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

Abstract

Young, low-mass Brown Dwarfs orbiting early-type stars, with low mass ratios (q≲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 16Myr substellar companion HD136164Ab (HIP75056Ab) 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±10MJ (q∼0.02), making HD136164Ab the youngest substellar companion with a dynamical mass estimate. The new mass and newly constrained orbital eccentricity (e=0.44±0.03) and separation (22.5±1au) could indicate that the companion formed via the low-mass tail of the Initial Mass Function. Our atmospheric fit to the SPHINX M-dwarf model grid suggests a sub-solar C/O ratio of 0.45, and 3× 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. HD136164Ab joins a select group of young substellar objects with dynamical mass estimates; epoch astrometry from future Gaia data releases will constrain the dynamical mass of this crucial object further.

William Otto Balmer

Ph.D. Candidate

I am an astronomer, science journalist, and author. I use ground and space based optical/near infrared telescopes to study exoplanets.