Precise Masses and Orbits for Nine Radial Velocity Exoplanets

Radial velocity (RV) surveys have discovered hundreds of exoplanetary systems but suffer from a fundamental degeneracy between planet mass Mp and orbital inclination i. In this paper we break this degeneracy by combining RVs with complementary absolute astrometry taken from the Gaia EDR3 version of the cross-calibrated Hipparcos-Gaia Catalog of Accelerations (HGCA). We use the Markov Chain Monte Carlo orbit code orvara to simultaneously fit literature RVs and absolute astrometry from the HGCA. We constrain the orbits, masses, and inclinations of nine single and massive RV companions orbiting nearby G and K stars. We confirm the planetary nature of six companions: HD 29021 b (4.47+0.67−0.65MJup), HD 81040 b (7.24+1.0−0.37MJup), HD 87883 b (6.31+0.31−0.32MJup), HD 98649 b (9.7+2.3−1.9MJup), HD 106252 b (10.00+0.78−0.73MJup), and HD 171238 b (8.8+3.6−1.3MJup). We place one companion, HD 196067 b (12.5+2.5−1.8MJup) on the planet-brown dwarf boundary, and two companions in the low mass brown dwarf regime: HD 106515 Ab (18.9+1.5−1.4MJup), and HD 221420 b (20.6+2.0−1.6MJup). The brown dwarf HD 221420 b, with a semi-major axis of 9.99+0.74−0.70 AU, a period of 27.7+3.0−2.5 years, and an eccentricity of 0.162+0.035−0.030 represents a promising target for high-contrast imaging. The RV orbits of HD 87883 b, HD 98649 b, HD 171238 b, and HD 196067 b are not fully constrained yet because of insufficient RV data. We find two possible inclinations for each of these orbits due to difficulty in separating prograde from retrograde orbits, but we expect this will change decisively with future Gaia data releases.