First spectroscopic observations of the substellar companion of the young debris disk star PZ Telescopii

Abstract

Context. In 2010 a substellar companion to the solar analog pre-main sequence star PZ Tel and member of the approximately 12 Myr old beta Pic moving group was found by high-contrast direct imaging independently by two teams. Aims. In order to determine the basic parameters of this companion more precisely and independent of evolutionary models, hence age-independent, we obtained follow-up spectroscopic observations of the primary and companion. Methods. We used the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the Very Large Telescope Unit 4/Yepun of ESO's Paranal Observatory in the H + K band and processed the data using the spectral deconvolution technique. The resulting spectrum of the companion was then compared to a grid of DRIFT-PHOENIX synthetic model spectra, a combination of a general-purpose model atmosphere code with a non-equilibrium, stationary cloud and dust model, using a chi(2) minimization analysis. Results. We find a best fitting spectral type of G6.5 for PZ Tel A. The extracted spectrum of the substellar companion, at a spatial position compatible with earlier orbit estimates, yields a temperature T-eff = 2500(-115)(+138) K, a visual extinction A(V) = 0.53(-0.53)(+0.84) mag, a surface gravity of log g = 3.50(-0.30)(+0.51) dex, and a metallicity at the edge of the grid of [M/H] = 0.30(-0.30) dex. Conclusions. We derive a luminosity of log(L-bol/L-circle dot) = -2.66(-0.08)(+0.06), a radius of R = 2.42(-0.34)(+0.28) R-Jup, and a mass of M = 7.5(-4.3)(+16.9) M-Jup for the PZ Tel companion, which is consistent with most earlier estimates using photometry alone. Combining our results with evolutionary models, we find a best-fitting mass of about 21 Jupiter masses at an age corresponding to the recently determined lithium depletion age of 7(-2)(+4) Myr. Hence, the PZ Tel companion is most likely a wide brown dwarf companion in the 12(-4)(+8) Myr old beta Pic moving group.