R4 and Its Circumstellar Nebula: Evidence for a Binary Merger?

We present new, NTT long-slit 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-1 on average. This implies that, for a measured radius of 1.2 pc, the outflow originated about 1.2 × 104 yr ago. The line flux ratio [N II] λ6584/[S II] λ6717 indicates that the nebula is nitrogen enriched and therefore 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, 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 ensuing B star wind. Since the bipolar morphology, the kinematics and the enriched chemical composition make the nebula surrounding R4 very similar to the observed LBV nebulae, our findings imply that at least a few LBV outbursts and nebulae might well be the result of the merging process of two massive stars.