Lucky Imaging survey for southern M dwarf binaries

Abstract

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.

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

“…The binary-fraction and width of the observed log-normal separation distributions in the Galactic field decline continuously with decreasing mass of the primary component (Duquennoy & Mayor 1991;Fischer & Marcy 1992;Delfosse et al 2004;Bergfors et al 2010;Raghavan et al 2010;Janson et al 2012;Jódar et al 2013;Tokovinin 2014;Duchêne & Kraus 2013;Reipurth et al 2014;Ward-Duong et al 2015;Cortés-Contreras et al 2017). This continuity across the mass-limit between stars and BDs was apparently supported by recent investigations of the late M-dwarf binary population in the Galactic field.…”

Characterization of close visual binaries from the AstraLux Large M Dwarf Survey

“…In order to derive approximate spectral types for the binary or multiple systems discovered in the AstraLux M dwarf survey and exclude background contaminants, each target was observed in SDSS i ′ -and z ′ -band and individual spectral types were derived from i ′ − z ′ colours and the integrated JHK spectral types, with estimated errors of ±1 spectral subtype (see Daemgen et al 2009;Bergfors et al 2010). Obtaining near-infrared spectral types allows us to Table 2.…”

“…better quantify the precision of the photometric spectral types, as derived for individual M dwarf binary components in Bergfors et al (2010); Janson et al (2012Janson et al ( , 2014a, and identify potential biases in the method. We find that, with the exception of J021330 B, all near-infrared spectral types agree with the photometrically derived spectral types within the photometric ±1 subclass error, and in most cases to within 0.5 subclasses (see Table 2).…”

The AstraLux Binary M Dwarfs Survey

“…In the first part of the southern sky survey, 124 young, nearby M dwarfs were observed with AstraLux Sur at NTT in November 2008 (Bergfors et al 2010). This subsample is in itself the largest M dwarf multiplicity survey to date.…”