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

TL;DR

This paper reports the direct imaging and characterization of a super metal-rich M-dwarf companion to HD 86006, demonstrating the synergy of radial velocity and direct imaging methods to study low-mass stars.

Contribution

First direct detection of a super metal-rich M-dwarf companion using ESO-SPHERE, combining spectral analysis and orbital modeling to characterize its properties.

Findings

Companion is an M4.1 star at ~25 AU from primary.

Spectral type and temperature measured as M4.1 and 3321 K.

No low-mass companions detected around HD 90520 within certain contrast limits.

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 $\sim$ 25 AU, with a $T_\mathrm{eff}$ and spectral type of 3321 $\pm$ 111 K and M 4.1 $\pm$ 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 $\Delta$H2 = 10.3 mag (or $1.3 * 10^{-4}$) at 0.2$''$ and 12.6 mag (or $10^{-5}$) at 0.5$''$, allowing us to rule out any low-mass companions with masses of 0.07 and 0.05 M$_{\odot}$ 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.