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2MASS J04435686+3723033 B: A Young Companion at the Substellar Boundary with Potential Membership in the $β$ Pictoris Moving Group

We present a detailed characterization of 2MASS J04435750+3723031, a low-mass companion orbiting the young M2 star, 2MASS J04435686+3723033, at 7.6 arcseconds (550 AU) with potential membership in the 23 Myr $β$ Pictoris moving group ($β$PMG). Using near-infrared spectroscopy of the companion from IRTF/SpeX we have found a spectral type of M6 $\pm$ 1 and indications of youth through age-sensitive absorption lines and a low surface gravity index (VL-G). A young age is supported by H$α$ 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 $β$PMG using $Gaia$ parallaxes and new radial velocities for the host and companion. If this system does belong to the $β$PMG, 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 ($\approx$52+52 M$_\mathrm{Jup}$ if equal-flux, compared with 99 $\pm$ 5 M$_\mathrm{Jup}$ 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 $\sim$3 AU. If 2M0443+3723 AB does not belong to any moving group then its age is more uncertain. In this case it is still young ($\lesssim$30 Myr), and the implied mass of the companion would be between $\sim$30--110 M$_\mathrm{Jup}$.