CEL O2+/H+ abundances agree to within 20-30 per cent with
published O2+/H+ abundances that were obtained from observations of infrared
fine-structure lines. Since the low excitation energies of the latter make them
insensitive to variations about typical nebular temperatures, fluctuations in
temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance
discrepancies. We present evidence that the observed OII ORLs from these HII
regions originate from gas of very similar density (<3500 cm-3) to that
emitting the observed heavy-element optical and infrared CELs, ruling out
models that employ high-density ionized inclusions in order to explain the
abundance discrepancy. We consider a scenario whereby much of the heavy-element
ORL emission originates from cold (<=500 K) metal-rich ionized regions.

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The categories of this paper are: ao-sci
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The categories of this paper are: quant-ph
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The categories of this paper are: cs
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The categories of this paper are: eess
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The categories of this paper are: atom-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-ex
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The categories of this paper are: acc-phys
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The categories of this paper are: cond-mat
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The categories of this paper are: funct-an
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The categories of this paper are: plasm-ph
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The categories of this paper are: nlin
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The categories of this paper are: math
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The categories of this paper are: math-ph
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The categories of this paper are: q-bio