WR 140 is the "canonical" massive colliding wind binary. It consists of a WC7 Wolf-Rayet (an evolved massive star whose atmosphere is highly enriched by carbon) and an O4-5 companion star gravitationally bound together in a long-period (7.9 year) highly eccentric (e=0.88) orbit. The O4-5 star is the more massive star, but the WC7 star has a very strong wind and this wind dominates the emission from the system. It's called a colliding wind binary because the strong wind from the WC7 star collides with the weaker wind from the O star companion and generates a variety of observables, notably strong X-ray emission produced by material in the "bow shock" where the two winds smash together. Peredur Williams has a nice movie of the orbit and the changing orientation of the bow shock as the two stars revolve around each other.
In addition to generating X-ray emission, the wind-wind collision is also responsible for the episodic production of dust. Dust apparently forms during the periastron passage of the two star. This probably has something to do with the changing density of the stellar winds from the WC star and the O star; it's expected that the wind densities fall off as 1/r2, where r is the radial distance from the center of the star. Thus when the stars are close to each other the density near the bow shock is higher than at other times. Somehow this triggers the formation of large (probably carbon-based) dust grains. Dust formation was first reported in 1978. Infrared observations in 1985 by P. M. Williams and M. G. Smith (U.K. Infrared Telescope) showed that the star had again produced a dust shell, "some 7-8 years since it last did so."
We've been monitoring WR 140 with RXTE since the 2001 periastron passage and have a web page where we post the updated lightcurve.
Upcoming WR140 Periastron passage: 2446147.4+3 \times 2899=2454844.400, or 2009-01-12 21:36:00.000 UTC
Upcoming X-ray Minimum (expected): 2009-01-26 06:20:09.600 UTC, based on phasing of the previous RXTE minimum
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