2005MNRAS.358..457Z

Electron temperatures derived from the HeI recombination line ratios, designated T_e(HeI), are presented for 48 planetary nebulae (PNe). We study the effect that temperature fluctuations inside nebulae have on the T_e(HeI) value. We show that a comparison between T_e(HeI) and the electron temperature derived from the Balmer jump of the HI recombination spectrum, designated T_e(HI), 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 and those deduced from recombination lines. We find that T_e(HeI) values are significantly lower than T_e(HI) values, with an average difference of <T_e(HI) -T_e(HeI)≥ 4000 K. The result is consistent with the expectation of the two-abundance nebular model but is opposite to the prediction of the scenarios of temperature fluctuations and/or density inhomogeneities. From the observed difference between T_e(HeI) and T_e(HI), 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 HeI recombination lines and thereby lead to apparently overestimated helium abundances for PNe.