We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO .
The 2 \farcs 4 ( \sim 120 AU ) pair is confirmed to be comoving from two epochs of high resolution imaging .
Follow-up low- and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0 ^ { +2 } _ { -1 } .
The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of \sim 10-100 Myr . UVW kinematics based the measured radial velocity of the primary and the system ’ s photometric distance ( 50 \pm 10 pc ) indicate it is likely a member of the \sim 50–150 Myr AB Dor moving group .
The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth , but its H -band morphology shows subtle hints of intermediate surface gravity .
The spectrum is also an excellent match to the \sim 200 Myr M9 brown dwarf LP 944-20 .
Assuming an age of 50–150 Myr , evolutionary models imply a mass of 32 \pm 6 M _ { \mathrm { Jup } } for the companion , making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD–35 2722 B and one of the few benchmark brown dwarfs known at young ages .