The mass of the candidate exoplanet companion to HD 33636 from Hubble Space Telescope astrometry and high-precision radial velocities

We have determined a dynamical mass for the companion to HD 33636 that indicates it is a low-mass star instead of an exoplanet. Our result is based on an analysis of Hubble Space Telescope (HST) astrometry and ground-based radial velocity data. We have obtained high-cadence radial velocity measurements spanning 1.3 yr of HD 33636 with the Hobby-Eberly Telescope at McDonald Observatory. We combined these data with previously published velocities to create a data set that spans 9 yr. We used this data set to search for, and place mass limits on, the existence of additional companions in the HD 33636 system. Our high-precision astrometric observations of the system with the HST Fine Guidance Sensor 1 r span 1.2 yr. We simultaneously modeled the radial velocity and astrometry data to determine the parallax, proper motion, and perturbation orbit parameters of HD 33636. Our derived parallax, pi(abs) = 35.6 +/- 0.2 mas, agrees within the uncertainties with the Hipparcos value. We find a perturbation period P = 2117.3 +/- 0.8 days, semi-major axis a(A) = 14.2 +/- 0.2 mas, and system inclination i = 4.1 degrees +/- 0.1 degrees. Assuming the mass of the primary star to be M-A = 1.02 +/- 0.03 Mo, we obtain a companion mass M-B = 142 11 M-Jup = 0.14 +/- 0.01 M-circle dot. The much larger true mass of the companion relative to its minimum mass estimated from the spectroscopic orbit parameters (M sin i = 9.3 M-Jup) is due to the nearly face-on orbit orientation. This result demonstrates the value of follow-up astrometric observations to determine the true masses of exoplanet candidates detected with the radial velocity method.