SCExAO/CHARIS Direct Imaging of A Low-Mass Companion At A Saturn-Like Separation from an Accelerating Young A7 Star

Abstract

We present the SCExAO direct imaging discovery and characterization of a low-mass companion to the nearby young A7IV star, HD 91312. SCExAO/CHARIS JHK (1.1-2.4 µm) spectra and SCExAO/HiCIAO H band imaging identify the companion over a two year baseline in a highly inclined orbit with a maximum projected separation of 8 au. The companion, HD 91312 B, induces an 8.8-σ astrometric acceleration on the star as seen with the Gaia & Hipparcos satellites and a long-term radial velocity trend as previously identified by Borgniet et al. (2019). HD 91312 B's spectrum is consistent with that of an early-to-mid M dwarf. Hipparcos and Gaia absolute astrometry, radial-velocity data, and SCExAO/CHARIS astrometry constrain its dynamical mass to be 0.337 +0.042 −0.044 M , consistent with -but far more precise than -masses derived from spectroscopy, and favors a nearly edge-on orbit with a semi-major axis of ∼9.7 au. This work is an example of precisely characterizing properties of low-mass companions at solar system-like scales from a combination of direct imaging, astrometry, and radial-velocity methods.