The ELODIE survey for northern extra-solar planets - I. Six new extra-solar planet candidates

Precise radial-velocity observations at Haute-Provence Observatory (OHP, France) with the ELODIE echelle spectrograph have been undertaken since 1994. In addition to several discoveries described elsewhere, including and following that of 51 Peg b, they reveal new sub-stellar companions with essentially moderate to long periods. We report here about such companions orbiting five solar-type stars (HD8574, HD23596, HD33636, HD50554, HD106252) and one sub-giant star (HD190228). The companion of HD8574 has an intermediate period of 227.55 days and a semi-major axis of 0.77AU. All other companions have long periods, exceeding 3 years, and consequently their semi-major axes are around or above 2AU. The detected companions have minimum masses m(2) sin i ranging from slightly more than 2M(Jup) to 10.6 M-Jup. These additional objects reinforce the conclusion that most planetary companions have masses lower than 5M(Jup) but with a tail of the mass distribution going up above 15 M-Jup. The orbits are all eccentric and 4 out of 6 have an eccentricity of the order of 0.5. Four stars exhibit solar metallicity, one is metal-rich and one metal-poor. With 6 new extra-solar planet candidates discovered, increasing their total known to-date number to 115, the ELODIE Planet Search Survey yield is currently 18. We emphasize that 3 out of the 6 companions could in principle be resolved by diffraction-limited imaging on 8 m-class telescopes depending on the achievable contrast, and therefore be primary targets for first attempts of extra-solar planet direct imaging.