The Extrasolar Planet ε Eridani b: Orbit and Mass

Hubble Space Telescope observations of the nearby (3.22 pc), K2 V starepsilon Eridani have been combined with ground-based astrometric and radialvelocity data to determine the mass of its known companion. We model theastrometric and radial velocity measurements simultaneously to obtain theparallax, proper motion, perturbation period, perturbation inclination, andperturbation size. Because of the long period of the companion, \eps b, weextend our astrometric coverage to a total of 14.94 years (including the threeyear span of the \HST data) by including lower-precision ground-basedastrometry from the Allegheny Multichannel Astrometric Photometer. Radialvelocities now span 1980.8 -- 2006.3. We obtain a perturbation period, P = 6.85+/- 0.03 yr, semi-major axis, alpha =1.88 +/- 0.20 mas, and inclination i =30.1 +/- 3.8 degrees. This inclination is consistent with a previously measureddust disk inclination, suggesting coplanarity. Assuming a primary mass M_* =0.83 M_{\sun}, we obtain a companion mass M = 1.55 +/- 0.24 M_{Jup}. Given therelatively young age of epsilon Eri (~800 Myr), this accurate exoplanet massand orbit can usefully inform future direct imaging attempts. We predict thenext periastron at 2007.3 with a total separation, rho = 0.3 arcsec at positionangle, p.a. = -27 degrees. Orbit orientation and geometry dictate that epsilonEri b will appear brightest in reflected light very nearly at periastron.Radial velocities spanning over 25 years indicate an acceleration consistentwith a Jupiter-mass object with a period in excess of 50 years, possiblyresponsible for one feature of the dust morphology, the inner cavity