Formation of Expanding Envelopes in Wolf-Rayet Stars

Some features of the envelopes of WR stars have not yet been explained in terms of the standard stellar wind. One of these features is the presence of gas condensations (clumps) in the envelopes. In this paper the possible reasons for clump formation are examined along with their role in the structure of envelopes. Clumps can be formed in the transitional zone between a star and its envelope because of instabilities in the gas. This zone lies much deeper than the "optical radus" of the star, so it cannot be observed. A clump expands as it moves under the action of radiation pressure from the star and its density decreases at the same time. The clump mixes with the surrounding gas if its mass is low. Large clumps can reach the visible region of the envelope.