Electron Temperatures of Planetary Nebulae Determined from the He I Discontinuities

TL;DR

This study measures electron temperatures in planetary nebulae using He I discontinuities and compares them with hydrogen-based temperatures, revealing discrepancies that suggest complex nebular structures like hydrogen-deficient inclusions.

Contribution

The paper introduces the use of He I discontinuities at 3421A to determine electron temperatures and demonstrates their significance in revealing nebular inhomogeneities beyond homogeneous models.

Findings

Te(He I) are generally lower than Te(H I)

Discrepancies are explained by hydrogen-deficient inclusions

Photoionization models support the presence of cold, hydrogen-deficient clumps

Abstract

We have used the He I discontinuities at 3421A to determine the electron temperatures, designated Te(He I), for a sample of five Galactic planetary nebulae (PNe). We compared Te(He I) with the electron temperatures derived from the hydrogen Balmer jump at 3646A, designated Te(H I), and found that Te(He I) are generally lower than Te(H I). There are two possible interpretations, a) the presence of substantial He+2 zone, or b) the presence of hydrogen-deficient cold clumps within diffuse nebulae. A series of photoionization models were constructed to test the two scenarios. We found that the observed Te(He I)/Te(H I) discrepancies are beyond the predictions of chemically homogeneous models. Our modelling shows that the presence of a small amount of hydrogen-deficient inclusions seems to be able to reproduce the observed intensities of He I discontinuities. We stress the value of He I discontinuities in investigating nebular physical conditions. Albeit with some observational and technical limitations, He I discontinuities should be considered in future modelling work.