A dusty pinwheel nebula around the massive star WR104

Wolf-Rayet (WR) stars are luminous, massive blue stars thought to be the immediate precursors to some supernovae. The existence of dust shells around such stars has been enigmatic since their discovery about 30 years ago, as the intense ultraviolet radiation from the star should be inimical to dust survival. Although dust creation models, including those involving interacting stellar winds, have been put forward to explain these dust shells, the high-resolution observations needed to distinguish between the models have hitherto been lacking. Here we present images of the dust outflow around WR104, obtained using a technique that allows us to resolve detail on scales of about 40AUat the distance of the star. Our images-taken at two epochs-show that the dust forms a spatially confined stream that follows precisely a linear (or archimedian) spiral trajectory with a rotation period of 220 +/- 30days. These results prove that, in this case, a binary companion is responsible for the creation of the circumstellar dust. Moreover, the spiral plume makes WR104 the prototype of a new class of circumstellar nebulae, which are unique to systems with interacting winds.