2025A&A...693A.298G

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.