Hipparcos preliminary astrometric masses for the two close-in companions to HD 131664 and HD 43848

Abstract

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 Ω, 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 Ω = 22 ± 28 deg. The resulting inferred true companion mass is then M c = 23 +26 −5 M J . For HD 43848, we find i = 12 ± 7 deg and Ω = 288 ± 22 deg, and a corresponding M c = 120 +167 −43 M J . Based on the statistical evidence from an F-test, the study of the joint confidence intervals of variation in i and Ω 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 (∼2 − σ). For HD 43848b, a true mass in the brown dwarf regime is ruled out at the 1 − σ 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.

Search for brown-dwarf companions of stars

“…The formal companion mass at moderate significance is 90 ± 16 M J . Although the orbit significance agrees with the result of Sozzetti & Desidera (2010), who obtain a companion mass of 23 +26 −5 M J with 95% confidence, the mass range derived from our analysis is significantly higher. -HD 154697: The companion is a low-mass star with a mass confined within 0.08-0.22 M .…”

“…-HD 43848: The orbit inclination is 165 +2 −3 deg and the companion is a very low mass star with mass 0.10 ± 0.01 M . This value is slightly lower and more precise than the one derived by Sozzetti & Desidera (2010), who obtain 0.11 +0.16 −0.04 M based on the radial velocities of Minniti et al (2009). This solution is found with the lower-eccentricity orbit (cf.…”

“…With the discovery of 9 new companions, we have increased the known number of these objects by about 20%. Ten companions in the Sozzetti & Desidera (2010) list are probably stars. We confirm the stellar nature in two cases (HD 43848B and HD 164427B) and detect 6 new stellar companions (HD 3277B, HD 17289B, HD 30501B, HD 53680B, HD 154697B, and HIP 103019B) at the 3-σ significance level.…”

“…Techniques to perform the simultaneous fitting to radialvelocity and Hipparcos astrometry data for substellar companions and verify the statistical confidence of the solution have been developed by Zucker & Mazeh (2000), , Halbwachs et al (2000), and Zucker & Mazeh (2001). Reffert & Quirrenbach (2006) and Sozzetti & Desidera (2010) used similar approaches to claim the detection of three browndwarf companions, although with a significance not exceeding 2-σ. Independently, successful detections of the astrometric signature of exoplanet candidates using the HST fine guidance sensor (HST FGS) optical interferometer are reported for a few stars with spectroscopic elements known from radial velocity (e.g.…”

A possible dividing line between massive planets and brown-dwarf companions

“…Precision-RV surveys have found ∼ 50 of these objects with M 2 sin i = 13 − 80 M J , which we adopt as working-definition of the brown-dwarf mass range, regardless of the object's formation history, composition, and membership in a multiple system (Sozzetti & Desidera 2010;Sahlmann et al 2010, henceforth SA10). Brown-dwarf (BD) companions have also been found in close orbit around M-dwarfs and in wide orbits (> 10 AU) around stars, but these are not discussed in this contribution.…”