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

¹ Astrophysikalisches Institut und Universitäts-Sternwarte, Universität Jena, Schillergäßchen 2-3, 07745 Jena, Germany
e-mail: tobi@astro.uni-jena.de
² Departamento de Física y Astronomía, Universidad de Valparaíso, Avenida Gran Bretaña 1111, Valparaíso, Chile
³ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
⁵ Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Ave., Chicago IL 60637, USA

Received: 2 April 2013
Accepted: 26 March 2014

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 β 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 χ² 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_⊙) = −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 β Pic moving group.