Geneva Observatory, ch des Maillettes 51, CH-1290 Sauverny
Four years after the discovery of the planet around 51 Peg, more than 20 extrasolar planets have been detected. These planets present a wide variety of (minimum) masses and kinematic properties, and show that extrasolar giant planets are rather common around solar-type stars.
Now, the CORALIE survey is adding another object to the extrasolar planetary list. The planet orbits the star HD 192263 (HIP 99711), a 7.79 apparent magnitude K2 dwarf in the constellation Aquila (The Eagle). This star has a parallax pi=50.27 +/- 1.13 mas (from the Hipparcos catalogue), corresponding to a distance of 19.9 pc. The derived absolute magnitude and luminosity are M_V=5.91 and L=0.34 solar luminosity. Its metallicity index, obtained from Stroemgren-uvby photometry, has a value [Fe/H]=-0.20. A similar value of -0.14 is obtained using a calibration of the CORAVEL cross-correlation dip, confirming its slight poor metal content.
40 high-precision radial-velocity (RV) measurements were obtained between May and September 1999, and cover about 7 orbital cycles, allowing us to derive an accurate orbital solution. The measurements have in average a precision of about 9 m/s due to photon noise. The planet orbits its parent star in a circular orbit, with a period of 23.87 days. The induced velocity semiamplitude amounts to 65 m/s. Given the orbital period and the mass of the parent star (0.75 M_Sun, inferred from its color index), the derived star-planet separation is 0.15 AU, and the minimum mass of the companion is 0.76 M_Jup. At such a distance from its parent star, the equilibrium temperature of the planet is about 480 K.
Activity-related processes can induce RV variations. Although HD 192263 is a slow rotator (Vsin i = 1.8 +/- 1.2 km/s) it is slightly active (log(R'_HK)=-4.37). The age derived from the Geneva evolutionary tracks suggests the star may be very young, which is consistent with its activity level. However, the analysis of the bisector of the cross-correlation dip shows no coherent variations. Furthermore, the Hipparcos and Geneva photometries show the star is stable. These facts rule out intrinsic stellar-activity as the source of the observed RV variation. Nevertheless, the observed residuals of 11 m/s around the orbital solution are close to the mean photon-noise value of 9 m/s and the small excess can be explained by the activity level of the star, leaving the planetary explanation as the only really capable of explaining the observed periodic RV variation.