Consequences of Mass Transfer in Close Binary Systems

Abstract

Of all aspects of stellar evolution the most important for close binary systems is that once a star has condensed out of a protostellar cloud it may repeatedly increase its dimensions by large factors. During such an expansion matter can reach a point where the attraction of a binary companion is strong enough to pull it toward the other star. Since the stellar radii that are reached in this expansion can be of the order of an astronomical unit, this kind of interaction may take place even in rather wide binaries with periods of the order of years. At least during such a stage binary evolution must follow a path different from that of a single star. There are other types of interaction between the two stars that come into play much earlier, such as energy transfer by radiation or by matter, which leaves the. star as a stellar wind, and tidal friction or interaction via magnetic fields, but because of a built-in instability mass transfer is by far the most important factor. In its simplest form this instability is caused by the fact that each gram of transferred matter adds to the gravitational pull of the mass-gaining companion while weakening the attractive properties of the expanding star. Since the components of a binary are not fixed in space, their separation may change as well, depending on the amount of angular momenl:um transferred. From observations of close binaries it is well known that many rotate in synchronism with their revolution. This is expected because of the tidal interaction in these systems. Hence for ratios of masses and radii that are not too extreme the rotational angular momentum of both components can be neglected compared to the orbital angular momentum. Under these conditions the individual contributions to the total angular momentum are fixed by the masses of the two stars. Nevertheless, the one-dimensional picture of single-star evolution cannot be retained, since the initial conditions must include not only mass and chemical composition, but also the distance and mass of the companion star. Further complications are met during the stages of mass transfer. However, we should not forget that binary systems in