An infrared imaging search for low-mass companions to members of the young nearby beta Pic and Tucana/Horologium associations

We present deep high dynamic range infrared images of young nearby stars in the Tucana/Horologium and beta Pic associations, all similar to 10 to 35 Myrs young and at similar to 10 to 60 pc distance. Such young nearby stars are well-suited for direct imaging searches for brown dwarf and even planetary companions, because young sub-stellar objects are still self-luminous due to contraction and accretion. We performed our observations at the ESO 3.5m NTT with the normal infrared imaging detector SofI and the MPE speckle camera Sharp-I. Three arc see north of GSC 8047-0232 in Horologium a promising brown dwarf companion candidate is detected, which needs to be confirmed by proper motion and/or spectroscopy. Several other faint companion candidates are already rejected by second epoch imaging. Among 21 stars observed in Tucana/Horologium, there are not more than one to five brown dwarf companions outside of 75 AU (1.5 at 50 pc); most certainly only less than or equal to 5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupiter masses) outside of 75 AU. For the first time, we can report an upper limit for the frequency of massive planets (similar to 10 M-jup) at wide separations (similar to 100 AU) using a meaningfull and homogeneous sample: Of 11 stars observed sufficiently deep in beta Pic (12 Myrs), not more than one has a massive planet outside of similar to 100 AU, i.e. massive planets at large separations are rare (less than or equal to 9 %).