Dust Formation in Wolf-Rayet Stellar Winds

Abstract

Wolf-Rayet stars are characterized by high mass-loss rates indicative of their transient evolutionary status. The late WC types are in addition surrounded by dust shells. Apart from episodic and variable dust formation around some late WC stars, most WC8, WC8.5, WC9, and WC10 stars show a constant termal IR excess over the history of IR astronomy.

In the past four years all 28 galactic WC stars of those types were monitored from ESO and UKIRT. The observations were supplemented with HAS data. Typical colour temperatures are of the order of 900 K. Because of the observed smooth energy distributions, the dust consists most likely of amorphous carbon. This is not surprising with WC stellar winds having a carbon mass fraction of .34.

Typical dust shell radii are of the order of 100 A.U., i e. in the region where the stellar winds have reached their terminal velocities (≈ 1500 km/s). Because of the large mass loss rates, of the order of 3.10^-5 M^⊙/yr, the high density winds carry the dust away. Since in most cases the IR characteristics are constant over many years, this implies that the dust is constantly being replenished.

In modeling the IR observations, (ρ,T) criteria are derived for the dust to form in the given carbon-rich environment. The derived carbon dust loss rates and the influence of the WC stars on the interstellar medium in this respect are compared with those of other dust producting stars.