Observations of the [O III] lambda4931/lambda4959 Line Ratio and O^+2 Abundances in Ionized Nebulae

Details Observations of the [O III] lambda4931/lambda4959 Line Ratio and O^+2 Abundances in Ionized Nebulae Observations of the [O III] lambda4931/lambda4959 Line Ratio and O^+2 Abundances in Ionized Nebulae

Aug 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...521..212m&link_type=abstract

The Astrophysical Journal, Volume 521, Issue 1, pp. 212-216.

Astronomy and Astrophysics

Astronomy

11

Ism: H Ii Regions, Ism: Abundances

Scientific paper

Recent abundance determinations using faint optical recombination lines suggest that the heavy element abundances in many ionized nebulae might be much higher than previously deduced from the standard collisional line method. In the case of the O^2+/H^+ abundance ratio, the collisional line [O III] (lambda4959+lambda5007) involved in the comparison is about 1000 times stronger than even the strongest O II optical recombination line lambda4649. An alternative comparison is to the [O III] lambda4931 line, with about the same strength as lambda4649. The I(lambda4931)/I(lambda4959) ratio, completely independent of all physical conditions because both lines arise from the same upper level, can be determined from both theory and nebular observations. We present new observations of the lambda4931/lambda4959 and other line ratios by Liu et al. and discuss older published values. The agreement of observations and theory is excellent, and I(lambda4931)/I(lambda4959)~4.1x10^-4. In the Orion Nebula (Esteban et al. 1998), the lambda4931.08 line is partially blended with the [Fe III] lambda4930.5 line. After correction, the observed O II I(lambda4649) line in two positions is, respectively, 15% lower and 40% higher than expected from [O III] lambda4931, which are rather modest discrepancies. The lambda4649 line in many planetary nebulae is significantly stronger than predicted from the lambda4931 line, corroborating results from comparison with the much stronger lambdalambda4959, 5007 lines. The observations of invariant line ratios presented here show that for lines close in wavelength, their intensity ratios can be measured with a precision better than 5%, even if their strengths differ by orders of magnitude.

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