We present the results of an extensive high-resolution imaging survey of M-dwarf multiplicity using the Lucky Imaging technique. The survey made use of the AstraLux Norte camera at the Calar Alto 2.2m telescope and the AstraLux Sur camera at the ESO New Technology Telescope in order to cover nearly the full sky. In total, 761 stars were observed (701 M-type and 60 late K-type), among which 182 new and 37 previously known companions were detected in 205 systems. Most of the targets have been observed during two or more epochs, and could be confirmed as physical companions through common proper motion, often with orbital motion being confirmed in addition. After accounting for various bias effects, we find a total M-dwarf multiplicity fraction of 27 ± 3% within the AstraLux detection range of 0.08-6 ′′ (semi-major axes of ∼3-227 AU at a median distance of 30 pc). We examine various statistical multiplicity properties within the sample, such as the trend of multiplicity fraction with stellar mass and the semi-major axis distribution. The results indicate that M-dwarfs are largely consistent with constituting an intermediate step in a continuous distribution from higher-mass stars down to brown dwarfs. Along with other observational results in the literature, this provides further indications that stars and brown dwarfs may share a common formation mechanism, rather than being distinct populations.

Context. Straight-forward derivation of planetary parameters can only be achieved in transiting planetary systems. However, planetary attributes such as radius and mass strongly depend on stellar host parameters. Discovering a transit host star to be multiple leads to a necessary revision of the derived stellar and planetary parameters. Aims. Based on our observations of 14 transiting exoplanet hosts, we derive parameters of the individual components of three transit host stars (WASP-2, TrES-2, and TrES-4) which we detected to be binaries. Two of these have not been known to be multiple before. Parameters of the corresponding exoplanets are revised. Methods. High-resolution "Lucky Imaging" with AstraLux at the 2.2 m Calar Alto telescope provided near diffraction limited images in i and z passbands. These results have been combined with existing planetary data in order to recalibrate planetary attributes. Results. Despite the faintness (Δmag ∼ 4) of the discovered stellar companions to TrES-2, TrES-4, and WASP-2, light-curve deduced parameters change by up to more than 1σ. We discuss a possible relation between binary separation and planetary properties, which -if confirmed -could hint at the influence of binarity on the planet formation process.

Context. 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. Aims. 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. Methods. The observations are carried out in the SDSS i and z band using the Lucky Imaging camera AstraLux Sur at the ESO 3.5 m 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. Results. In the first part of the survey, we observed 124 M dwarfs of integrated spectral types M 0−M 6 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 52 pc and with angular separations of 0.1 −6.0 , corresponding to projected separations of 3−180 AU at median distance 30 pc. Compared to earlytype M dwarfs (M > ∼ 0.3 M ), later-type (and hence lower mass) M dwarf binaries appear to have closer separations, and more similar masses.

Observed properties of stars and planets in binary/multiple star systems provide clues to planet formation and evolution. We extended our survey for visual stellar companions to the hosts of transiting exoplanets by 21 stars, using the Lucky Imaging technique with the two AstraLux instruments: AstraLux Norte at the Calar Alto 2.2-m telescope, and AstraLux Sur at the ESO 3.5-m New Technology Telescope at La Silla. Typically a sensitivity to companions of magnitude difference ∆z ′ ≈ 4 is achieved at angular separation ρ = 0.5 ′′ and ∆z ′ 6 for ρ = 1 ′′ .We present observations of two previously unknown binary candidate companions, to the transiting planet host stars HAT-P-8 and WASP-12, and derive photometric and astrometric properties of the companion candidates. The common proper motions of the previously discovered companion candidates with the exoplanet host stars TrES-4 and WASP-2 are confirmed from follow-up observations. A Bayesian statistical analysis of 31 transiting exoplanet host stars observed with AstraLux suggests that the companion star fraction of planet hosts is not significantly different from that of solar-type field stars, but that the binary separation is on average larger for planet host stars.

We present near-infrared high-contrast imaging photometry and integral field spectroscopy of ROXs 42B, a binary M0 member of the 1-3 Myr-old ρ Ophiuchus star-forming region, from data collected over 7 years. Each data set reveals a faint companion -ROXs 42Bb -located ∼ 1.16 ′′ (r proj ≈ 150 AU ) from the primaries at a position angle consistent with a point source identified earlier by Ratzka et al. (2005). ROXs 42Bb's astrometry is inconsistent with a background star but consistent with a bound companion, possibly one with detected orbital motion. The most recent data set reveals a second candidate companion at ∼ 0. ′′ 5 of roughly equal brightness, though preliminary analysis indicates it is a background object. ROXs 42Bb's H and K s band photometry is similar to dusty/cloudy young, low-mass late M/early L dwarfs. K-band VLT/SINFONI spectroscopy shows ROXs 42Bb to be a cool substellar object (M8-L0; T ef f ≈ 1800-2600 K), not a background dwarf star, with a spectral shape indicative of young, low surface gravity planet-mass companions. We estimate ROXs 42Bb's mass to be 6-15 M J , either below the deuterium burning limit and thus planet mass or straddling the deuterium-burning limit nominally separating planet-mass companions from other substellar objects. Given ROXs 42b's projected separation and mass with respect to the primaries, it may represent the lowest mass objects formed like binary stars or a class of planet-mass objects formed by protostellar disk fragmentation/disk instability, the latter slightly blurring the distinction between non-deuterium burning planets like HR 8799 bcde and low-mass, deuterium-burning brown dwarfs.