Problems for the WELS classification of planetary nebulae central stars: Self-consistent nebular modelling of four candidates

TL;DR

This study uses integral field spectroscopy and self-consistent photoionization modeling to analyze four planetary nebulae with supposed weak emission-line central stars, revealing that the WELS classification may be spurious and that emission lines originate from nebular gas.

Contribution

It provides the first spatially-resolved analysis of WELS candidates, challenging the WELS classification and offering detailed nebular models and chemical abundances.

Findings

WELS lines originate in nebular gas, not central stars

Three nebulae are hydrogen-rich O(H)-type

All four nebulae are high excitation, optically-thin, with similar evolutionary stages

Abstract

We present integral field unit (IFU) spectroscopy and self-consistent photoionisation modelling for a sample of four southern Galactic planetary nebulae (PNe) with supposed weak emission-line (WEL) central stars. The Wide Field Spectrograph (WiFeS) on the ANU 2.3 m telescope has been used to provide IFU spectroscopy for NGC 3211, NGC 5979, My 60, and M 4-2 covering the spectral range of 3400-7000{\AA}. All objects are high excitation non-Type I PNe, with strong He II emission, strong [Ne V] emission, and weak low-excitation lines. They all appear to be predominantly optically-thin nebulae excited by central stars with $T_{\rm eff} > 10^5$K. Three PNe of the sample have central stars which have been previously classified as weak emission-line stars (WELS), and the fourth also shows the characteristic recombination lines of a WELS. However, the spatially-resolved spectroscopy shows that rather than arising in the central star, the C IV and N III recombination line emission is distributed in the nebula, and in some cases concentrated in discrete nebular knots. This may suggest that the WELS classification is spurious, and that, rather, these lines arise from (possibly chemically enriched) pockets of nebular gas. Indeed, from careful background subtraction we were able to identify three of the sample as being hydrogen rich O(H)-Type. We have constructed fully self-consistent photoionization models for each object. This allows us to independently determine the chemical abundances in the nebulae, to provide new model-dependent distance estimates, and to place the central stars on the H-R diagram. All four PNe have similar initial mass ($1.5 < M/M_{\odot} <2.0$) and are at a similar evolutionary stage.