2. Observing program
The sample contains the 127 M dwarfs listed in the third edition of the nearby star catalog (CNS3 preliminary version, Gliese & Jahreiss, 1991) with a distance closer than 9 pc, a B1950.0 declination above -16 degrees, brighter than V=15, and without a close much brighter primary. Observations have been carried out since September 1995 with the ELODIE fiber-fed spectrograph (Baranne et al., 1996) on the 1.93m telescope at Observatoire de Haute Provence (OHP). The R=42000 spectra are wavelength calibrated through simultaneous observations of a thorium lamp. Since June 1998 some southern stars have also been observed with the nearly identical CORALIE spectrograph on the recently commissioned swiss 1.2m telescope at la Silla (Chile). CORALIE mostly differs from the older ELODIE instrument by its spectral resolution of R=50000, better sampling of the spectrograph PSF by the CCD camera pixels, and significantly better temperature control. The first indications are that these modifications together result in a substantially improved intrinsic stability (Queloz et al., in preparation).
The extracted M dwarf spectra are analysed through cross-correlation with a binary (0/1) template constructed from an observed ELODIE spectrum of Barnard's star, Gl699 (Delfosse et al., 1998b). For slowly rotating stars the resulting velocities have internal standard errors (photon noise plus low level uncalibrated instrumental instabilities) which typically range from 10-15 m.s-1 for bright M dwarfs (V10) to 75 m.s-1 at the magnitude limit of the sample. For Gl 876 (V=10.2) typical standard errors are 10 to 20 m.s-1, depending on airmass and seeing conditions. Magnetic activity is common in M type dwarfs, and may further degrade the measurement accuracy (Saar et al., 1998). This potential error source is still uncompletely characterised for M dwarfs, but for slowly rotating stars () we can already bound it to 20m.s-1 for our cross-correlation analysis with the M4 binary template. Within the brighter two thirds of the sample, a conservative assumption at the present time is that we will detect all variables with semi-amplitudes larger than 40m.s-1. Assuming for illustration a 5 years period, this corresponds to a 1 Jupiter mass (MJ) planet orbiting a 0.25 M4V primary (at 1.8 AU), or to a 2 MJ planet orbiting a 0.6 M0V primary (at 2.5 AU).
© European Southern Observatory (ESO) 1998
Online publication: September 14, 1998