Orbital and Atmospheric Modeling of H <scp>ii</scp> 1348B: An Eccentric Young Substellar Companion in the Pleiades

Abstract Brown dwarfs with known physical properties (e.g., age and mass) are essential for constraining models of the formation and evolution of substellar objects. We present new high-contrast imaging observations of the circumbinary brown dwarf H ii 1348B—one of the few known substellar companions in the Pleiades cluster. We observed the system in the infrared <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>L</mml:mi> <mml:mi>′</mml:mi> </mml:math> band with the Large Binocular Telescope Interferometer in dual-aperture direct-imaging mode (i.e., with the two telescope apertures used separately) on 2019 September 18. The observations attained a high signal-to-noise ratio (SNR &gt; 150) photometric detection and relative astrometry with uncertainties of ∼5 mas. This work presents the first model of the companion’s orbital motion using relative astrometry from five epochs across a total baseline of 23 yr. Orbital fits to the compiled data show the companion’s semimajor axis to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>a</mml:mi> <mml:mo>=</mml:mo> <mml:mn>14</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>30</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>130</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">au</mml:mi> </mml:math> with an eccentricity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.7</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.29</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.12</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . We infer that H ii 1348B has a mass of 60–63 ± 2 M J from comparison to brown dwarf evolutionary models given the well-constrained distance and age of the Pleiades. No other objects were detected in the H ii 1348 system, and through synthetic planet injection and retrievals we establish detection limits at a cluster age of 112 ± 5 Myr down to ∼10–30 M J for companions with projected separations of 21.5–280 au. With this work, H ii 1348B becomes the second directly imaged substellar companion in the Pleiades with measured orbital motion after HD 23514B.