EVOLUTION IN CLOSE BINARY SYSTEMS

Abstract

3. A hypothesis that this situation is the result of a changeover in the role caused by transfer of mass between them can be made compatible with the observed absence of a transient phase in which the more massive star has reached the Roche limit first only if a mass transfer at a rate of 10~5 to 10~4 O /yr is limited to KH-IO5 years, and occurs with velocities generally less than 102 km/sec. This would, however, require the density of such temporary gas streams to be of the order of IO^-IO21 particles per cm3 (i.e., 106 to 103 g/cm3 for hydrogen), and optically completely thick. 4. The gas streams actually observed by spectroscopic means in many semidetached systems fall short of these requirements by several orders of magnitude; and any mass transfer by these would become noticeable only on a long (nuclear) time scale. 5. The existence of binary systems like Sirius or Procyon (consisting of a main -sequence star attended by a white dwarf) - in which very wide separation of both components largely precludes mass transfer - suggests that a loss of mass of their white dwarfs must have occurred with velocities sufficient to remove it altogether from the system - as it happens in Wolf-Rayet stars or novae. β. Another significant fact concerning close binaries in their post -main -sequence evolutionary stage is a virtually complete absence of such binaries with mass ratios close to one - in contrast with their relatively high frequency encountered on the main sequence, and in spite of an increased probability of discovery. The reason why these should become extinct in the post -main -sequence stage is unknown. 7. Close binaries of the W UMa type are much too numerous in space to be regarded as evolutionary descendants of detached main -sequence systems of comparable masses and dimensions. A hypothesis is, therefore, advanced that their origin was by contraction towards two preexisting mass centers in close proximity of each other - a contraction which became arrested (by the onset of hydrogen burning) at, or even beyond, their respective Roche limits - thus resulting in the formation of systems with properties as we see them today. 8. The observed excesses of photometric over dynamical ellipticities of W UMa-type systems point to the conspicuous gravity -darkening operative over their surfaces - not smaller than that to be expected for the case of radiative equilibrium in subsurface layers.