6. Concluding remarks
We have presented two giant-planet candidates, around HD 75289 and HD 130322, detected with the CORALIE echelle spectrograph mounted on the 1.2-m Euler Swiss telescope at La Silla Observatory. Both planets are on quasi-circular orbits close to their parent stars, they have periods of 3.51 and 10.72 days. The inferred minimum masses are 0.42 (the lightest found to date) and 1.02 , respectively. These objects were discovered as part of a large radial-velocity monitoring of more than 1650 southern dwarf stars of the solar vicinity, forming a volume-limited sample well-defined for statistical studies as e.g. the distribution of planetary orbital elements or the distribution of (substellar) companions to solar-type stars.
With 4 CORALIE planetary candidates (Gl 86, HD 75289, HD 130322, HD 192263) announced in about 1 year, our observing facility shows its efficiency (precision + observing time availability) for giant-planet detection. Out of our sample we expect to obtain several tens of planet candidates. This will bring stronger statistical constraints for the formation and evolution theories of these systems. The availability of a dedicated telescope is of prime importance for achieving this efficiency. If the stellar variability is rapidly established with the commonly achieved precision, the orbital parameter determination (especially the period and eccentricity) is more affected by the data sampling induced by the observation time windows. This may explain the presently observed lack of systems with semi-amplitude radial-velocity variations smaller than about 35 m s-1 which moreover require a large number of measurements and are more sensitive to activity-related noise.
Concerning activity-related noise, it is worth noticing that one of the planetary candidates presented here orbits a fairly active K0 star (HD 130322; ). This is also the case for HD 192263 (Santos et al. 2000) which is an active K2 dwarf (). In both cases the radial-velocity jitter induced by activity is small compared to the orbital radial-velocity semi-amplitude and thus did not prevent us from detecting the planet. This illustrates the result pointed out by Saar et al. (1998) and Santos et al. (1999b) that activity-induced radial-velocity noise is becoming smaller when going from late F to K dwarfs. The latters remain thus suitable targets for planet-search programmes even if they show a fair activity level. This is especially true for non- or slowly-rotating stars, knowing that rotation amplifies the spectral line asymmetries induced by the change in the stellar disk illumination due to the presence of stellar spots.
Finally, the combination of bisector analysis and cross-correlation technique (Queloz et al. in prep) provides a robust diagnostic which allows us to confidently rule out intrinsic phenomena for the source of observed radial-velocity variations in the case of short-period planetary companions, as illustrated on our two candidates.
© European Southern Observatory (ESO) 2000
Online publication: April 10, 2000