R4 and its circumstellar nebula: evidence for a binary merger?

Abstract

We present new, NTT longslit spectroscopy of the B[e] supergiant in the binary system R4 in the Small Magellanic Cloud. The data show extended, forbidden N and S emissions which are typical signatures of circumstellar matter. Their extension along the space axis of the slit defines an angular size of 8.6'' which translates into a linear size of 2.4 pc. The N emission lines also show the velocity structure of a bipolar outflow expanding at 100 km/s on average. This implies that, for a measured radius of 1.2 pc, the outflow originated about 12000 yrs ago. The line flux ratio [NII]6584/[SII]6717 indicates that the nebula is nitrogen enriched and therefore it has been ejected from the central star. This is the first bipolar, ejection nebula detected around a well-established B[e] supergiant. The bipolar morphology and the chemical enrichment shown by the nebula associated with R4 are consistent with the picture of a binary merger (Langer & Heger 1998), in which R4 was originally a system composed by a close pair and a third star (the observed A companion). The close pair merged into a single star and the merging process produced a circumstellar nebula that was later shaped by the ensueing B star wind.