Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star HIP 39017
Salama, Maïssa; Currie, Thayne; Lewis, Briley L.; Tamura, Motohide; Uyama, Taichi; Kuzuhara, Masayuki; Brandt, Timothy D.; Williams, Jonathan P.; Li, Yiting; Chilcote, Jeffrey; Groff, Tyler; Guyon, Olivier; Lozi, Julien; Vievard, Sebastien; Deo, Vincent; Ahn, Kyohoon; Skaf, Nour; Kasdin, N. Jeremy; Chen, Minghan; Vincent, Maria; El Morsy, Mona; Lacy, Brianna; Bowens-Rubin, Rachel; An, Qier; Gibbs, Aidan; Tobin, Taylor L.
United States, Japan
Abstract
We present the direct-imaging discovery of a substellar companion (a massive planet or low-mass brown dwarf) to the young, γ Doradus (γ Dor)-type variable star HIP 39017 (HD 65526). The companion's SCExAO/CHARIS JHK (1.1–2.4 μm) spectrum and Keck/NIRC2 $L^{\prime} $ photometry indicate that it is an L/T transition object. A comparison of the JHK+L $^{\prime} $ 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 semimajor axis of ${23.8}_{-6.1}^{+8.7}$ au, a dynamical companion mass of ${30}_{-12}^{+31}$ M J, 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 M J 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 ${23.6}_{-7.4}^{+9.1}$ M J and a mass ratio of ∼1.4%. Analysis of the host star's multifrequency γ Dor-type pulsations, astrometric monitoring of HIP 39017 b, 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.