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 κ Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for understanding the formation and evolution of gas giant planets and brown dwarfs on wide orbits. <BR /> Aims: We present new angular differential imaging (ADI) images of the system at 2.146 (K[SUB]s[/SUB]), 3.776 (L'), 4.052 (NB_4.05), and 4.78 μm (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 of the companion and use it to characterize the object. <BR /> Methods: We used analysis methods adapted to ADI to extract the companion flux. We compared the photometry of the object to reference young, and 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. <BR /> Results: We derive a more accurate J = 15.86 ± 0.21, H = 14.95 ± 0.13, K[SUB]s[/SUB] = 14.32 ± 0.09 mag for κ And b. We detect the companion in all our high-contrast observations. We confirm previous contrasts obtained at K[SUB]s[/SUB] and L' band. We derive NB_4.05 = 13.0 ± 0.2, and M' = 13.3 ± 0.3 mag and estimate log [SUB]10[/SUB](L/L[SUB]⊙[/SUB]) = -3.76 ± 0.06. Atmospheric models yield T[SUB]eff[/SUB] = 1900[SUP]+100[/SUP][SUB]-200[/SUB] K. They do not set any constraint on the surface gravity. "Hot-start" evolutionary models predict masses of 14[SUP]+25[/SUP][SUB]-2[/SUB] M[SUB]Jup[/SUB] based on the luminosity and temperature estimates, and when considering a conservative age range for the system (30[SUP]+120[/SUP][SUB]-10[/SUB] Myr), "warm-start" evolutionary tracks constrain the mass to M ≥ 10M[SUB]Jup[/SUB]. <BR /> Conclusions: The mass of κ Andromedae b mostly falls in the brown-dwarf regime, owing to remaining uncertainties in age and in mass-luminosity models. According to the formation models, disk instability in a primordial disk may account for the position and a wide range of plausible masses of κ And b. The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Instituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of the University of Notre Dame, University of Minnesota, and University of Virginia.Appendices are available in electronic form at <A href="http://www.aanda.org/10.1051/0004-6361/201322119/olm">http://www.aanda.org</A>