2007A&A...472..321K

Direct imaging using various techniques for suppressing the stellar halo nowadays can achieve the contrast levels required to detect and characterize the light of substellar companions at orbital distances greater than a few astronomical units from their host stars. The method nicely complements the radial velocity surveys that provide evidence that giant extrasolar planets in close-in orbits are relatively common. The paper presents results from a small survey of 22 young, nearby stars that was designed to detect substellar companions and ultimately giant extrasolar planets down to Jupiter masses. The targets are members of the Tucana and β Pictoris moving groups, apart from the somewhat older star HIP 71395, which has a radial velocity trend suggesting a massive planet in large orbit. The survey was carried out in the L-band using adaptive optics-assisted imaging with NAOS-CONICA (NACO) at the VLT. The chosen observation wavelength is well-suited to searching for close companions around young stars and it delivers unprecedented detection limits. The presented technique reaches some of the best sensitivities as of today and is currently the most sensitive method for the contrast-limited detection of substellar companions that are cooler than about 1000K. The companion to 51 Eri, GJ 3305, was found to be a very close binary on an eccentric orbit. No substellar companions were found around the target stars, although the method permitted companions to be detected down to a few Jupiter masses at orbital distances typically of 5 astronomical units. A planet with a mass ≥1M_Jup at distances ≥5AU around AU Mic can be excluded at the time of our observations. The absence of detected planets sets constraints on the frequency distribution and maximum orbital distance of giant exoplanets. For example, a radial distribution power law index of 0.2 in combination with a maximum orbital radius exceeding 30AU can be rejected at a 90% confidence level.