2020A&A...633L...2M

Context. HD 72946 is a bright and nearby solar-type star hosting a low-mass companion at long period (P∼16yr) detected with the radial velocity (RV) method. The companion has a minimum mass of 60.4∼2.2M_J and might be a brown dwarf. Its expected semi-major axis of ∼243mas makes it a suitable target for further characterization with high-contrast imaging, in particular to measure its inclination, mass, and spectrum and thus definitely establish its substellar nature. Aims. We aim to further characterize the orbit, atmosphere, and physical nature of HD 72946B. Methods. We present high-contrast imaging data in the near-infrared with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. We also use proper motion measurements of the star from HIPPARCOS and Gaia. Results. The SPHERE data reveal a point source with a contrast of ∼9mag at a projected separation of ∼235mas. No other point sources are detected in the field of view. By jointly fitting the RV, imaging, and proper motion data, we constrain all the orbital parameters of HD 72946B and assess a dynamical mass of 72.4±1.6M_J and a semi-major axis of 6.456.45^+0.08_–0.07au. Empirical comparison of its SPHERE spectrum to template dwarfs indicates a spectral type of L5.0±1.5. The J-H3 color is close to the expectations of the DUSTY models and suggests a cloudy atmosphere. Comparison with atmospheric models of the spectrophotometry suggests an effective temperature of ∼1700K. The bolometric luminosity (log(L/L_☉)=-4.11±0.10dex) and dynamical mass of HD 72946B are more compatible with evolutionary models for an age range of ∼0.9-3Gyr. The formation mechanism of the companion is currently unclear as the object appears slightly away from the bulk of model predictions. HD 72946B is currently the closest benchmark brown dwarf companion to a solar-type star with imaging, RV, and proper motion measurements.