Precise radial velocity measurements from McDonald Observatory establish the presence of a planet orbiting the primary component of the gamma Cephei binary star system, as indicated by earlier data from the CFHT (Walker et al. 1992). This companion is the first extrasolar planet in a relatively close stellar binary system (the two stars at their closest approach are only 12 AU apart), and thus has far-reaching implications for our understanding how planets form in binary systems and for the overall frequency of planetary systems in our galaxy (since the majority of stars exist in binary or multiple systems).
Residual radial velocity variations of gamma Cephei after subtraction of the binary orbital motion (click to enlarge). The planetary signal (solid black line) with a semi-amplitude of 26 m/s is present over 20 years consistent in amplitude and phase. The color coding follows the same scheme as described above.
Gamma Cephei is a K1 sub-giant in a long-period (P> 40 yrs) binary system. Walker et al. (1992) first reported short-term periodic radial velocity (RV) variations in the residuals after subtracting the velocity contribution due to the stellar companion. These residual variations had a period of 2.52 yr period and were consistent with a Jupiter-mass companion in orbit at approximately 2 AU from the primary. Walker et al. dismissed this hypothesis because they detected weak variations in the Ca II emission line index with the same period as the planet.
Precise stellar radial velocity measurements for gamma Cep made at McDonald Observatory now extend the time baseline of observations to more than 20 years. The combined dataset shows that the planet period has been present and coherent over the past 20 years. The long lived nature of the residual RV variations make it unlikely that they are due to stellar rotation. Our Ca II S-index measurements show no periodic variations at the 2.5 yr period. Furthermore, no appropriate period is found in (contemporaneous) Hipparcos photometry.
We conclude that a planet with a projected mass of 1.76 Jupitermasses in orbit 1.8 AU from the primary star is the most likely explanation for the short term RV variations in gamma Cep. This planet is in a binary star system with the shortest binary period found so far and should provide an interesting case study for understanding how binary stars influence the planet formation process.
P = 903 +/- 6 days (orbital period)
T = 53156.85 +/- 52 days (time of periastron in Julian Date)
e = 0.2 +/- 0.07 (orbital eccentricity)
w = 76 +/- 19 deg (angle of periastron)
K = 26.3 m/s (radial velocity semi-amplitude)
P = 74 +/- 16 yrs (orbital period)
T = 48506 +/- 38 days (time of periastron in Julian Date)
e = 0.44 +/- 0.06 (orbital eccentricity)
w = 162.1 +/- 1.8 (angle of periastron)
K = 1980 m/s (radial velocity semi-amplitude)
M2 sin i = 0.4 solar masses (M1V?)
a1 sin i = 4.21 AU (semimajor axis of the primary)
a2 sin i = 17.16 AU (semimajor axis of the secondary)
McDonald Observatory press release (9th.Oct.2002)
AG Tagung press release (in german)
TLS Tautenburg Observatory