SAFARI - I. A SPHERE discovery of a super metal-rich M-dwarf companion to the star HD 86006

Abstract

We report the direct detection of a fully convective, early-to-mid M-dwarf companion orbiting the star HD 86006, using ESO-SPHERE during Science Verification as part of the SAFARI program. HARPS+CORALIE radial velocity measurements first indicated a possible companion. Such work highlights the synergies that are now possible between these two observing methods. We studied the companion by comparing our observed spectra with BT-Settl models and template spectra, measuring spectral indices to obtain a spectral type, and used a joint radial velocity and astrometric fit to simulate the companion's orbit. The companion was found to be 4.14 mag fainter than the primary in the H2 band, residing at a physical separation of ∼25 au, with a T_eff and spectral type of 3321 ± 111 K and M 4.1 ± 1.1, respectively. We note that the age derived from BT-Settl models for such a star is too low by over two orders of magnitude, similar to other known field mid-M stars. We searched for the radial velocity companion to HD 90520 without any clear detection; however, we reached a low contrast level of ΔH2 = 10.3 mag (or 1.3 * 10^-4) at 0.2 arcsec and 12.6 mag (or 10^-5) at 0.5 arcsec, allowing us to rule out any low-mass companions with masses of 0.07 and 0.05 M_⊙ at these separations. This discovery provides us with the exciting opportunity to better constrain the mass-luminosity relation for low-mass stars in the super metal-rich domain, expanding our understanding of the most-common types of stars and substellar objects.