Hipparcos preliminary astrometric masses for the two close-in companions to HD 131664 and HD 43848 A brown dwarf and a low-mass star

Context. Several mechanisms for forming brown dwarfs have been proposed, which today are not believed to be mutually exclusive. Among the fundamental characteristics of brown dwarfs that are intrinsically tied to their origins, multiplicity is particularly relevant. Any successful determination of the actual mass for such objects in any systems is thus worthwhile, as it allows one to improve on the characterization of the multiplicity properties (e. g., frequency, separation, mass-ratio distribution) of sub-stellar companions. Aims. We attempt to place better constraints on the masses of two Doppler-detected substellar companions to the nearby G dwarfs HD 131664 and HD 43848. Methods. We carried out orbital fits to the Hipparcos Intermediate Astrometric Data (IAD) for the two stars, taking advantage of the knowledge of the spectroscopic orbits, and solving for the inclination angle i and the longitude of the ascending node Omega, the two orbital elements that can be determined in principle solely by astrometry, A number of checks were carried out to assess the reliability of the orbital solutions thus obtained. Results. The best-fit solution for HD 131664 yields i = 55 +/- 33 deg and Omega = 22 +/- 28 deg. The resulting inferred true companion mass is then M(c) = 23(-5)(+26) M(J). For HD 43848, we find i = 12 +/- 7 deg and Omega = 288 +/- 22 deg, and a corresponding M(c) = 120(-43)(+167) M(J). Based on the statistical evidence from an F-test, the study of the joint confidence intervals of variation in i and Omega and the comparison of the derived orbital semi-major axes with a distribution of false astrometric orbits obtained for single stars observed by Hipparcos, the astrometric signal of the two companions to HD 131664 and HD 43848 is then considered detected in the Hipparcos IAD, with a level of statistical confidence not exceeding 95%. Conclusions. We constrain the true mass of HD 131664b to that of a brown dwarf to within a somewhat statistically significant degree of confidence (similar to 2 - sigma). For HD 43848b, a true mass in the brown dwarf regime is ruled out at the 1 - sigma confidence level. The results are discussed in the context of the properties of the (few) close substellar and massive planetary companions to nearby solar-type stars and their implications for proposed models of formation and structure of massive planets and brown dwarfs.