6 - The Formation and Evolution of Symbiotic Stars

Summary

Introduction

In the preceding Chapters, we have seen that symbiotic stars are neither an entirely homogeneous group of variables, nor a random collection of stellar misfits. Although they possess an exciting diversity of hot white dwarfs, hot subdwarfs, and accreting main sequence stars, the unifying factor among symbiotic systems is the evolved red giant star losing mass via stellar wind or tidal overflow. This giant, whether it be a Mira, a semi-regular, or a non-variable red star, severely limits the length of the symbiotic relationship, as the evolutionary timescale for this star is < 10⁶ yr (Becker 1979; Iben 1967). This is a very small fraction of the lifetime of any binary system, and it is therefore important to understand (i) how a given binary manages to become symbiotic, and, once it finds itself in this unusual state of affairs, (ii) how the binary extricates itself from a period of symbiosis.

The formation of binary stars is a process that is not understood in detail, although considerable progress has been made in recent years. The currently most popular scenario for the formation of binary stars begins with the collapse of a rotating interstellar cloud threaded by a magnetic field. The loss of angular momentum by magnetic braking allows the cloud to contract, and instabilities encountered during the early phases of the collapse result in the formation of dense fragments (cf. Mouschovias 1978). Although detailed calculations for the subsequent evolution of the cloud are not yet possible, it is believed that the rapidly rotating fragments undergo fission into smaller components which then contract into normal stars.