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Constraining the orbit of the planet-hosting binary tau Bootis Clues about planetary formation and migration

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Context. The formation of planets in compact or highly eccentric binaries and the migration of hot Jupiters are two outstanding problems in planet formation. Detailed characterisation of known systems is important for informing and testing models. The hot Jupiter tau Boo Ab orbits the primary star in the long-period (P greater than or similar to 1000 yr), highly eccentric (e similar to 0.9) double star system tau Bootis. Due to the long orbital period, the orbit of the stellar binary is poorly constrained.

Aims. Here we aim to constrain the orbit of the stellar binary tau Boo AB in order to investigate the formation and migration history of the system. The mutual orbital inclination of the stellar companion and the hot Jupiter has important implications for planet migration. The binary eccentricity and periastron distance are important for understanding the conditions under which tau Boo Ab formed.

Methods. We combine more than 150 yr 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.

Results. We determine the orbit of tau Boo B and find an orbital inclination of 47.2(-3.7)(+2.7 degrees), a periastron distance of 28.3(-3.0)(+2.3)au, and an eccentricity of 0.87(-0.03)(+0.04). 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 similar to 45 degrees, a result consistent with the assumption of a well-aligned, coplanar system.

Conclusions. 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 approximate to 6 au. 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.

TidsskriftAstronomy & Astrophysics
Antal sider9
StatusUdgivet - 10 maj 2019

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