The carbon isotope ratio of beta Pic b with high-resolution spectroscopy

Isotopic ratios trace the formation and evolution of planets and link their atmospheres to the chemistry of their natal protoplanetary discs. We measure 12 C/13 C = 58+18-15 in the atmosphere of the young super-Jupiter beta Pic b from 11 nights of CRIRES+ K-band spectroscopy (R about 100, 000) at the Very Large Telescope (VLT). We detect both 12 CO and 13 CO and constrain 12 C/13 C with a Bayesian retrieval jointly fitted with near-infrared photometry. The inferred 12 C/13 C is consistent with the present-day interstellar medium (ISM), is below the solar value, and is comparable to measurements in other young super-Jupiters. We also retrieve T eff = 1629+30-28, K, near-solar to mildly super-solar metallicity ([M/H] = 0.20+0.16-0.12), a solar-like carbon-to-oxygen ratio (C/O = 0.52 plus or minus 0.03), and tentative evidence for thick clouds. We analyse each night independently and combine the results of the six epochs with the highest signal-to-noise ratio (S/N), propagating night-to-night scatter into the final uncertainties. This provides an isotopic benchmark for a directly imaged planet interior to the CO snow line.