A probable close brown dwarf companion to GJ 1046 (M 2.5V)

Context. Brown dwarf companions to stars at separations of a few AU or less are rare objects, and none have been found so far around early-type M dwarfs (M 0V-M 5V). With GJ 1046 (M 2.5V), a strong candidate for such a system with a separation of $0.42~\mathrm{AU}$ is presented. Aims. We aim at constraining the mass of the companion in order to decide whether it is a brown dwarf or a low-mass star. Methods. We employed precision RV measurements to determine the orbital parameters and the minimum companion mass. We then derived an upper limit to the companion mass from the lack of disturbances of the RV measurements by a secondary spectrum. An even tighter upper limit is subsequently established by combining the RV-derived orbital parameters with the recent new version of the Hipparcos Intermediate Astrometric Data. Results. For the mass of the companion, we derive $m \ge 26.9~{M}_\mathrm{Jup}$ from the RV data. Based on the RV data alone, the probability that the companion exceeds the stellar mass threshold is just $6.2\% $. The absence of effects from the secondary spectrum lets us constrain the companion mass to $m \le 229~{M}_\mathrm{Jup}$. The combination of RV and Hipparcos data yields a $3\sigma $ upper mass limit to the companion mass of $112~{M}_\mathrm{Jup}$ with a formal optimum value at $m=47.2~{M}_\mathrm{Jup}$. From the combination of RV and astrometric data, the chance probability that the companion is a star is $2.9\% $. Conclusions. We have found a low-mass, close companion to an early-type M dwarf. While the most likely interpretation of this object is that it is a brown dwarf, a low-mass stellar companion is not fully excluded