Mass Transfer After the Main-Sequence Phase

Abstract

The striking feature of semi-detached systems had been noticed for a long time, that the expanded component is the less massive one, without exception. Crawford (1955) was the first to suggest that the apparent violation of the best-established rules of star evolution calls for an enormous mass transfer which reverses the roles of primary and secondary stars. The pioneering quantitative study by Morton (1960) confirmed the observational conclusion that the process is very rapid, at least in the sense that its initial phase performs the major part of transfer. It is mostly completed on a time-scale of 10⁵ yr, and possibly less in many instances. No pair is known to show a bilateral post-main sequence mass exchange. The difference of evolutionary speed — although small for nearly equal components — thus still suffices to outlast the rapid transfer. And no pair is known which shows, by way of period changes, the maximum theoretical rates of mass change per time. What is known, is the low-mass object remaining a bit later. The spectral features, when discerned, resemble those of a subgiant, and the great dissimilarity of masses between normal class IV subgiants and mass-loss stars is revealed by the low mass functions of pairs of the latter kind. The rejuvenated secondaries which were recipients of mass transfer could likewise hardly be recognised from the spectrum if it were not for evidence of binary nature.