13 CO and potential variability in beta Pictoris b with GRAVITY+

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 beta 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+24-17, 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 beta Pictoris b; we report a tentative constraint on the variability amplitude of about 1.4+0.6-0.7 %.