The ESO SupJup Survey. IV. Unveiling the carbon isotope ratio of GQ Lup B and its host star

The carbon isotope ratio ( ) 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 in a young system. We aim to characterise GQ Lup B’s atmosphere, determining its temperature, chemical composition, spin, surface gravity, and while also measuring the of its host star, GQ Lup A. 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 of GQ Lup A was derived using isotope-dependent PHOENIX models. We measured atmospheric abundances in GQ Lup B, including 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 $ for GQ Lup B and $51^ $ for GQ Lup A. Strong veiling of GQ Lup A’s photospheric lines was also identified and accounted for. The similar 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.