Helium recombination spectra as temperature diagnostics for planetary nebulae

Abstract

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 and 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 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. © 2005 RAS.