Abstract
Context. The first substellar companion of possibly planetary mass around a normal star, GQ Lup, has been directly imaged (Neuhauser et al. 2005, A&A, 435, L13). Besides the unknown formation history, the mass of such an object is a criterion to decide about its true nature. Aims. We aim to determine the physical properties of the GQ Lup companion - effective temperate (T eff ) and surface gravity (log g), and thus its mass independently from evolutionary models. Methods. We use the adaptive optics integral-field spectrograph SINFONI at the VLT for near-infrared spectroscopy from 1.1 to 2.5 μm with a resolution of R 2500-4000. We compare these spectra with synthetic atmospheric models (GAIA v2.0 cond). Results. From the complete set of spectra we find a consistent effective temperature and surface gravity of Ten = 2650 ± 100 K and log g = 3.7 ± 0.5 dex. Combined with a slightly revised luminosity of log L/L ⊙ = -2.25 ± 0.24 dex for the companion, we determine a radius of R = 3.50+1.50 R jup and thus a mass of ∼25 M Jup . The uncertainty of this value is rather high. Due to the large uncertainty of the surface gravity, the mass could range from 4 to 155 M Jup . By comparing the paramaters of the companion of GQ Lup to the ones of 2MASS J05352184-0546085, published by Stassun et al. (2006, Nature, 440, 311), we conclude that the companion to GQ Lup A has a mass lower than 36 ± 3 M Jup .
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