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

The 12 CO/^13CO ratio was introduced as an indicator for where in the disk a planet has formed. Previously, a lower value compared to that of the host star was suggested to indicate that a planet accreted CO ice beyond the disk’s CO ice line. In this Letter, we aim to determine the ^12CO/^13CO value of the directly imaged planet β$ Pictoris b and whether we can link it to its formation. Its apparent brightness results in an exceptional signal-to-noise ratio 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 analysed with petitRADTRANS . Our retrievals robustly indicate ^13CO with a ^12CO/^13CO ratio of 91^ +24 _ -17 , consistent with both a solar to interstellar-medium-like value. Our ^12CO/^13CO value corroborates recent interpretations that ^13CO 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 β Pictoris b; we report a tentative constraint on the variability amplitude of about 1.4$^ +0.6 _ -0.7 %.