THE DISTRIBUTIONS OF BINARY-SYSTEM MASS RATIOS - A LESS BIASED SAMPLE

Different investigators have obtained very different forms for the distribution function, N(q), of the ratios of the masses of stars in binary systems, q=M2 \M1. Some are consistent with the component stars having been drawn at random from a standard initial mass function; others are not. The difference arises more from the selection of systems to be analysed than from the methods of analysis. The present sample includes 132 SBls and 32 SB2s, whose orbits have been obtained by R. F. Griffin and his colleagues using radial velocity spectrometers. Many of them (about 106) are K giants. The systems have smaller velocity amplitudes (median 9 km s-1) and longer periods (median 590 d) than samples previously analysed. The distribution of their mass ratios has been derived both directly, assuming an average value of sin3 i, and indirectly, using a mathematically more correct model-fitting approach. The distribution, N(q), in both cases is best fit by a power law near q-1 over the range q = 0.1-1.0, after allowance is made for observational selection proportional to the sin or sin2 of the angle of inclination. This is rather flatter than the slope of around two generally found for initial mass functions of 0.8-5 M⊗ stars. A somewhat peculiar pattern of mass loss by the KIII primaries would be required to account for the difference. Models of binary formation by the fragmentation process can account for a wide range of mass ratios, but do not yet predict a unique initial distribution function. © Royal Astronomical Society • Provided by the NASA Astrophysics Data System.