JWST observations of ¹³CO₂ ice

Tracing the chemical environment and thermal history of ices in protostellar envelopes

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
e-mail: brunken@strw.leidenuniv.nl
² Department of Physics and Astronomy, The University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA
³ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁵ University of Massachusetts Amherst, Amherst, MA, USA
⁶ Tata Institute of Fundamental Research, Mumbai 400005, India
⁷ Academia Sinica Institute of Astronomy & Astrophysics, 11F of Astro-Math Bldg., No.1, Sec. 4, Roosevelt Rd., Taipei, Taiwan
⁸ Department of Physics and Astronomy, University of Rochester, 500 Wilson Boulevard, Rochester, NY 14611, USA
⁹ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
¹⁰ Department of Astronomy, University of Illinois, 1002 W. Green St., Urbana, IL 61801, USA
¹¹ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903, USA
¹² INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹³ Max Planck Institute for Astronomy, Heidelberg, Baden Wuerttemberg, Germany
¹⁴ Friedrich-Schiller-Universität, Jena, Thüringen, Germany
¹⁵ United Kingdom Astronomy Technology Centre, Edinburgh, UK
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁷ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁸ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK
¹⁹ Caltech/IPAC, Pasadena, CA, USA
²⁰ Jet Propulsion Laboratory, Pasadena, USA
²¹ University of Michigan, Ann Arbor, MI, USA
²² Space Science Institute, Boulder, CO, USA
²³ Center for Astrophysics Harvard & Smithsonian, Cambridge, MA, USA
²⁴ Gemini South Observatory, La Serena, Chile
²⁵ Northwestern University, Evanston, IL, USA
²⁶ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
²⁷ European Southern Observatory, Garching bei München, Germany
²⁸ RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0106, Japan
²⁹ NV5 Geospatial Solutions, Inc. 385 Interlocken Crescent, Suite 300 Broomfield, CO 80021, USA

Received: 23 November 2023
Accepted: 5 February 2024

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 ¹²CO₂ 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 ¹³CO₂ 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 ¹³CO₂ ice in five deeply embedded Class 0 sources that span a wide range in masses and luminosities (0.2–10⁴ 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 ¹³CO₂. We then combined these findings with laboratory data and show that a 15.2 µm ¹²CO₂ bending-mode-inspired five-component decomposition can be applied to the isotopologue band, with each component representative of CO₂ ice in a specific molecular environment. The final solution consists of cold mixtures of CO₂ with CH₃OH, H₂O, and CO as well as segregated heated pure CO₂ ice at 80 K. Our results are in agreement with previous studies of the ¹²CO₂ ice band, further confirming that ¹³CO₂ is a useful alternative tracer of protostellar heating and ice composition. We also propose an alternative solution consisting only of heated mixtures of CO₂:CH₃OH and CO₂:H₂O ices and warm pure CO₂ ice at 80 K (i.e., no cold CO₂ 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.