High-excitation nebulae around Magellanic Wolf-Rayet stars

TL;DR

This paper reports the discovery of highly excited nebulae around Wolf-Rayet stars in the SMC, revealing new insights into their ionizing capabilities, nebular characteristics, and potential populations of faint or binary-related hot stars.

Contribution

It presents the discovery of a rare HeIII region around a weak-lined WN star, compares WR atmosphere models, and identifies new blue nebulae possibly linked to faint or binary Wolf-Rayet stars.

Findings

Discovery of a HeIII region around SMC WR10.

Identification of new blue [OIII] nebulae around WR stars.

Evidence suggesting faint or binary Wolf-Rayet stars contribute to nebular ionization.

Abstract

The SMC harbours a class of hot nitrogen-sequence Wolf-Rayet stars (WNE) that display only relatively weak broad emission lines. This indicates low mass-loss rates and makes them also hard to detect. However, such stars are possible emitters of strong He+ Lyman continua that in turn could ionize observable HeIII regions, i.e. highly excited HII regions emitting nebular HeII4686 emission. We here report the discovery of a rare HeIII region in the SMC which is located in the OB association NGC249 around the weak-lined WN star SMC WR10. WR10 is particularly interesting since it is a single star showing the presence of atmospheric hydrogen. While analysing the spectrum in the framework of two popular WR atmosphere models, we found for the same input parameters strongly discrepant predictions (by 1 dex) for the He+ Lyman continuum. A second aspect of the work reported here concerns the beautiful MCELS images which clearly reveal a class of strongly [OIII]5007 emitting (blue-coded) nebulae. Not unexpectedly, most of the 'blue' nebulae are known Wolf-Rayet bubbles, but new bubbles around a few WRs are also detected. Moreover, we report the existence of blue nebulae without associated known WRs and discuss the possibility that they reveal weak-wind WR stars with very faint stellar HeII4686 emission. Alternatively, such nebulae might hint at the hitherto missing population of relatively low-mass, hot He stars predicted by massive binary evolution calculations. Such a binary system is probably responsible for the ionization of the unique HeII4686-emitting nebula N44C.