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

Abstract The L9 dwarf SDSS J100711.74+193056.2 is situated 7 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>°</mml:mi> </mml:mrow> </mml:mover> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> pc (~1.6 tidal radii from Regulus), and shares tangential motion within 2 km s −1 , 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.