1999ApJ...521..212M

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 O²⁺/H⁺ abundance ratio, the collisional line [O III] (λ4959+λ5007) involved in the comparison is about 1000 times stronger than even the strongest O II optical recombination line λ4649. An alternative comparison is to the [O III] λ4931 line, with about the same strength as λ4649. The I(λ4931)/I(λ4959) 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 λ4931/λ4959 and other line ratios by Liu et al. and discuss older published values. The agreement of observations and theory is excellent, and I(λ4931)/I(λ4959)~4.1x10^–4. In the Orion Nebula ((ref???)Esteban et al. 1998), the λ4931.08 line is partially blended with the [Fe III] λ4930.5 line. After correction, the observed O II I(λ4649) line in two positions is, respectively, 15% lower and 40% higher than expected from [O III] λ4931, which are rather modest discrepancies. The λ4649 line in many planetary nebulae is significantly stronger than predicted from the λ4931 line, corroborating results from comparison with the much stronger λλ4959, 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.