A reexamination of electron density diagnostics for ionized gaseous nebulae

¹ Department of Astronomy, Peking University, Beijing 100871, PR China e-mail: bwaw@bac.pku.edu.cn
² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 15 June 2004
Accepted: 2 August 2004

Abstract

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 .