A reexamination of electron density diagnostics for ionized gaseous nebulae
Department of Astronomy, Peking University, Beijing 100871, PR China e-mail: firstname.lastname@example.org
2 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
Accepted: 2 August 2004
We present a comparison of electron densities derived from optical forbidden line diagnostic ratios for a sample of over a hundred nebulae. We consider four density indicators, the  ,  ,  and  doublet ratios. Except for a few H ii regions for which data from the literature were used, diagnostic line ratios were derived from our own high quality spectra. For the  doublet ratio, we find that our default atomic data set, consisting of transition probabilities from Zeippen ([CITE]) and collision strengths from Pradhan ([CITE]), fit the observations well, although at high electron densities, the  doublet ratio yields densities systematically lower than those given by the  doublet ratio, suggesting that the ratio of transition probabilities of the  doublet, , given by Zeippen ([CITE]) may need to be revised upwards by approximately 6 per cent. Our analysis also shows that the more recent calculations of  transition probabilities by Zeippen ([CITE]) and collision strengths by McLaughlin & Bell ([CITE]) are inconsistent with the observations at the high and low density limits, respectively, and can therefore be ruled out. We confirm the earlier result of Copetti & Writzl ([CITE]) that the  transition probabilities calculated by Wiese et al. ([CITE]) yield electron densities systematically lower than those deduced from the  doublet ratio and that the discrepancy is most likely caused by errors in the transition probabilities calculated by Wiese et al. ([CITE]). Using our default atomic data set for , we find that .
Key words: atomic data / ISM: lines and bands / ISM: planetary nebulae: general
© ESO, 2004