Hunting for complex cyanides in protostellar ices with the JWST

A tentative detection of CH₃CN and C₂H₅CN

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: nazari@strw.leidenuniv.nl
² Laboratory for Astrophysics, Leiden Observatory, Universiteit Leiden, Leiden, Zuid-Holland, The Netherlands
³ University of Rochester, Rochester, NY, USA
⁴ Netherlands Organization for Applied Scientific Research (TNO), 2628 CK Delft, The Netherlands
⁵ Max-Planck Institut für Extraterrestrische Physik, Garching bei München, Germany
⁶ University of Toledo, Toledo, OH, USA
⁷ University of Massachusetts Amherst, Amherst, MA, USA
⁸ Department of Astronomy and Astrophysics Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
⁹ Academia Sinica Institute of Astronomy & Astrophysics, 11F of Astro-Math Bldg., No.1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan, ROC
¹⁰ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, Texas 78712-1205, USA
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹³ Max Planck Institute for Astronomy, Heidelberg, Baden Wuerttemberg, Germany
¹⁴ United Kingdom Astronomy Technology Centre, Edinburgh, UK
¹⁵ Department of Astronomy, University of Illinois, 1002 West Green St, Urbana, IL 61801, USA
¹⁶ RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan

Received: 21 November 2023
Accepted: 15 January 2024

Abstract

Nitrogen-bearing complex organic molecules have been commonly detected in the gas phase but not yet in interstellar ices. This has led to the long-standing question of whether these molecules form in the gas phase or in ices. The James Webb Space Telescope (JWST) offers the sensitivity, spectral resolution, and wavelength coverage needed to detect them in ices and investigate whether their abundance ratios are similar in gas and ice. We report the first tentative detection of CH₃CN, C₂H₅CN and the simple molecule, N₂O, based on the CN-stretch band in interstellar ices toward three (HOPS 153, HOPS 370, and IRAS 20126+4104) out of the five protostellar systems observed as part of the Investigating Protostellar Accretion (IPA) GO program with JWST-NIRSpec. We also provide upper limits for the two other sources with smaller luminosities in the sample. We detect OCN⁻ in the ices of all sources with typical CH₃CN/OCN⁻ ratios of around 1. Ice and gas column density ratios of the nitrogen-bearing species with respect to each other are better matched than those with respect to methanol, which are a factor of ~5 larger in the ices than the gas. We attribute the elevated ice column densities with respect to methanol to the difference in snowline locations of nitrogen-bearing molecules and of methanol, biasing the gas-phase observations toward fewer nitrogen-bearing molecules. Moreover, we find tentative evidence of the enhancement of OCN⁻, CH₃CN, and C₂H₅CN in warmer ices; although, the formation of these molecules likely starts along with methanol in the cold prestellar phase. Future surveys combining NIRSpec and MIRI, and additional laboratory spectroscopic measurements of C₂H₅CN ice, are necessary for robust detection and conclusions on the formation history of complex cyanides.