The Gaia Ultracool Dwarf Sample – IV. GTC/OSIRIS optical spectra of Gaia late-M and L dwarfs

As part of our comprehensive, ongoing characterisation of the low-mass end of the mainsequence in the Solar neighbourhood, we used the OSIRIS instrument at the 10.4 m Gran Tele-scopio Canarias to acquire low- and mid-resolution (R≈300 and R≈2500) optical spectroscopyof 53 late-M and L ultracool dwarfs. Most of these objects are known but poorly investigatedand lacking complete kinematics. We measured spectral indices, determined spectral types(six of which are new) and inferred effective temperature and surface gravity from BT-Settlsynthetic spectra fits for all objects. We were able to measure radial velocities via line centrefitting and cross correlation for 46 objects, 29 of which lacked previous radial velocity mea-surements. Using these radial velocities in combination with the latest Gaia DR3 data, we alsocalculated Galactocentric space velocities. From their kinematics, we identified two candidatesoutside of the thin disc and four in young stellar kinematic groups. Two further ultracool dwarfsare apparently young field objects: 2MASSW J1246467+402715 (L4ß), which has a potential,weak lithium absorption line, and G 196–3B (L3ß), which was already known as young dueto its well-studied primary companion.