0 Ja n 20 05 Helium recombination spectra as temperature diagnostics for planetary nebulae

Electron temperatures derived from the He I recombination line ratios, designated T e (He I), are presented for 48 planetary nebulae (PNe). We study the effect that temperature fluctuations inside nebulae have on the T e (He I) value. We show that a comparison between T e (He I) and the electron temperature derived from the Balmer jump of the H I recombination spectrum, designated T e (H I), provides an opportunity to discriminate between the paradigms of a chemically homogeneous plasma with temperature and density variations, and a two-abundance nebular model with hydrogen-deficient material embedded in diffuse gas of a " normal " chemical composition (i.e. ∼ solar), as the possible causes of the dichotomy between the abundances that are deduced from collisionally excited lines to those deduced from recombination lines. We find that T e (He I) values are significantly lower than T e (H I) values, with an average difference of T e (H I)-T e (He I) = 4000 K. The result is consistent with the expectation of the two-abundance nebular model but is opposite to the prediction of the scenarios – 2 – of temperature fluctuations and/or density inhomogeneities. From the observed difference between T e (He I) and T e (H I), we estimate that the filling factor of hydrogen-deficient components has a typical value of 10 −4. In spite of its small mass, the existence of hydrogen-deficient inclusions may potentially have a profound effect in enhancing the intensities of He I recombination lines and thereby lead to apparently overestimated helium abundances for PNe.