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 μ m), dual-band imaging (2.1–2.25 μ m), 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.