JOYS+ study of solid-state 12C/13C isotope ratios in protostellar envelopes: Observations of CO and CO2 ice with the James Webb Space Telescope

Context. The carbon isotope ratio is a powerful tool for studying the evolution of stellar systems due to its sensitivity to the local chemical environment. Recent detections of CO isotopologs in disks and exoplanet atmospheres revealed a high variability in the isotope abundance, ponting towards significant fractionation in these systems. In order to fully understand the evolution of this quantity in stellar and planetary systems, however, it is crucial to trace the isotope abundance from stellar nurseries to the time of planet formation. During the protostellar stage, the multiple vibrational modes of CO₂ and CO ice, which peak in the near- and mid-infrared, provide a unique opportunity to examine the carbon isotope ratio in the solid state. With the current sensitivity and wide spectral coverage of the James Webb Space Telescope, the multiple weak and strong absorption features of CO₂ and CO have become accessible at a high signal-to-noise ratio in solar-mass systems. Aims. We aim to study the carbon isotope ratio during the protostellar stage by deriving column densities and ratios from the various absorption bands of CO₂ and CO ice, and by comparing our results with the ratios derived in other astronomical environments. Methods. We quantify the ¹²CO₂/¹³CO₂ and the ¹²CO/¹³CO isotope ratios in 17 class 0/I low-mass protostars from the ¹²CO₂ ν₁ + ν₂ and 2ν₁ + ν₂ combination modes (2.70 µm and 2.77 µm), the ¹²CO₂ ν₃ stretching mode (4.27 µm), the ¹³CO₂ ν₃ stretching mode (4.39 µm), the ¹²CO₂ ν₂ bending mode (15.2 µm), the ¹²CO 1-0 stretching mode (4.67 µm), and the ¹³CO 1-0 stretching mode (4.78 µm) using the James Webb Space Telescope NIRSpec and MIRI observations. We also report a detection of the 2-0 overtone mode of ¹²CO at 2.35 µm. Results. The column densities and ¹²CO₂/¹³CO₂ ratios derived from the various CO₂ vibrational modes agree within the reported uncertainties, and we find mean ratios of 85 ± 23, 76 ± 12, and 97 ± 17 for the 2.70 µm band, the 4.27 µm band, and the 15.2 µm band, respectively. The main source of uncertainty on the derived values stems from the error on the band strengths; the observational errors are negligible in comparison. Variation of the ¹²CO₂/¹³CO₂ ratio is observed from one source to the next, which indicates that the chemical conditions of their envelopes might be genuinely different. The ¹²CO/¹³CO ratios derived from the 4.67 µm band are consistent, albeit elevated with respect to the ¹²CO₂/¹³CO₂ ratios, and we find a mean ratio of 165 ± 52. Conclusions. These findings indicate that ices leave the prestellar stage with elevated carbon isotope ratios relative to the overall values found in the interstellar medium, and that fractionation becomes a significant factor during the later stages of star and planet formation.