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Astron. Astrophys. 354, 99-102 (2000)

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1. Introduction

Almost 20 planetary companions with minimum masses ([FORMULA]) less than 10 [FORMULA] have so far been detected by very high-precision radial velocity surveys (Butler et al. 1999; Marcy et al. 1999a; Fischer et al. 1999 and Marcy et al. 1999bfor a review of older detections). The semi-major axes of their orbits range from very small (0.05 AU) to 3 AU. Some have eccentric orbits, others have secondary more massive companions, some have both. The large observed spread in orbital characteristics of all known planetary candidates causes some difficulties in understanding their formation process in comparison with our own solar system. It also raises the issue of the real nature of these objects, particularly the more massive ones.

In June 1998 we initiated a systematic and large scale exoplanet search survey (1600 nearby G and K stars) in the southern hemisphere with the new 1.2 m alt-azimuth Euler Swiss telescope at La Silla, ESO Chile. The technique we are using to detect planets is to look for a stellar reflex motion due to an orbiting planet by very precise radial velocity measurements. The CORALIE echelle spectrograph is used to measure star spectra from which the Doppler effect is then computed.

CORALIE is an improved version of the ELODIE spectrograph (Baranne et al. 1996) with which, 4 years ago, the first extra-solar planet orbiting a star (51 Peg) was discovered (Mayor & Queloz 1995). The CORALIE front-end adaptator is located at the Nasmyth focus of the Euler telescope. Two sets of two fibers can alternatively feed the spectrograph which is located in an isolated and temperature controled room. The set of fibers used for high precision radial velocity measurements includes a double scrambler device designed by Dominique Kohler (see Queloz et al. (1999) for references) to improve the stability of the input illumination of the spectrograph. Thanks to a slightly different optical combination at the entrance of the spectrograph and the use of a 2k by 2k CCD camera with smaller pixels (15µm), CORALIE has a larger resolution than ELODIE. A resolving power of 50,000 ([FORMULA]) is observed with a 3 pixel sampling. As with the ELODIE spectrograph, CORALIE makes use of on-line reduction software that computes the radial velocity of stars several minutes after their observation. (See Baranne et al. (1996) for details about the reduction process). The simultaneous thorium technique is used to correct any instrumental drifts occuring during the star exposure (see Queloz (1999) for details). The many improvements carried out in the thermal control and the resolution of the instrument, as well as in the reduction software, yield a factor two improvement in the instrument precision compared with ELODIE.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000