Dust formation in binaries with OB and WR components in a two-phase stellar-wind model

We describe the formation of carbon dust in binary systems with hot components as a result of the collisions of clouds in a two-phase stellar-wind model. Calculations are made for the well studied system WR 140. The collisions lead to the formation of composite clouds and shock waves, with the temperature at the shock front equal to about 3×10⁸ K along both sides of the interface boundary. During isobaric deexcitation to (0.5–0.7)×10⁴ K, the cloud density increases by a factor of several thousand; its thickness in the direction of the shock decreases by the same factor. After deexcitation, the hydrogen inside the composite cloud is in its atomic state, while the carbon remains ionized. The deexcitation is followed by expansion of the cloud, which moves away from both stars. During the first 10⁶ s, its thickness remains relatively small, so that the expansion is one-dimensional. The radiation field inside the cloud decays, resulting in the recombination of the carbon. Further expansion of the cloud leads to adiabatic cooling, and the formation of dust particles becomes possible. After the dimensions of the cloud have become roughly the same in all directions, its expansion is isotropic, so that it becomes transparent within approximately 10⁶ s, and the dust is heated to (1.0–1.4)×10³ K, observed as an IR “lare.” The time required for the cloud to move from the exciting star and heat the dust is comparable to the observed delay in the increased IR emission relative to the time of periastron.