VLTI/GRAVITY Provides Evidence the Young, Substellar Companion HD 136164 Ab formed like a "Failed Star"

Young, low-mass Brown Dwarfs orbiting early-type stars, with low mass ratios ($q\lesssim 0.01$), appear intrinsically rare and present a formation dilemma: could a handful of these objects be the highest mass outcomes of planetary" formation channels (bottom up within a protoplanetary disk), or are they more representative of the lowest mass failed binaries" (formed via disk fragmentation, or core fragmentation)? Additionally, their orbits can yield model-independent dynamical masses, and when paired with wide wavelength coverage and accurate system age estimates, can constrain evolutionary models in a regime where the models have a wide dispersion depending on initial conditions. We present new interferometric observations of the $16\mathrm{M}\mathrm{y}\mathrm{r}$ substellar companion HD~136164~Ab (HIP~75056~Ab) with VLTI/GRAVITY and an updated orbit fit including proper motion measurements from the Hipparcos-Gaia Catalogue of Accelerations. We estimate a dynamical mass of $35\pm 10{\mathrm{M}}_{\mathrm{J}}$ ($q\sim 0.02$), making HD~136164~Ab the youngest substellar companion with a dynamical mass estimate. The new mass and newly constrained orbital eccentricity ($e=0.44\pm 0.03$) and separation ($22.5\pm 1\mathrm{a}\mathrm{u}$) could indicate that the companion formed via the low-mass tail of the Initial Mass Function. Our atmospheric fit to the $\mathtt{\text{SPHINX}}$ M-dwarf model grid suggests a sub-solar C/O ratio of $0.45$, and $3\times$ solar metallicity, which could indicate formation in the circumstellar disk via disk fragmentation. Either way, the revised mass estimate likely excludes ``bottom-up" formation via core accretion in the circumstellar disk. HD~136164~Ab joins a select group of young substellar objects with dynamical mass estimates; epoch astrometry from future $\mathit{\text{Gaia}}$ data releases will constrain the dynamical mass of this crucial object further.