2010A&A...520A..54B

While M dwarfs are the most abundant stars in the Milky Way, there is still large uncertainty about their basic physical properties (mass, luminosity, radius, etc.) as well as their formation environment. Precise knowledge of multiplicity characteristics and how they change in this transitional mass region, between Sun-like stars on the one side and very low mass stars and brown dwarfs on the other, provide constraints on low mass star and brown dwarf formation. In the largest M dwarf binary survey to date, we search for companions to active, and thus preferentially young, M dwarfs in the solar neighbourhood. We study their binary/multiple properties, such as the multiplicity frequency and distributions of mass-ratio and separation, and identify short period visual binaries, for which orbital parameters and hence dynamical mass estimates can be derived in the near future. The observations are carried out in the SDSS i' and z' band using the Lucky Imaging camera AstraLux Sur at the ESO 3.5m New Technology Telescope. Lucky Imaging is a very efficient way of observing a large sample of stars at an angular resolution close to the diffraction limit. In the first part of the survey, we observed 124 M dwarfs of integrated spectral types M0-M6 and identified 34 new and 17 previously known companions to 44 stars. We derived relative astrometry and component photometry for these binary and multiple systems. More than half of the binaries have separations smaller than 1'' and would have been missed in a simply seeing-limited survey. Correcting our sample for selection effects yields a multiplicity fraction of 32±6% for 108 M dwarfs within 52pc and with angular separations of 0.1''-6.0'', corresponding to projected separations of 3-180AU at median distance 30pc. Compared to early-type M dwarfs (M>0.3M_☉), later-type (and hence lower mass) M dwarf binaries appear to have closer separations, and more similar masses.