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

Abstract

The carbon isotope ratio (${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$) is a potential tracer of giant planet and brown dwarf formation. The GQ Lup system, hosting the K7 T Tauri star GQ Lup A and its substellar companion GQ Lup B, offers a unique opportunity to investigate this ratio in a young system. We aim to characterise the atmosphere of GQ Lup B, determining its temperature, chemical composition, spin, surface gravity, and ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$, while also measuring the same ratio for its host star, GQ Lup A. High-resolution K-band spectra of GQ Lup were obtained using CRIRES+ at the VLT. We modelled the starlight contribution from GQ Lup A and fitted GQ Lup Bś spectrum with atmospheric models from petitRADTRANS. The ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ ratio for GQ Lup A was derived using isotope-sensitive PHOENIX models. Atmospheric analysis of GQ Lup B revealed abundances of H${}_2$O, ${}^{12}$CO, ${}^{13}$CO, HF, Na, Ca, and Ti, along with a C/O ratio of $0.50\pm 0.01$, consistent with the solar value. The carbon isotope ratio was measured as ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}=53_{-6}^{+7}$ for GQ Lup B and ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}=51_{-8}^{+10}$ for GQ Lup A. Strong veiling of the photospheric lines of GQ Lup A was identified and accounted for. The similar ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ values in GQ Lup A and B suggest a common origin from a shared material reservoir, supporting formation via disc fragmentation or gravitational collapse.

Accepted for publication in A&A, 20 pages, 14 figures

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