SDSS J100711.74+193056.2: A Candidate Common Motion Substellar Companion to the Nearest B-Type Star Regulus

The L9 dwarf SDSS J100711.74+193056.2 is situated 7$^{\circ}$.5 north of the nearest B-type star Regulus (d = 24.3+-0.2 pc), part of a stellar quadruplet. The object is at similar distance (d = 21.9+-1.0 pc) as Regulus, with a 3D separation of 3.9+0.6-0.5 pc ($\sim$1.6 tidal radii from Regulus), and shares tangential motion within 2 km/s, hinting at a physical connection. Near-infrared spectroscopy with Keck/NIRES finds that SDSS J100711.74+193056.2 also has a comparable radial velocity as Regulus A and B, a metallicity similar to Regulus B, and a spectral morphology consistent with the estimated 1--2 Gyr total age of Regulus's close pre-white dwarf companion. Taken together, these observations indicate that SDSS J100711.74+193056.2 is a very widely-separated and potentially physically-bound companion to Regulus, with a binding energy and mass ratio comparable to other wide star-brown dwarf systems. It joins a growing list of brown dwarfs at the L dwarf/T dwarf transition with independent constraints on physical properties such as age and metallicity.