Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star HIP 39017
Taylor Tobin, Thayne Currie, Yiting Li, Jeffrey Chilcote, Timothy D. Brandt, Brianna Lacy, Masayuki Kuzuhara, Maria Vincent, Mona El Morsy, Vincent Déo, Jonathan P. Williams, Olivier Guyon, Julien Lozi, Sébastien Vievard, Nour Skaf, Kyohoon Ahn, Tyler D. Groff, N. Jeremy Kasdin, Taichi Uyama, Motohide Tamura, Aidan Gibbs, B. Lewis, Rachel Bowens-Rubin, Maïssa Salama, Qier An, Minghan Chen
Abstract
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 μ m) spectrum and Keck/NIRC2 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>L</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> photometry indicate that it is an L/T transition object. A comparison of the JHK + L <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo accent="false">′</mml:mo> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>23.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>8.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> au, a dynamical companion mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>30</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>12</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>31</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>23.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>7.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>9.1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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.