2007A&A...463..309S

The first substellar companion of possibly planetary mass around a normal star, GQ Lup, has been directly imaged (Neuhaeuser et al., 2005A&A...435L..13N). Besides the unknown formation history, the mass of such an object is a criterion to decide about its true nature. We aim to determine the physical properties of the GQ Lup companion - effective temperate (T_eff) and surface gravity (logg), and thus its mass independently from evolutionary models. We use the adaptive optics integral-field spectrograph SINFONI at the VLT for near-infrared spectroscopy from 1.1 to 2.5µm with a resolution of R=2500-4000. We compare these spectra with synthetic atmospheric models (GAIA v2.0 cond). From the complete set of spectra we find a consistent effective temperature and surface gravity of Teff=2650±100K and logg=3.7±0.5dex. Combined with a slightly revised luminosity of logL/L_☉=-2.25±0.24dex for the companion, we determine a radius of R=3.50^+1.50_–1.03R_Jup and thus a mass of ∼25M_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. (2006Natur.440..311S), we conclude that the companion to GQ Lup A has a mass lower than 36±3M_Jup.