High-exitation nebulae around Magellanic Wolf-Rayet stars

Abstract

AbstractThe SMC harbours a class of hot nitrogen-sequence Wolf-Rayet stars (WNE) that display only relatively weak broad Heiiλ4686 emission indicative of their low mass-loss rates and which are therefore hard to detect. However, such stars are possible emitters of strong He+Lyman continua which in turn could ionize observable Heiiiregions, i.e. highly excited Hiiregions emitting nebular Heiiλ4686 emission. We here report the discovery of a second Heiiiregion in the SMC within OB association NGC 249 within which the weak-lined WN star SMC WR10 is embedded. SMC WR10 is of special importance since it is a single star showing the presence of atmospheric hydrogen. While analysing the spectrum in the framework of two popular, independent WR atmosphere models we found strongly discrepant predictions (by 1 dex) of the He+continuum for the same input parameters. A second interesting aspect of the work reported here concerns the beautiful MCELS images which clearly reveal a class of strongly Oiiiλ5007 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 Heiiλ4686 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 Heiiλ4686-emitting nebula N 44C.