${}^{13}$CO and potential variability in ${\hat{I}}^2$ Pictoris b with GRAVITY+

Abstract

The ${}^{12}$CO/${}^{13}$CO ratio was introduced as an indicator for where in the disk a planet has formed. Previously a lower value compared to the host star's was suggested to show that a planet accreted CO ice beyond the disk's CO ice line. In this letter we aim to determine the ${}^{12}$CO/${}^{13}$CO value of the directly imaged planet ${\hat{I}}^2$ Pictoris b, and whether we can link it to its formation. Its apparent brightness results in an exceptional S/N of up to ~60 per wavelength point. We present the first science observations with the upgraded GRAVITY+ instrument at a spectral resolution of R ~ 4000, which we analyse with petitRADTRANS. Our retrievals robustly indicate the presence of ${}^{13}$CO with a ${}^{12}$CO/${}^{13}$CO ratio of 91${}_{-17}^{+24}$, consistent with both a solar to ISM-like value. Our ${}^{12}$CO/${}^{13}$CO value corroborates recent interpretations that ${}^{13}$CO may be a less useful tracer of formation location in the disk than previously thought; nonetheless, we discuss theories with which this value is consistent. As our observations span ~7 hours, this enabled us to search for atmospheric variability in ${\hat{I}}^2$ Pictoris b; we report a tentative constraint on the variability amplitude of about 1.4${}_{-0.7}^{+0.6}$%.

Accepted for publication in A&A (26th of May 2026), data products and models are available at https://doi.org/10.5281/zenodo.20608083