Helium Recombination Lines as a Probe of Abundance and Temperature Problems

Abstract

The paper presents a simplified formula to determine an electron temperature, Te(He I), for planetary nebulae (PNe) using the He I 7281/6678 line flux ratio. In our previous studies of Te(He I) (Zhang et al. 2005), we used the He I line emission coefficients given by Benjamin et al. (1999). Here we examine the results of using more recent atomic data presented by Porter et al. (2005). A good agreement is shown, suggesting that the effect of uncertainties of atomic data on the resultant Te(He I) is negligible. We also present an analytical formula to derive electron temperature using the He I discontinuity at 3421 A. Our analysis shows that Te(He I) values are significantly lower than electron temperatures deduced from the Balmer jump of H I recombination spectra, Te(H I), and that inferred from the collisionally excited [O III] nebular-to-auroral forbidden line flux ratio, Te([O III]). In addition, Te(H I) covers a wider range of values than either Te(He I) or Te([O III]). This supports the two-abundance nebular model with hydrogen-deficient material embedded in diffuse gas of a ``normal'' chemical composition (i.e. ~solar).