The q = 1 peak in the mass-ratios for Hipparcos visual binaries

Abstract

Aims:The magnitude differences for Hipparcos visual binaries give important information about the upper part of their mass-ratio distribution, and in previous studies, the author found a narrow peak at q=1 This excess of equal-mass, wide pairs has not been confirmed in other studies, and the present work aims to strengthen the Hipparcos results.
Methods: We construct a Galaxy model with binaries of known properties, filter it through a "Hipparcos-like" selection model, and then compare the model with the actual Hipparcos observations. By changing the input distributions in the model, the fit to the observations can be improved, enabling some conclusions about the mass-ratio distribution f_q(q) and the distribution of semi-major axes f_a(a). The most important weak point is the modelling of the selection effects in the Hipparcos Input Catalog, but by testing different biases, typical effects can be quantified.
Results: From fittings in different color(mass)-intervals, I find that the shapes of the distributions of orbit-sizes and mass-ratios vary with mass, even in the small (1.9-5.5 Msun) mass-sum interval well covered by the Hipparcos data. In particular, the excess of "twins" in f_q(q) is definitely present, but the peak amplitude diminishes at higher masses. There is normally also a decrease in the number of binaries with increasing mass-ratio in the interval 0.5<q<0.9, but below 2 Msun, this slope suddenly reverses, giving a broad peak towards 1.0 below the narrow one. For typical wide orbits (100-1000 au), the f_a(a)-distribution shows a steeper decline for lower masses, while almost reaching the scale-free 1/a power-law for the highest ones.