Active dust formation by population I Wolf-Rayet stars

We review studies of heated dust formation around Wolf-Rayet stars, observed in the near-IR with groundbased JHKL'M photometry and in the IR With ISO-SWS spectroscopy. Episodes of fresh dust formation with intervals of the order of ten years has been discovered for three WC+O colliding wind binaries, episodic and variable persistent dust formation has been found for four other WC+O candidate binaries, and persistent dust formation is known for 19 WC8-9 stars. Of the last two categories two stars have been imaged in the near-IR, showing pin-wheels in the sky with rotation periods of the order of 1-2 yr. This suggests that perhaps all dusty WC stars are binaries. Dust formation is the least understood of all phenomena associated with colliding stellar winds in WC+OB binaries, including non-thermal radio emission and variable X-ray and gamma -ray emission. While the latter two phenomena are associated with the apex of the wind-wind collision cones, dust formation occurs in the wake of the collision cones, at distances of a few hundred stellar radii away from these hot evolved massive binaries. After formation, the dust is being carried away by the WC stellar winds and cools gradually to interstellar temperatures. The cooling dust radiation emission affects the wavelength regions where FIRST will observe.