摘要：Context. We present the results of a survey to detect low-mass
companions of Ursa Major (UMa) group members, carried out in 2003–2006 with NACO at the
ESO VLT. While many extra-solar planets and planetary candidates have been found in close
orbits around stars by the radial velocity and the transit methods, direct detections at
wider orbits are rare. The UMa group, a young nearby stellar association at an age of
about 200–600 Myr, has not yet been addressed as a whole although its members represent a
very interesting sample to search for and characterize substellar companions by direct
imaging.
Aims. Our goal was to find or to provide detection limits on wide
substellar companions around nearby UMa group members using high-resolution imaging.
Methods. We searched for faint companions around 20 UMa group members
within 30 pc. The primaries were placed below a semi-transparent coronagraph, a rarely
used mode of NACO, to increase the dynamic range of the images. In most cases, second
epoch images of companion candidates were taken to check whether they share common proper
motion with the primary.
Results. Our coronagraphic images rule out substellar companions around
the stars of the sample. A typical dynamical range of 13–15 mag in the Ks band was
achieved at separations beyond 3′′ from the star. Candidates as faint as Ks ≈ 20 were
securely identified and measured. The survey is most sensitive between separations of 100
and 200 au but only on average because of the very different target distance. Field
coverage reaches about 650 au for the most distant targets. Most of the 200 candidates
detected in the covered fields are visible in two epochs and were rejected because they
are distant background objects.
