A Planetary Companion to Γ Cephei A

We report the detection of a planetary companion in orbit around the primary star of the binary system γ Cephei. High precision radial velocity measurements using 4 independent data sets spanning the time interval 1981–2002 reveal long-lived residual radial velocity variations (after subtracting the contribution due to the stellar companion) that are coherent in phase and amplitude with a period or 2.47 years and a semi-amplitude of 27 m s. These residual radial velocity variations are most likely caused by a planetary mass companion with M sin i = 1.59 MJupiter and an orbital semi-major axis of a2 = 2.03 AU. The conclusion regarding the planet hypothesis was made after a careful analysis of all the available photometric and spectroscopic data. Our Ca II H & K S-index measurements taken during 1998-2002 show no variations with the planet period. One of our data sets spanning the time 1988-1995 consists of high resolution (R = 200,000) spectra suitable for line bisector measurements. This analysis shows that the changes in the mean bisector velocity span and curvature for this star is less than 5 m s. The Hipparcos photometry for this star made during 1989 – 1992 is constant to less than 0.001 mag. An analysis of the Walker et al. Ca II λ8662 Å data shows that the possible long-period variations found by Walker et al. were only present during 1986.5 – 1992 and absent during 1981–1986.5. While these Ca II variations are not long-lived, as opposed to the radial velocity variations which have been completely stable in period and phase over 20 years, it is unlikely that rotation modulation is causing the radial velocity signal attributed to the planet. Furthermore, a refined period for the Ca II λ8662 Å variations is 2.1 yrs, significantly less than residual radial velocity period. The lack of Ca II, line bisector, and photometric variations for this star is inconsistent with the rotational modulation hypothesis for the residual RV variations and confirms the planet hypothesis first proposed by Walker et al. (1992). The long duration of our radial velocity measurements enable us to obtain a better estimate of the binary period or PBinary = 73.9 ± 15.8 yrs. γ Cep is the shortest period binary for which an extrasolar planet has been found and it may provide insights into the relationship between planetary and binary star formation. Subject headings: planetary systems — techniques: radial velocities