The Possible Influence of Interstellar Clouds on Stellar Velocities.

Gravitational encounters between stars and interstellar clouds produce a much shorter relaxation time of the galaxy in the solar neighborhood than do star-star encounters. This result is caused by much larger masses of the interstellar clouds as compared with stars. In the extreme case that the largest cloud complexes acting as gravitational units should have masses of the order of a million solar masses it is found that low-velocity stars may have been speeded up appreciably by star-cloud encounters during 3 X lO years. This speedup of the stars, which is the same for stars of all masses, arises from the tendency of the encounters to act toward equipartition of energy between clouds and stars, though at the present time equipartition must be far from reached. If the masses of the large cloud complexes are, in fact, high enough to make the star-cloud encounters sufficiently effective, one may suppose that all low-velocity population I stars have been formed from interstellar clouds with initial average velocities equal to those of the present clouds and that the present differences in the velocity dispersions of population I stars have been caused entirely by star-cloud encounters. Under this assumption, the encounters would have increased the average velocity of older groups (late dwarfs and red giants) by about a factor of 2, while they would not have had time to affect the velocities of the younger stars (early main sequence). Even under extreme assumptions the star-cloud encounters are found to be incapable of changing noticeably the velocities of the fast population II stars. This may indicate that all population II stars were formed from the interstellar matter at an early stage, when the velocities of the primeval clouds were still high.