Spectral analysis of two directly imaged benchmark L dwarf companions at the stellar-substellar boundary^★

¹ Départment d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
⁵ Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
⁶ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA

^★ Corresponding author: william.ceva@unige.ch

Received: 26 April 2025
Accepted: 3 July 2025

Abstract

We used multiple epochs of high-contrast imaging spectrophotometric observations to determine the atmospheric characteristics and thermal evolution of two previously detected benchmark L dwarf companions, HD 112863 B and HD 206505 B. We analyzed IRDIS and IFS data from VLT/SPHERE of each companion, both of which have dynamical masses near the stellar-substellar boundary. We compared each companion with empirical spectral standards, and constrained their physical properties through atmospheric model fits. From these analyses, we estimate that HD 112863 B is spectral type L3 ± 1 and that HD 206505 B is spectral type L2 ± 1. Using the BT-Settl atmospheric model grids, we found a bimodal solution for the atmospheric model fit of HD 112863 B where T_eff = 1757₋₃₆⁺³⁷ K or 2002₋₂₄⁺²³ K and log g = 4.973_−0.063^+0.057 or 5.253_−0.033^+0.037, while for HD 206505 B, T_eff = 1754₋₁₃⁺¹³ K and log g = 4.919_−0.029^+0.031. The results of a comparison of the bolometric luminosities of the companions with evolutionary models imply that both companions are likely above the hydrogen burning limit.