Direct Imaging Discovery of a Young Brown Dwarf Companion to an A2V Star

Abstract

We present the discovery and spectroscopy of HIP 75056Ab, a companion directly imaged at a very small separation of 0"125 to an A2V star in the Scorpius-Centaurus OB2 association. Our observations utilized Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet Research Experiment between 2015 and 2019, enabling low-resolution spectroscopy (0.95-1.65 μm), dual-band imaging (2.1-2.25 μm), and relative astrometry over a four-year baseline. HIP 75056Ab is consistent with spectral types in the range of M6-L2 and T_eff ∼ 2000-2600 K. A comparison of the companion's brightness to evolutionary tracks suggests a mass of ∼20-30 M_Jup. The astrometric measurements are consistent with an orbital semimajor axis of ∼15-45 au and an inclination close to face-on (i ≲ 35°). In this range of mass and orbital separation, HIP 75056Ab is likely at the low-mass end of the distribution of companions formed via disk instability, although a formation of the companion via core accretion cannot be excluded. The orbital constraints are consistent with the modest eccentricity values predicted by disk instability, a scenario that can be confirmed by further astrometric monitoring. HIP 75056Ab may be utilized as a low-mass atmospheric comparison to older, higher-mass brown dwarfs, and also to young giant planets. Finally, the detection of HIP 75056Ab at 0"125 represents a milestone in detecting low-mass companions at separations corresponding to the habitable zones of nearby Sun-like stars.