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Astron. Astrophys. 356, 590-598 (2000)

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2. The CORALIE planet-search sample

The diversity of characteristics presented by the new planetary systems (e.g. MCM99) makes clear that existing ideas of planetary formation need serious revision. Enlarging the number of detected systems is now necessary to obtain proper statistical distributions of orbital elements, planetary masses, etc, fundamental for a better understanding of the formation and evolution of these systems.

Since the summer of 1998, a large high-precision radial-velocity programme is being carried out with the CORALIE echelle spectrograph on the 1.2-m Euler Swiss telescope at La Silla. Velocities are measured by numerically cross-correlating high-resolution spectra with a template appropriate for G-K stars. Further information on the method as well as technical and instrumental details and a description of the first discoveries of the programme are given in Baranne et al. (1996) and in Queloz et al. (1999, 2000).

The overall sample consists of about 1650 solar-type stars selected according to distance in order to obtain a well-defined volume-limited set of dwarf stars. As very briefly described in Udry et al. (1999), the global sample selection was performed applying the following criteria:

  • Selection of stars closer than 50 pc ([FORMULA] mas) with precise parallaxes ([FORMULA] mas) and F8 [FORMULA] Spectral type [FORMULA] M1 in the HIPPARCOS catalogue.

  • Photometric cut off of giant stars (2.5 mag off a "mean" main sequence). This allows us to remove most evolved stars while keeping stars with misclassified luminosity classes like the two candidates presented here (see below).

  • Removal of the fainter cool dwarfs, by a colour-dependent distance cut off ([FORMULA] = [FORMULA]), taking into account that our cross-correlation technique needs less signal for K dwarfs 1.

The final sample is presented in Fig. 1 which shows the stellar apparent magnitude and distance distributions. We note that [FORMULA]80% of the selected stars are brighter than [FORMULA] and that faint objects are mostly K dwarfs for which our technique is the most efficient.

[FIGURE] Fig. 1. Apparent magnitude and distance distributions of the CORALIE planet-search sample. 80% of the stars are brighter than [FORMULA] the faintest ones being mainly K dwarfs for which our technique is the most efficient

For the planet search itself, large amplitude binaries and potentially intrinsic variable stars (large activity index and fast rotators, Saar et al. 1998, Santos et al. 1999b, see Sect. 5) have to be avoided. The stability of a planetary system may be perturbed and even destroyed by the close proximity of a stellar companion. Moreover, the simultaneous determinations of the large amplitude and planetary orbits would require an additional observational effort. Most large-amplitude systems were detected with the CORAVEL spectrographs (Baranne et al. 1979) at a lower precision level (300m s-1). Long-period binaries showing only linear drifts in radial velocity remain nevertheless good candidates for the planet search as the gravitational perturbation of the stellar companion is weak and the long-period orbital effect on the radial velocities can be easily corrected (e.g. Gl 86, Paper I). The CORAVEL measurements also provide us with the stellar rotational velocity (Benz & Mayor 1984) often used as an activity indicator (Saar et al. 1998). The fast rotators and large amplitude binary systems are collected in low-priority subprogrammes used as observing backup programmes; however, they still belong to the global volume-limited sample to be used for statistical purpose. We are left in the end with about 1000 high-priority candidates for the core of the planet-search programme itself.

The CORALIE survey offers a unique opportunity to explore the relation between activity and other stellar characteristics like rotational velocity, rotational period or intrinsic radial-velocity and photometric variations (Santos et al. 1999b).

Because of its size and the quality of the measurements, the above defined planet-search programme will also significantly improve the data available for stellar binary systems, reactualizing the Duquennoy & Mayor (1991) type of work. In particular, the determination of a good distribution of masses of companions of solar-type stars down to the planet regime will bring some further insights in the brown dwarf - planet transition zone. This will further support or invalidate the so-called "brown-dwarf desert" suspected by the lack of brown-dwarf candidates in the high-precision radial-velocity surveys (Marcy & Butler 1995) and recently emphasized by the study of Halbwachs et al. (2000) which combines CORAVEL and ELODIE radial-velocity measurements with HIPPARCOS astrometric data to derive real masses for a set of spectroscopic brown-dwarf candidates (Mayor et al. 1997).

Finally, the survey will naturally provide good candidates for the future ground-based (VLTI) and spatial (SIM) interferometric programmes.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000