tau Boo Radial velocities & astrometric data

The binary eccentricity and periastron distance are important for understanding the conditions under which {tau} Boo formed. We combine more than 150 years of astrometric data with twenty-five years of high-precision radial velocities. The combination of sky-projected and line-of-sight measurements places tight constraints on the orbital inclination, eccentricity, and periastron distance of {tau} Boo AB.

We determine the orbit of {tau} Boo B and find an orbital inclination of 47.2^+2.7^_-3.7_{deg}, a periastron distance of 28.3^+2.3^_-3.0_au, and an eccentricity of 0.87^+0.04^_-0.03_. We find that the orbital inclinations of {tau} Boo Ab and {tau} Boo B, as well as the stellar spin-axis of {tau} Boo A coincide at ~45 degrees, a result consistent with the assumption of a well-aligned, coplanar system. The likely aligned, coplanar configuration suggests planetary migration within a well-aligned protoplanetary disc.

Due to the high eccentricity and small periastron distance of {tau} Boo B, the protoplanetary disc was tidally truncated at ~6au. We suggest that {tau} Boo Ab formed near the edge of the truncated disc and migrated inwards with high eccentricity due to spiral waves generated by the stellar companion.