The hunt for formamide in interstellar ices

Abstract

Context. Although solid-state pathways are expected to dominate the formation mechanisms of many complex organic molecules (COMs), very few COMs have been securely identified in interstellar ices, in stark contrast with the many COM detections in the gas phase. The launch of the James Webb Space Telescope (JWST) and its increase in sensitivity and spectral resolution opens the possibility of identifying more COMs in ices, but additional laboratory data are necessary. Formamide (NH2CHO) is one such COM that is of great interstellar and prebiotic relevance where more laboratory data are needed in the hunt for its presence in interstellar ices. Aims. This work aims to characterize the mid-IR spectra of formamide in its pure form as well as in mixtures of the most abundant interstellar ices via laboratory simulation of such ices, as well as to demonstrate how these laboratory spectra can be used to search for formamide in ice observations. Methods. Mid-IR spectra (4000–500 cm−1/2.5–20 μm) of formamide, both in its pure form as well as in binary and tertiary mixtures with H2O, CO2, CO, NH3, CH3OH, H2O:CO2, H2O:NH3, CO:NH3, and CO:CH3OH, were collected at temperatures ranging from 15–212 K. Results. Apparent band strengths and positions of eight IR bands of pure amorphous and crystalline formamide at various temperatures are provided. Three of these bands are identified as potential formamide tracers in observational ice spectra: the overlapping C = O stretch and NH2 scissor bands at 1700.3 and 1630.4 cm−1 (5.881 and 6.133 μm), the CH bend at 1388.1 cm−1 (7.204 μm), and the CN stretch at 1328.1 cm−1 (7.529 μm). The relative apparent band strengths, positions, and full width half maxima (FWHM) of these features in mixtures at various temperatures were also determined. All of the laboratory spectra are available to the community on the Leiden Ice Database for Astrochemistry (LIDA) for use in the interpretation of both observations (e.g., from JWST) and laboratory spectroscopic data. Finally, the laboratory spectra are compared to observational spectra of a variety of low- and high-mass young stellar objects as well as prestellar cores observed with the Infrared Space Observatory, the Spitzer Space Telescope, and JWST. A comparison between the formamide CH bend in laboratory data and the 7.24 μm band in the observations tentatively indicates that, if formamide ice is contributing significantly to the observed absorption, it is more likely in a polar matrix. Upper limits ranging from 0.35-5.1% with respect to H2O were calculated via scaling the formamide:H2O laboratory spectrum to the observations. These upper limits are in agreement with gas-phase formamide abundances and take into account the effect of a H2O matrix on formamide’s band strengths.

JWST Observations of Young protoStars (JOYS+): Detecting icy complex organic molecules and ions

“…In addition, because of the ice matrix in which the molecules are embedded, the shape of the absorption bands change, with mixing ratio and temperature (e.g., Öberg et al 2007;Bouwman et al 2007). While ice COMs show peculiar band shapes at high temperatures (>70 K), they are very similar at lower temperatures (see Terwisscha van Scheltinga et al 2018Rachid et al 2020Rachid et al , 2021Rachid et al , 2022Slavicinska et al 2023). In this sense, we add to the criteria presented by Boogert et al (2015) and Jørgensen et al (2020) that a degeneracy analysis of molecules sharing similar functional groups is needed in order to claim a firm ice COM detection.…”

“…The typical band strength uncertainties are around 15% and 30% for pure and mixed ices, respectively (Rachid et al 2022;Slavicinska et al 2023).…”

“…These two COMs were also tentatively identified in the JWST high spectral resolution and sensitivity spectrum of IRAS 15398-3359 (Yang et al 2022) and toward two background stars (McClure et al 2023), albeit at lower spectral resolution. Other upper limits have been determined for methyl formate (CH 3 OCHO; Terwisscha van Scheltinga et al 2021), methylamine (CH 3 NH 2 ; Rachid et al 2021), methyl cyanide (CH 3 CN; Rachid et al 2022), and formamide (NH 2 CHO; Schutte et al 1999;Slavicinska et al 2023). Boudin et al (1998) also estimated upper limits for molecules such as ethane (C 2 H 6 ), acetylene (C 2 H 2 ), hydrazine (N 2 H 4 ), hydrogen peroxide (H 2 O 2 ) and the hydrozonium ion (N 2 H + 5 ).…”