Inhibition of diffusion by the reflection effect in binary Hg-Mn stars

The Hg–Mn stars constitute a sub-group of the peculiar A stars which have abundance anomalies in their atmospheres. Their distinctive properties are excesses of P, Mn, Ga, Y and Hg 1 , slow rotation 2 , a high frequency of double-lined spectroscopic binaries 3,4 , and a lack of detectable magnetic fields 5 . It is widely believed that the abundance anomalies are at least partly due to the separation of elements by diffusion under the influence of gravitation and radiation pressure 6,7 . For the diffusion mechanism to operate the atmosphere has to be extremely quiescent with circulation velocities ≲10 −3 m s −1 . We point out here that in a spectroscopic binary system with similar components surface streaming velocities greatly in excess of this value are expected due to the mutual irradiation of the components.