2016MNRAS.456.2576B

We present Very Large Telescope/Spectrograph for INtegral Field Observations in the Near Infrared (VLT/SINFONI) J, H + K spectra of seven close visual pairs in M dwarf binary/triple systems, discovered or observed by the AstraLux M dwarf survey. We determine the spectral types to within ±1.0 subclasses from comparison to template spectra and the strength of K-band water absorption, and derive effective temperatures. The results are compared to optical spectral types of the unresolved binary/multiple systems, and we confirm that our photometric method to derive spectral types in the AstraLux M dwarf survey is accurate. We look for signs of youth such as chromospheric activity and low surface gravity, and find an age in the range 0.25-1 Gyr for the GJ 852 system. Strong Li absorption is detected in optical spectra of the triple system J024902 obtained with the Fiberfed Extended Range Optical Spectrograph (FEROS) at the European Southern Observatory (ESO)-Max-Planck-Gesellschaft (MPG) 2.2 m telescope. The equivalent width of the absorption suggests an age consistent with the β Pic moving group. However, further observations are needed to establish group membership. Ongoing orbital monitoring will provide dynamical masses and thus calibration of evolutionary models for low mass stars.