Dust Formation in Massive WR+O Binaries: Recent Results

The massive, luminous Population I Wolf-Rayet stars can be considered as stars with the highest known sustained mass loss rates. Around 10% of WR stars may form carbon-rich dust in their dense and inhomogeneous winds. Though we are yet to find how dust is formed in such an extremely hostile environment, we have made substantial progress over the past decade. Here we discuss the results of recent high-resolution mid-infrared imaging of a sample of the most prodigious WR ‘dustars’. This allows one to map rapidly changing dust-forming regions and derive some basic properties of the freshly formed dust. 1. The Wolf-Rayet ‘Dustars’ The hot, massive Population I Wolf-Rayet stars are the evolved descendents of O-type stars. WR stars live on the verge of exploding as Supernovae (even hypernovae), thus providing vitally important information about the very final stages of massive star evolution, possibly leading to Gamma-Ray Bursts. The extremely luminous (L∼10 5 L⊙) and hot (Teff ≫ 20000K) hydrostatic cores of WR stars drive fast dense winds: average mass-loss rates Ṁ ∼ 10 M⊙/yr and terminal velocities v∞ ∼ 1000 − 4500 km/s. There are three evolutionarysuccessive WR phases: WN, WC (carbon-rich, dust-producing class we are going to discuss here), and WO. Even though present-epoch dust production output for all Galactic WC stars is ∼1% of the total Galactic rate (Dwek 1985; Cohen 1991), the dust-generating WC stars are regarded as outstanding for three main reasons: (i) The absolute rate of dust production is extraordinarily high, reaching Ṁ ∼ 10M⊙yr −1 (van der Hucht et al. 1987; Williams 1995) in some stars. (ii) The dust is formed in a hot, extremely hostile environment, posing a formidable theoretical problem. (iii) In the early (age∼1 Gyr) universe, WR stars could be very common, but unique sources of dust, along with subsequent dust-generating SN events, since WR stars evolve much more rapidly than any lower-mass stars, commonly associated with dust production in the ‘modern’ universe. It is not clear how much (and what kind of) dust can be produced in a SN explosion (Dwek 2004). However, it is quite clear that the copious amounts