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 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).
Gamma Cephei is a K1 sub-giant in a long-period (P> 50 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 obtained with the Harlan J. Smith 2.7 m telescope 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 minimum mass of 1.7 Jupitermasses in orbit 2.13 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.
Parameters of the planet and its orbit:
Period = 905.57 ± 3.08 [days]
T periastron = 253121.925 ± 66.9 [JD]
e = 0.12 ± 0.05
omega = 49.6 ± 25.6 [deg]
K = 27.5 ± 1.5 [m/s]
M sin i = 1.7 ± 0.4 [Jupiter]
a = 2.13 ± 0.05 [AU]
Parameters of the binary orbit:
Period = 20750.7 ± 1568.6 [days]
T periastron = 248429.03 ± 27.0 [JD]
e = 0.361 ± 0.023
omega = 158.8 ± 1.2 [deg]
K = 1.82 ± 0.049 [km/s]
f(m) = 0.0106 ± 0.0012 M [Sun]
a = 18.5 ± 1.1 [AU]
Properties of the host star: (derived by Klaus Fuhrmann)
Spectral type = K1 IV
V = 3.22 [mag]
M bol = 2.14 [mag]
d = 13.79 [parsec] (based on Hipparcos parallax)
M = 1.59 ± 0.12 M [Sun]
Teff = 4888 [K]
log g = 3.33
Radius = 4.66 [R Sun]
References:
Hatzes A.P., Cochran W.D., Endl M., McArthur B., Paulson D.B., Walker, G.A.H., Campbell, B., Stephenson Y. 2003, ApJ, 599, 1383
paper (pdf-file)
Walker, G.A.H., Bohlender, D.A., Walker, A.R., Irwin, A.W., Stephenson, L.S., Larson, A.,
1992, ApJ, 396, L91-94
link to ADS service
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