Near-infrared integral-field spectroscopy of the companion to GQ Lupi

Context: The first substellar companion of possibly planetary mass around a normal star, GQ Lup, has been directly imaged (Neuhäuser 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 T_eff = 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_-1.03 {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}.

Based on observations collected at the European Southern Observatory, Chile, in programmes 275.C-5033(A) and 077.C-0264(A).