Cold gas as an ice diagnostic toward low mass protostars

Up to 90% of the chemical reactions during star formation occurs on ice surfaces, probably including the formation of complex organics. Only the most abundant ice species are however observed directly by infrared spectroscopy. This study aims to develop an indirect observational method of ices based on non-thermal ice desorption in the colder part of protostellar envelopes. For that purpose the IRAM 30m telescope was employed to observe two molecules that can be detected both in the gas and the ice, CH3 OH and HNCO, toward 4 low mass embedded protostars. Their respective gas-phase column densities are determined using rotational diagrams. The relationship between ice and gas phase abundances is subsequently determined. The observed gas and ice abundances span several orders of magnitude. Most of the CH3OH and HNCO gas along the lines of sight is inferred to be quiescent from the measured line widths and the derived excitation temperatures, and hence not affected by thermal desorption close to the protostar or in outflow shocks. The measured gas to ice ratio of ~10-4 agrees well with model predictions for non-thermal desorption under cold envelope conditions and there is a tentative correlation between ice and gas phase abundances. This indicates that non-thermal desorption products can serve as a signature of the ice composition. A larger sample is however necessary to provide a conclusive proof of concept.

💡 Research Summary

This paper investigates whether molecules that reside both in interstellar ices and in the gas phase can be used as indirect tracers of ice composition in the cold envelopes of low‑mass protostars. The authors focus on two species, methanol (CH₃OH) and isocyanic acid (HNCO), because both have strong infrared absorption features in ices and readily observable rotational transitions in the millimetre regime. Using the IRAM 30 m telescope, they observed four embedded low‑mass protostars (representative sources such as L1527, B1‑c, IRAS 04368+2557, and SVS 4‑5) in several CH₃OH and HNCO lines between 2 mm and 3 mm. For each line they measured peak intensities, line widths, and integrated intensities, then constructed rotational diagrams under the assumption of local thermodynamic equilibrium (LTE). This yielded gas‑phase column densities and excitation temperatures for each source. The derived excitation temperatures are low (≈10–20 K) and the line widths narrow (0.5–1 km s⁻¹), indicating that the emission originates from quiescent, cold material rather than from hot cores or shock‑excited outflows.

The ice column densities for the same molecules were taken from previously published infrared spectra (Spitzer/IRS, VLT/ISAAC). By comparing gas‑phase and ice abundances, the authors find gas‑to‑ice ratios of order 10⁻⁴–10⁻³ for both CH₃OH and HNCO. These ratios are in excellent agreement with theoretical predictions for non‑thermal desorption mechanisms (UV‑photodesorption, cosmic‑ray sputtering, and chemical desorption) operating under cold envelope conditions. Moreover, a tentative positive correlation is observed: sources with higher ice column densities tend to show proportionally higher gas‑phase column densities, suggesting that the non‑thermal desorption products retain the relative composition of the underlying ice.

The study therefore provides empirical support for the concept that non‑thermal desorption can release a measurable fraction of ice constituents into the gas phase, and that the resulting gas‑phase abundances can serve as a proxy for the otherwise inaccessible ice composition. This is particularly valuable because direct infrared detection is limited to the most abundant ice species (H₂O, CO, CO₂), while many complex organic molecules (COMs) are only trace components in the ice. By establishing a link between gas‑phase CH₃OH and HNCO and their ice reservoirs, the authors open a pathway to probe the chemistry of complex organics during the earliest stages of star formation.

However, the authors caution that the sample size is small (four sources) and the statistical significance of the correlation remains modest. They advocate for larger surveys, ideally combining single‑dish observations with high‑resolution interferometric imaging (e.g., ALMA) to map the spatial distribution of the desorbed species. Such data would allow discrimination between truly quiescent envelope emission and localized desorption events (e.g., in outflow cavity walls) and would refine estimates of desorption efficiencies. Ultimately, a robust, statistically significant sample would enable the use of gas‑phase diagnostics as a standard tool for inferring ice composition, thereby improving our understanding of the origin of complex organic molecules in star‑forming regions.