The AstraLux Multiplicity Survey: Extension to Late M-dwarfs

TL;DR

This study extends the multiplicity survey of low-mass stars to include 286 mid/late M-dwarfs, revealing new companions and providing detailed statistical analysis of their binary properties.

Contribution

It presents the first comprehensive multiplicity analysis of mid/late M-dwarfs, bridging the gap between earlier M-type and brown dwarf studies with new observational data.

Findings

Detected 68 companions, 41 of which are new discoveries.

Multiplicity fraction is 21-27%, depending on mass ratio assumptions.

Semi-major axis distribution follows a narrower log-normal function than for Sun-like stars.

Abstract

The distribution of multiplicity among low-mass stars is a key issue to understanding the formation of stars and brown dwarfs, and recent surveys have yielded large enough samples of nearby low-mass stars to study this issue statistically to good accuracy. Previously, we have presented a multiplicity study of ~700 early/mid M-type stars observed with the AstraLux high-resolution Lucky Imaging cameras. Here, we extend the study of multiplicity in M-type stars through studying 286 nearby mid/late M-type stars, bridging the gap between our previous study and multiplicity studies of brown dwarfs. Most of the targets have been observed more than once, allowing us to assess common proper motion to confirm companionship. We detect 68 confirmed or probable companions in 66 systems, of which 41 were previously undiscovered. Detections are made down to the resolution limit of ~100 mas of the instrument. The raw multiplicity in the AstraLux sensitivity range is 17.9%, leading to a total multiplicity fraction of 21-27% depending on the mass ratio distribution, which is consistent with being flat down to mass ratios of ~0.4, but cannot be stringently constrained below this value. The semi-major axis distribution is well represented by a log-normal function with mu_a = 0.78 and sigma_a = 0.47, which is narrower and peaked at smaller separations than for a Sun-like sample. This is consistent with a steady decrease in average semi-major axis from the highest-mass binary stars to the brown dwarf binaries.