Draft version April 17, 2020 Typeset using L A T E X twocolumn style in AASTeX62 2MASS J04435686+3723033 B: A Young Companion at the Substellar Boundary with Potential Membership in the β Pictoris Moving Group Caprice L. Phillips, 1, 2 Brendan P. Bowler, 3 Gregory Mace, 4 Michael C. Liu, 5 and Kimberly Sokal 4 1 Department of Astronomy, The Ohio State University, Columbus, OH, 43210,USA 2 LSSTC DSFP Fellow 3 Department of Astronomy, The University of Texas at Austin, Austin, TX, 78712, USA 4 McDonald Observatory & Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA 5 Institute for Astronomy, University of Hawai‘i at M¯ anoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA (Accepted by ApJ 7th April 2020) ABSTRACT We present a detailed characterization of 2MASS J04435750+3723031, a low-mass companion orbit- ing the young M2 star ” 2MASS J04435686+3723033 at 7. ′′ 6 (550 AU) with potential membership in the 23 Myr β Pictoris moving group (βPMG). Using near-infrared spectroscopy of the companion from IRTF/SpeX we have found a spectral type of M6 ± 1 and indications of youth through age-sensitive absorption lines and a low surface gravity index (VL-G). A young age is supported by Hα emission and lithium absorption in the host. We re-evaluate the membership of this system and find that it is a marginally consistent kinematic match to the βPMG using Gaia parallaxes and new radial velocities for the host and companion. If this system does belong to the βPMG, it would be a kinematic outlier and the companion would be over-luminous compared to other similar ultracool objects like PZ Tel B; this would suggest 2M0443+3723 B could be a close brown dwarf binary (≈52+52 M Jup if equal-flux, compared with 99 ± 5M Jup if single), and would make it the sixth substellar companion in this group. To test this hypothesis, we acquired NIR AO images with Keck II/NIRC2, but they do not resolve the companion to be a binary down to the diffraction limit of ∼3 AU. If 2M0443+3723 AB does not belong to any moving group then its age is more uncertain. In this case it is still young (30 Myr), and the implied mass of the companion would be between ∼30–110 M Jup . Keywords: binaries: close – stars: brown dwarfs, imaging, individual (2MASS J04435750+3723031 , 2MASS J04435686+3723033), low-mass 1. INTRODUCTION The study and characterization of the lowest-mass stars and brown dwarfs (BDs) is a relatively new field, with the first brown dwarfs, Gliese 229 B (Nakajima et al. 1995; Oppenheimer et al. 1995), Teide 1 (Rebolo et al. 1995), and PPL 15 (Basri et al. 1996) having been discovered less than a quarter century ago. BDs ( 75 M Jup ) are not massive enough to stably burn hydrogen in their cores and thus represent the transition region be- tween gas giant planets and low–mass stars. Substellar objects that fall below the hydrogen burning limit grad- ually cool and grow dimmer as they age and thus follow a degenerate mass-age-luminosity relationship (Burrows et al. 2001; Saumon & Marley 2008). This is contrary to low–mass stars which have stable luminosities over the main course of their lifetime. These objects are gener- ally defined based on the limit for the onset of hydrogen and deuterium burning, not necessarily their formation: stars have masses greater than 75 M Jup , BDs span 13–75 M Jup , and objects between 0.2 M Jup and 13 M Jup are considered gas giant planets if they are companions to stars (Burrows et al. 2001). Benchmark systems are objects that have two or more measured fundamental quantities such as luminosity and age; for brown dwarfs these parameters can be used to infer other properties like mass, temperature, and ra- dius using substellar evolutionary models (Burrows et al. 2001). For example, BDs that are companions to stars and members of young moving groups have ages that can be determined from the age of the host star or from age-dating the moving group. Benchmark brown dwarfs provide valuable tests for substellar atmospheric and arXiv:2004.07250v1 [astro-ph.SR] 15 Apr 2020