JWST observations of ¹³CO₂ ice

Abstract

The structure and composition of simple ices can be severely modified during stellar evolution by protostellar heating. Key to understanding the involved processes are thermal and chemical tracers that can be used to diagnose the history and environment of the ice. The 15.2 µm bending mode of 12 CO 2 in particular has proven to be a valuable tracer of ice heating events but suffers from grain shape and size effects. A viable alternative tracer is the weaker 13 CO 2 isotopologue band at 4.39 µm, which has now become accessible at high S/N with the James Webb Space Telescope (JWST). In this study, we present JWST NIRSpec observations of 13 CO 2 ice in five deeply embedded Class 0 sources that span a wide range in masses and luminosities (0.2–10 4 L ⊙ ) taken as part of the Investigating Protostellar Accretion Across the Mass Spectrum (IPA) program. The band profiles vary significantly depending on the source, with the most luminous sources showing a distinct narrow peak at 4.38 µm. We first applied a phenomenological approach with which we demonstrate that a minimum of three to four Gaussian profiles are needed to fit the absorption feature of 13 CO 2 . We then combined these findings with laboratory data and show that a 15.2 µm 12 CO 2 bending-mode-inspired five-component decomposition can be applied to the isotopologue band, with each component representative of CO 2 ice in a specific molecular environment. The final solution consists of cold mixtures of CO 2 with CH 3 OH, H 2 O, and CO as well as segregated heated pure CO 2 ice at 80 K. Our results are in agreement with previous studies of the 12 CO 2 ice band, further confirming that 13 CO 2 is a useful alternative tracer of protostellar heating and ice composition. We also propose an alternative solution consisting only of heated mixtures of CO 2 :CH 3 OH and CO 2 :H 2 O ices and warm pure CO 2 ice at 80 K (i.e., no cold CO 2 ices) for decomposing the ice profiles of HOPS 370 and IRAS 20126, the two most luminous sources in our sample that show strong evidence of ice heating resulting in ice segregation.