Geneva Observatory, ch des Maillettes 51, CH-1290 Sauverny
After the detection of 2 extra-solar planets in the Southern sky around Gl 86 and HD 75289 the high-resolution CORALIE spectrograph on the 1.2-m Euler Swiss telescope at La Silla Observatory is yielding its 3rd planetary candidate in less than 1 year.
The new extra-solar planet has been detected around the star HD 130322 in the VIRGO constellation. HD 130322 is a K0 dwarf of apparent magnitude V=8.04 and color index B-V=0.781. Its metallicity is solar and the luminosity is about half the solar one. The HIPPARCOS catalogue lists a precise astrometric parallax pi=33.6 +/- 1.5 mas setting the star at a distance of about 30 pc from the Sun. The derived absolute magnitude M_V=5.67 invalidates the earlier giant classification generally found in the literature.
A set of 111 high-precision radial-velocity (RV) measurements over 15 cycles has allowed us to determine an accurate orbital solution for the system. The planet orbits its parent star in 10.7 days inducing a RV semi-amplitude variation of 116 m/s for the star which is easily detected with the precision level obtained with CORALIE (6-7 m/s). With a small but significant eccentricity e=0.06 the orbit is almost circular. From the period and mass of the primary star (0.79 M_Sun), the separation between the planet and the star is estimated to be 0.088 AU and the inferred mass of the planetary companion about 1.08 M_Jup. With a period of 10.7 days, the planet belongs to the "hot Jupiter" family. The temperature estimate at the planet surface is slightly higher than 1000 deg K. This new candidate furthermore increases the observed piling up of planets with small orbital separations.
Since, for short-orbital periods, RV periodic variations could be induced by motions of the photosphere due to stellar activity-related variations over a rotational period, it is important to eliminate this possibility by objective criteria. Although HD 130322 is a slow rotator (Vsini<2km/s), the star is slightly active as indicated by the emission flux in the core of the CaII H and K absorption lines. However, neither a careful analysis of the mean bisector of the stellar lines nor the Geneva and the Hipparcos photometries do exhibit coherent variations ruling thus out any possible explanations of the observed periodic RV variations involving intrinsic stellar phenomena. This leaves the planetary explanation as the most likely. The fair activity level can however be invoked to explain the relatively high residues around the derived orbital solution (O-C=15 m/s) compared with the CORALIE precision.