Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star, HIP 39017

We present the direct-imaging discovery of a substellar companion (a massive planet or low-mass brown dwarf) to the young, γ Doradus-type variable star, HIP 39017 (HD 65526). The companion's SCExAO/CHARIS JHK (1.1-2.4μm) spectrum and Keck/NIRC2 L^ photometry indicate that it is an L/T transition object. A comparison of the JHK+L^ spectrum to several atmospheric model grids finds a significantly better fit to cloudy models than cloudless models. Orbit modeling with relative astrometry and precision stellar astrometry from Hipparcos and Gaia yields a semi-major axis of 23.8+8.7-6.1 au, a dynamical companion mass of 30⁺³¹₋₁₂~MJ, and a mass ratio of ~1.9%, properties most consistent with low-mass brown dwarfs. However, its mass estimated from luminosity models is a lower ~13.8 MJ due to an estimated young age ( 115 Myr); using a weighted posterior distribution informed by conservative mass constraints from luminosity evolutionary models yields a lower dynamical mass of 23.6-7.4+9.1~MJ and a mass ratio of ~1.4%. Analysis of the host star's multi-frequency γ Dor-type pulsations, astrometric monitoring of HIP 39017b, and Gaia Data Release 4 astrometry of the star will clarify the system age and better constrain the mass and orbit of the companion. This discovery further reinforces the improved efficiency of targeted direct-imaging campaigns informed by long-baseline, precision stellar astrometry.