On the Statistical Significance of the Hipparcos Astrometric Orbit of rho Coronae Borealis

Abstract

Recently Gatewood, Han & Black presented an analysis of the Hipparcos stellar positions and their own ground-based measurements of rho CrB, suggesting an astrometric orbit of 1.5 mas with an extremely small orbital inclination of 0.5 degrees. This indicates that the planet-candidate secondary might be a late M star. We used the Hipparcos data of rho CrB together with the individual radial velocities of Noyes et al. to independently study the stellar orbit and to assess its statistical significance. Our analysis yielded the same astrometric orbit. However, a permutation test we performed on the Hipparcos measurements indicated that the statistical significance of the astrometric orbit is only 2-sigma. Therefore, we can expect about one out of 40 systems with a similar data set to show a false astrometric orbit.