Detections of Complex Molecules in Interstellar Ices

The James Webb Space Telescope (JWST) illustrates its potential through three new detections of biological precursors in interstellar ices. Will Rocha and team, part of the JWST Observations of Young protoStars (JOYS+) project, detected a total of ten molecules in their ice form around forming stars. Previously inaccessible to humankind, the recently launched JWST provides astronomers access to infrared radiation at the highest levels of sensitivity and resolution. By looking at infrared radiation, astronomers can detect ices in space. The surfaces of these ices are where scientists believe to be the birthplace of complex molecules in space. The detections of these complex molecules on ices is further evidence of their formation on ice surfaces rather than solely in the gas-phase.

Rocha and team detected a collection of simple molecules, ions, and complex organic molecules towards two star-forming regions, IRAS 2A and IRAS 23385, low-mass and high-mass forming stars, respectively, called protostars. The team detected simple ices such as water (H₂O), formic acid (HCOOH), methane (CH₄), and sulfur dioxide (SO₂). Their detections of ions OCN^- and HCOO^- are of astrobiological importance as OCN^- ions are formed from molecules in acid-base reactions and the chemically-related molecule, HNCO, serves as a peptide bond between two single amino acids. Concerning complex organic molecules, carbon-bearing molecules consisting of six or more atoms, the most complex molecule in ice-form detected to date was methanol (CH₃OH). However, Rocha and team report for the first time the secure detection of solid-phase molecules more complex than methanol such as acetaldehyde (CH₃CHO), ethanol (CH₃CH₂OH), and methyl formate (CH₃OCHO), all molecules of prebiological importance. 

Figure 1: Fits of JWST spectral data of high-mass protostar IRAS 23385 (top) and low-mass protostar IRAS 2A (bottom) to laboratory infrared data. Legend corresponds to the ice mixture used in the fits. Ice mixtures are representative of astronomical conditions of molecules on ice surfaces that have a varying degree of solid ices present. Credit: Rocha et al. 2023

Since the launch of JWST, two other programs (Yang et al. 2022 and McClure et al. 2023), made tentative detections of acetaldehyde (CH₃CHO) and ethanol (CH₃CH₂OH). These detections were classified as tentative because there were not enough spectral features to confirm them. When data is collected from telescopes such as JWST, it is collected in the form of signals. Scientists then take this jumble of signals and compare with laboratory infrared experiments that identify the unique signals that come from each molecule. By disentangling all the signals from each other, detections of molecules are then possible (Figure 1). However, if the signal is not strong enough or there are not enough features present for that specific molecule, the detection is then labeled as tentative. In the work by Rocha and team, they report tentative detections of molecules acetic acid (CH₃COOH) and formaldehyde (H₂CO). The authors state that more comparisons with JWST data is needed to elucidate at what levels these molecules may be present in interstellar ices.

This article made use of the following publications:

Rocha et al. 2023, A&A

McClure et al. 2023, Nature Astronomy, 7, 431-443

Yang et al. 2022, ApJL, 941, L13