Near-IR integral-field spectroscopy of the companion to GQ Lup

The first substellar companion of possibly planetary mass around a normal star, GQ Lup, has been directly imaged (Neuhaeuser et al., 2005). 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 and surface gravity, 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 2.5 um 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 of Teff = 2600 +/- 100 K and surface gravity of log{g} = 3.7 +/- 0.5. Combined with a slightly revised luminosity of log(L/L_(\sun)) = -2.25 +/- 0.24 for the companion, we determine a radius of R = 3.50 (+1.50/-1.03) Jupiterradii and thus a mass of ~ 25 Jupitermasses. Due to the large uncertainty of the surface gravity, the mass could range from 4 to 155 Jupitermasses. By comparing the paramaters of the companion of GQ Lup to the ones of 2MASS J05352184-0546085, published by Stassun et al. (2006), we conclude that the companion to GQ Lup A has a mass lower than 36 Jupitermasses.