Characterization of the gaseous companion κ Andromedae b - New Keck and LBTI high-contrast observations

Context. We previously reported the direct detection of a low mass companion at a projected separation of 55 2 AU around the B9 type star Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for the understanding of the formation and evolution of gas giant planets and brown dwarfs on wide-orbits. Aims. We present new angular di erential imaging (ADI) images of the system at 2.146 (Ks), 3.776 (L’), 4.052 (NB 4:05) and 4.78 m (M’) obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We aim to determine the near-infrared spectral energy distribution (SED) of the companion and use it to characterize the object. Methods. We used analysis methods adapted to ADI to extract the companion flux. We compared the photometry of the object to reference young/old objects and to a set of seven PHOENIX-based atmospheric models of cool objects accounting for the formation of dust. We used evolutionary models to derive mass estimates considering a wide range of plausible initial conditions. Finally, we used dedicated formation models to discuss the possible origin of the companion. Results. We derive a more accurate J = 15:86 0:21, H = 14:95 0:13, Ks = 14:32 0:09 mag for And b. We redetect the companion in all our high contrast observations. We confirm previous contrasts obtained at Ks and L’ band. We derive NB 4:05 = 13:0 0:2 and M 0 = 13:3 0:3 mag and estimate Log10(L=L ) = 3:76 0:06. Atmospheric models yield Te = 1900 +100 K. They do not set constrains on the surface gravity. “Hot-start” evolutionary models predict masses of 14 +25 MJup based on the luminosity and temperature estimates, and considering a conservative age range for the system (30 +120 Myr). “warm-start” evolutionary tracks constrain the mass to M 11MJup. Conclusions. The mass of Andromedae b mostly falls in the brown-dwarf regime, due to remaining uncertainties in age and mass-luminosity models. According to the formation models, disk instability in a primordial disk could account for the position and a wide range of plausible masses of And b.