Orbital coplanarity in solar-type binary systems: Implications for planetary system formation and detection

The equatorial inclinations of solar-type stars within visual binary systems are computed by combining upsilon sin i measurements with rotational period measurements, or with expected rotational velocities based upon the age of the star in question. These inclinations are then compared with the orbital inclinations of the systems to test the alignment between the equatorial and orbital planes, and how the tendency for or against coplanarity varies as a function of parameters such as spectral type, separation, eccentricity. The results are extended to planetary systems in order to determine the appropriateness of basing planetary search strategies upon a parent star's equatorial inclination, and to address issues in planetary system formation and evolution, including the stability of planetary orbits within binary systems. During the course of this project new or improved upsilon sin i measurements are made for over 30 solar-type stars within binary systems, and provisional orbits are calculated for 13 binary systems. The results suggest that approximate coplanarity between the equatorial and orbital planes exists for solar-type binary systems with separations less than 30-40 AU. The coplanarity tendency, as well as this 'critical separation,' are not significantly affected by most of the other parameters studied. The one significant exception occurs with hierarchical multiple systems, where noncoplanarity may exist at relatively small separations. Assuming planetary distances in our solar system are typical, the results suggest there is no reason to expect planets to orbit in planes significantly different from that of the parent star's equator, in turn suggesting that planetary formation models and search strategies dependent upon this assumption are valid from this standpoint. The results also suggest that noncoplanarity between the components of a binary system is not a significant issue in addressing the stability of planetary orbits within the system. As an application of these results, the equatorial inclination of HD 114762 is measured in order to determine the nature of the low-mass companion reported by Latham et al. (1989), a candidate brown dwarf. Although no definitive statements can be made, the analysis supports the view that the companion is a low-mass M star.