Low-Mass Companions to Solar-Type Stars

We present preliminary results from a coronagraphic survey of young nearby Sun-like stars using the Palomar and Keck adaptive optics systems. We have targeted 251 solar analogs (F5-K5) at 20-160 pc from the Sun, spanning the 3-3000 Myr age range. The youngest (<500 Myr) 100 of these have been imaged with deeper exposures to search for sub-stellar companions. The deep survey is sensitive to brown-dwarf companions at separations >0.5" from their host stars, with sensitivity extending to planetary-mass (5-15 Mjup) objects at wider (>3") separations. Based on the discovery of a number of new low-mass (<0.2 Msun) stellar companions, we infer that their frequency at >20 AU separations (probed via direct imaging) may be greater (12%) than that found from radial velocity surveys probing <4 AU separations (6%; Mazeh et al. 2003). We also report the astrometric confirmation of the first sub-stellar companion from the survey - an L4 brown dwarf at a projected distance of 44 AU from the 500 Myr-old star HD 49197. Based on this detection, we estimate that the frequency of sub-stellar companions to solar-type stars is at least 1%, and possibly of order a few per cent.