The ESO SupJup Survey

IV. Unveiling the carbon isotope ratio of GQ Lup B and its host star

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
² Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain

^★ Corresponding author; picos@strw.leidenuniv.nl

Received: 20 August 2024
Accepted: 16 December 2024

Abstract

Context. The carbon isotope ratio (¹²C/¹³C) is a potential tracer of giant planet and brown dwarf formation. The GQ Lup system, which hosts the K7 T Tauri star GQ Lup A and its substellar companion GQ Lup B, provides a unique opportunity to investigate ¹²C/¹³C in a young system.

Aims. We aim to characterise GQ Lup B’s atmosphere, determining its temperature, chemical composition, spin, surface gravity, and ¹²C/¹³C, while also measuring the ¹²C/¹³C of its host star, GQ Lup A.

Methods. We obtained high-resolution K-band spectra of GQ Lup using CRIRES⁺ at the VLT. We modelled GQ Lup A’s starlight contribution and fitted GQ Lup B’s spectrum using atmospheric models from petitRADTRANS. The ¹²C/¹³C ofGQ Lup A was derived using isotope-dependent PHOENIX models.

Results. We measured atmospheric abundances in GQ Lup B, including H₂O, ¹²CO, ¹³CO, HF, Na, Ca, and Ti, and determined a C/O ratio of 0.50 ± 0.01, which is consistent with the solar value. The carbon isotope ratio is ¹²C/¹³C = 53₋₆⁺⁷ for GQ Lup B and 51₋₈⁺¹⁰ for GQ Lup A. Strong veiling of GQ Lup A’s photospheric lines was also identified and accounted for.

Conclusions. The similar ¹²C/¹³C values in GQ Lup A and B suggest a common origin from a shared material reservoir, supporting the idea of formation via disc fragmentation or gravitational collapse.