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${\fdg}$5 north of the nearest B-type star Regulus ($d\,=\,24.3\pm0.2$\,pc), part of a stellar quadruplet. The object is at similar distance ($d\,=\,21.9\pm1.0$\,pc) as Regulus, with a 3D separation of $3.87^{+0.61}_{-0.47}$\,pc ($\sim$1.6 tidal radii from Regulus), and shares tangential motion within 2~\kms, 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 1--2~Gyr age of Regulus's close white dwarf companion. Taken together, these observations indicate that SDSS~J100711.74+193056.2 is a very widely-separated but physically bound companion to Regulus, with a binding energy and mass ratio comparable to other wide star-brown dwarf systems. It is also one of the few brown dwarfs at the L dwarf/T dwarf transition with independent age and metallicity constraints.