v 1 8 A pr 1 99 9 A dusty pinwheel nebula around the massive star WR 104

Wolf-Rayet (WR) stars are luminous massive blue stars thought to be immediate precursors to the supernova terminating their brief lives. The existence of dust shells around such stars has been enigmatic since their discovery some 30 years ago; the intense radiation field from the star should be inimical to dust survival [1]. Although dust-creation models, including those involving interacting stellar winds from a companion star [2], have been put forward, high-resolution observations are required to understand this phenomena. Here we present resolved images of the dust outflow around Wolf-Rayet WR 104, obtained with novel imaging techniques, revealing detail on scales corresponding to about 40 AU at the star. Our maps show that the dust forms a spatially confined stream following precisely a linear (or Archimedian) spiral trajectory. Images taken at two separate epochs show a clear rotation with a period of 220±30 days. Taken together, these findings prove that a binary star is responsible for the creation of the circumstellar dust, while the spiral plume makes WR 104 the prototype of a new class of circumstellar nebulae unique to interacting wind systems. Observations of WR 104 were made with the Keck I telescope on 14 April and 4 June 1998, and employed the technique of aperture masking interferometry in order to recover information out to the diffraction limit of the 10 m Keck aperture [3, 4]. We present, in Figure 1, reconstructed images taken at 1.65 & 2.27 µm (∆λ = 0.33 & 0.16 µm respectively) for both observing epochs. As infrared emission from the hot circumstellar dust dominates the infrared region of the spectrum, we may interpret the highly asymmetric curved plumes evident in the maps as tracing the distribution of this material. Previous high-resolution efforts have been restricted to partially-resolved one-dimensional visibility curves interpreted in the context of spherically symmetric outflow models [7, 8]. As a comparison with these earlier results, we have fitted a uniform-disk model to our visibilities, azimuthally averaged and cropped to the resolutions then obtained, finding perfect agreement with the 130 mas diameter disk reported in 1981 [7]. This similarity over a timescale of decades is in accord with the inclusion of WR 104 in the small handful of 'persistent' dust producing WR's [9]. Additional interferometric observations at 3.08 µm (∆λ = 0.1 µm) show no evidence of the marked enlargement towards longer wavelengths reported by Dyck et. al. [8]. However, as 1