Orbitally modulated dust formation by the WC7+O5 colliding-wind binary WR 140

We present high-resolution infrared (2–18 μm) images of the archetypal periodic dust-making Wolf–Rayet binary system WR 140 (HD 193793) taken between 2001 and 2005, and multi-colour (J–[19.5]) photometry observed between 1989 and 2001. The images resolve the dust cloud formed by WR 140 in 2001, allowing us to track its expansion and cooling, while the photometry allows tracking the average temperature and total mass of the dust. The combination of the two data sets constrains the optical properties of the dust, and suggests that they differ from those of the dust made by the WC9 dust-makers, including the classical 'pinwheel', WR 104. The photometry of individual dust emission features shows them to be significantly redder in (nbL'–[3.99]) , but bluer in ([7.9]–[12.5]), than the binary, as expected from the spectra of heated dust and the stellar wind of a Wolf–Rayet star. The most persistent dust features, two concentrations at the ends of a 'bar' of emission to the south of the star, were observed to move with constant proper motions of 324 ± 8 and 243 ± 7 mas yr^−1. Longer wavelength (4.68 and 12.5 μm) images show dust emission from the corresponding features from the previous (1993) periastron passage and dust formation episode, showing that the dust expanded freely in a low-density void for over a decade, with dust features repeating from one cycle to the next. A third persistent dust concentration to the east of the binary (the 'arm') was found to have a proper motion ∼320 mas yr^−1, and a dust mass about one-quarter that of the 'bar'. Extrapolation of the motions of the concentrations back to the binary suggests that the eastern 'arm' began expansion four to five months earlier than those in the southern 'bar', consistent with the projected rotation of the binary axis and wind-collision region (WCR) on the sky. A comparison of model dust images and the observations constrains the intervals when the WCR was producing sufficiently compressed wind for dust nucleation in the WCR, and suggests that the distribution of this material was not uniform about the axis of the WCR, but more abundant in the following edge in the orbital plane.