The Planet Host Star Gamma Cephei: Physical Properties, the Binary Orbit, and the Mass of the Substellar Companion

Abstract

The bright K1 III-IV star gamma Cep has been reported previously to have a companion in a 2.5-yr orbit that is possibly substellar, and also has a stellar companion at a larger separation that has never been seen. Here we determine for the first time the three-dimensional orbit of the stellar companion accounting also for the perturbation from the closer object. We combine new and existing radial velocity measurements (of both classical precision and high precision) with intermediate astrometric data from the Hipparcos mission (abscissa residuals) as well as ground-based positional observations going back more than a century. The orbit of the secondary star is eccentric and has a period of 66.8 yr. We establish the primary star to be on the first ascent of the giant branch, and to have a mass of 1.18 M_Sun, an effective temperature of 4800 K, and an age around 6.6 Gyr (for an assumed metallicity [Fe/H] = +0.01). The unseen secondary star is found to be an M4 dwarf with a mass of 0.362 M_Sun, and is expected to be about 6.4 mag fainter than the primary in K. The minimum mass of the putative planetary companion is M_p sin i = 1.43 M_Jup. Taking advantage again of the high-precision Hipparcos observations we are able to place a dynamical upper limit on this mass of 13.3 M_Jup (95% confidence level) and 16.9 M_Jup (99.73%, or 3 sigma) thus confirming that it is indeed substellar in nature. (Abridged)