A Planetary Companion to gamma Cephei A

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Abstract
We report the detection of a planetary companion in orbit around the primary star of the binary system gamma~Cephei. High precision radial velocity measurements using 4 independent data sets spanning the time interval 1981 to 2002 reveal long-lived residual radial velocity variations 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 Msin i = 1.59 M_Jupiter and an orbital semi-major axis of 2.03AU. We have carefully analyzed of all the available photometric and spectroscopic data. Our CaII H&K S-index measurements taken during 1998-2002 show no variations with the planet period. Analysis of high resolution spectra taken 1988-1995 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 to 1992 is constant to less than 0.001 mag. An analysis of the Walker etal CaII 8662A data shows that the possible long-period variations found by Walker etal were only present during 1986.5 to 1992 and absent during 1981 to 1986.5. While these CaII 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 CaII 8662A variations is 2.1 yrs, significantly less than residual radial velocity period. The lack of CaII, 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 etal (1992).

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