Near - IR integral - field spectroscopy of the companion to GQ Lup ⋆ ( Research Note )

Context. The first substellar companion of possibly planetary mass around a normal star, GQ Lup, has been directly imaged (Neuhäuser et al., 2005). 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 (Teff) 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 Teff = 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 −1.03 RJup and thus a mass of ∼ 25MJup. 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 MJup. 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 ± 3 MJup.