The frequencies of binaries on the main sequence

Abstract

Solar-type dwarfs and their evidence are discussed for two types of binaries: (1) the bifurcation doubles with periods less than about 100 y and frequencies of secondary masses that vary as M/sub 2//sup 0.4/ and (2) the independent-condensation doubles of longer periods having secondaries that follow the van Rhijn frequency distribution. Integrations of these distributions of secondaries indicate that more than half of the primaries have nearby stellar companions and more than half have distant stellar companions. The high-velocity stars have few nearby companions but a normal frequency of distant companions. The most likely explanation for these properties are that metal-poor protostars are slow in radiating away excess energy and therefore slow in contracting; they may be able to lose angular momentum fast enough so that they need not bifurcate to dispose of the initial excess. The properties of binaries among B2--B5 dwarfs are very similar to those of the solar-type dwarfs in (1) their distribution of orbital periods, (2) the occurrence of two types of binaries, (3) the break period between the two types, and (4) the frequency distribution of secondary masses for bifurcation doubles. These results imply that the binary characteristics of normal stars may not depend sensitivelymore » upon primary mass. The published data on the binary frequencies in clusters of rapidly-rotating and slowly-rotating stars are reviewed in a different way and the evidence implies the existence of an inverse correlation between duplicity and mean rotational velocity. Whether clusters are rich or poor in binaries compared with field stars yields the provisional answer that they are somewhat deficient in binaries.« less