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# The 13CO-rich atmosphere of a young accreting super-Jupiter

## Abstract

Isotope abundance ratios have an important role in astronomy and planetary sciences, providing insights into the origin and evolution of the Solar System, interstellar chemistry and stellar nucleosynthesis1,2. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (around 89) in the Solar System1,3, but do vary on galactic scales with a 12C/13C isotopologue ratio of around 68 in the current local interstellar medium4,5,6. In molecular clouds and protoplanetary disks, 12CO/13CO ratios can be altered by ice and gas partitioning7, low-temperature isotopic ion-exchange reactions8 and isotope-selective photodissociation9. Here we report observations of 13CO in the atmosphere of the young, accreting super-Jupiter TYC 8998-760-1 b, at a statistical significance of more than six sigma. Marginalizing over the planet’s atmospheric temperature structure, chemical composition and spectral calibration uncertainties suggests a 12CO/13CO ratio of $${31}_{-10}^{+17}$$(90% confidence), a substantial enrichment in 13C with respect to the terrestrial standard and the local interstellar value. As the current location of TYC 8998-760-1 b at greater than or equal to 160 astronomical units is far beyond the CO snowline, we postulate that it accreted a substantial fraction of its carbon from ices enriched in 13C through fractionation.

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## Data availability

The data are publicly available from the ESO Science Archive with the programme ID 2103.C-5012(C).

## Code availability

The data analysis was performed with custom Python scripts following the standard procedure. The code and reduced spectrum are available from https://gitlab.strw.leidenuniv.nl/yzhang/yses1b-sinfoni. The atmospheric retrieval models use petitRADTRANS, which is available from https://petitradtrans.readthedocs.io/, and the nested sampling tool PyMultiNest, which is available from https://johannesbuchner.github.io/PyMultiNest/.

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## Acknowledgements

We thank E. van Dishoeck, A. Cridland and A. Miotello for discussions on carbon fractionation in protoplanetary disks. We thank K. Chubb for a 13CO line list comparison. Based on observations collected at the European Southern Observatory (ESO) under ESO programme 2103.C-5012(C). Y.Z. and I.A.G.S. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement number 694513. The research of A.J.B. and F.S. leading to these results has received funding from the European Research Council under ERC Starting Grant agreement 678194 (FALCONER). P.M. acknowledges support from the European Research Council under the European Union’s Horizon 2020 research and innovation programme under grant agreement number 832428. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

## Author information

Authors

### Contributions

Y.Z. and I.A.G.S. performed the data analysis and wrote the manuscript. A.J.B. led the SINFONI proposal, planned the observations and commented on the manuscript, and is the principal investigator of the Young Suns Exoplanet Survey (YSES) that led to the discovery of the TYC 8998 system. P.M. developed the retrieval models and assisted the data analysis. C.G., M.A.K., E.E.M., T.M., M.R. and F.S. constitute the core team of YSES, contributed to the SINFONI proposal and commented on the manuscript. H.J.H. helped the preparations of the observations and commented on the manuscript.

### Corresponding author

Correspondence to Ignas A. G. Snellen.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Drake Deming and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Schematics of the observations of TYC 8998-760-1 b using SINFONI at the Very Large Telescope.

The background image is captured by the SPHERE instrument on the VLT (Credit: ESO/Bohn et al.). The small blue box marks the FOV of SINFONI observations targeting the planet b. Both the host star and planet c are outside the FOV. An example of the wavelength-collapsed image is shown in the enlarged blue box, showing negligible contribution from starlight.

### Extended Data Fig. 2 Posteriors of retrieved parameters.

a, Posteriors of the retrieved parameters and temperature structure for the full (cyan) and reduced (red) models. The vertical dashed lines denote the 5%, 50% and 95% quantiles (90% uncertainties) of the distribution. b, TP profile. The shaded regions with decreasing colour saturation show 1σ, 2σ and 3σ temperature uncertainty envelopes, respectively. The black dashed line shows the flux average of the emission contribution function. The opaqueness of the temperature uncertainty envelopes has been scaled by this contribution function. c, Fitting statistics of the full and reduced retrieval model, where ln(Z) and ln(Bm) represent the logarithm of Bayesian evidence and Bayes factor, respectively.

### Extended Data Fig. 3 Posteriors of the retrieved parameters for the data of individual nights.

Similar to Extended Data Fig. 2a.

### Extended Data Fig. 4 Cross-correlation signal of 13CO from individual nights and bandheads.

a, Observational residuals of the two nights separately. b, Cross-correlation signal from the individual nights. c, Filtered observational residuals of the two 13CO bandheads separately. d, Cross-correlation signal from the individual bandheads.

### Extended Data Fig. 5 Impact of telluric absorption lines and cross-correlation signal of 13CO at the extended wavelength region.

a, Comparison of the telluric transmission model with residuals. Some noise is attributed to imperfect telluric correction as noted by dotted grey lines. b, Cross-correlation function between the telluric model and the 13CO model, showing no correlation between them.

### Extended Data Fig. 6 K-band spectrum of the brown dwarf 2M0355 taken by Keck/NIRSPEC and the cross-correlation signal of 13CO.

The black line shows the observed spectrum and the orange line is the best-fit model obtained by retrieval analysis. Bottom: CCF between the 13CO model and observational residuals. The peak at zero velocity clearly shows the detection of 13CO.

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Zhang, Y., Snellen, I.A.G., Bohn, A.J. et al. The 13CO-rich atmosphere of a young accreting super-Jupiter. Nature 595, 370–372 (2021). https://doi.org/10.1038/s41586-021-03616-x

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