We present high-resolution infrared ( 2–18 \mu m ) 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 datasets constrains the optical properties of the dust , and suggest that they differ from those of the dust made by the WC9 dust-makers , including the classical ‘ pinwheel ’ , WR 104 .
Photometry of individual dust emission features shows them to be significantly redder in ( nbL ^ { \prime } – [ 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 \pm 8 and 243 \pm 7 mas y ^ { -1 } .
Longer wavelength ( 4.68- \mu m and 12.5- \mu m ) images shows 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 \sim 320 mas y ^ { -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 4–5 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 .
Comparison of model dust images and the observations constrain 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 .