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Near-infrared integral-field spectra of the planet/brown dwarf companion AB Pictoris b
Context. We have already imaged a co-moving companion at a projectedseparation of ~260 AU from the young star AB Pic A. Evolutionary modelpredictions based on JHK photometry of AB Pic b suggest a mass of ~13-14MJup, placing the object at the deuterium-burning boundary. Aims: We aim to determine the spectral type, the surface gravity,and the effective temperature of AB Pic b. From the comparison of ourabsolute photometry to surface fluxes generated by atmospheric models,we also aim at deriving mass and radius estimates that are independentof evolutionary model predictions to test and refine them. Methods: We used the adaptive-optics-fed integral field spectrographSINFONI to obtain high-quality, medium-resolution spectra of AB Pic b(R_? = 1500-2000) over the 1.1-2.5 ?m range. Our analysisrelie on comparing our spectra to young standard templates and to thelatest libraries of synthetic spectra developed by the Lyon group. Results: AB Pic b is confirmed as a young early-L dwarf companion. Wederive a spectral type L0-L1 and find several features indicative of anintermediate gravity atmosphere. A comparison to synthetic spectrayields Teff = 2000+100-300 K and log(g)= 4 ± 0.5 dex. Determination of the derived atmosphericparameters of AB Pic b is limited by an imperfect match of currentatmosphere spectra with our near-infrared observations of AB Pic b. Thecurrent treatment of dust settling and the missing molecular opacitylines in the atmosphere models could be responsible. By combining theobserved photometry, the surface fluxes from atmosphere models and theknown distance of the system, we derive new mass, luminosity, and radiusestimates of AB Pic b. They independently confirm the evolutionary modelpredictions. We finally review the current methods used forcharacterizing planetary mass companions and discuss them in theperspective of future planet deep-imaging surveys that will be facedwith the same limitations.Based on service-mode observations (080.C-0590(A)) collected at theEuropean Organization for Astronomical Research in the SouthernHemisphere, Chile.
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