The GUAPOS project. VI: the chemical inventory of shocked gas

The study of the chemical composition of star-forming regions is key to understanding the chemical ingredients available during the formation of planetary systems. Because the chemical inventory of interstellar dust grains in prestellar phases might be altered by protostellar warming, an alternative to inferring the chemical composition of the grains might be to observe regions that are affected by shocks associated with molecular outflows. These shocks can desorb the molecules and might produce less chemical processing because the timescales are shorter. We present a detailed study of the chemical reservoir of a shocked region located in the G31.41+0.31 protocluster using data from the G31.41+0.31 Unbiased ALMA sPectral Observational Survey (GUAPOS). We report the detection of 30 molecular species (plus 18 isotopologs). We compared the molecular ratios in the shocked region with those derived toward the hot core of G31.41+0.31. They are poorly correlated, with the exception of N-bearing species. Our results confirm observationally that a different level of chemical alteration is present in hot cores and in shocks. While the former likely alter the molecular ratios by thermal processing during longer timescales, the latter might represent freshly desorbed material that constitutes a better proxy of the composition of the ice mantle. The similarity of the molecular ratios of the N-bearing species in the G31.41+0.31 shock and the hot core suggests that these species are predominantly formed at early evolutionary stages. Interestingly, the abundances in the G31.41+0.31 shock are better correlated with other shock-dominated regions (two protostellar outflows and a molecular cloud in the Galactic center). This suggests that gas-phase chemistry after shock-induced ejection from grains is negligible and that the composition of the ice mantle is similar regardless of the Galactic environment.

💡 Research Summary

The sixth paper of the GUAPOS (G31.41+0.31 Unbiased ALMA Spectral Observational Survey) project presents a comprehensive astrochemical analysis of a shock‑dominated region located ∼19 000 au southwest of the hot core in the massive star‑forming complex G31.41+0.31. Using ALMA Band 3 (86–116 GHz) data from Cycle 5, the authors obtained an unbiased spectral survey with a spatial resolution of 1.2″ (≈4500 au) and a velocity resolution of ∼1.6 km s⁻¹. After continuum subtraction with STATCONT, line identification and LTE modelling were performed with the MADCUBA‑SLIM tool, employing CDMS, JPL and LSD spectroscopic catalogs. AUTOFIT was used to derive column densities, excitation temperatures, systemic velocities, and line widths. When convergence failed for Tex, a representative value of 20 K was adopted; the systemic velocity was fixed at 94 km s⁻¹ and the line width at 6.5 km s⁻¹ (based on CH₃CHO). Optically thick main isotopologues were treated by scaling from the thin isotopologues using standard isotopic ratios (e.g., ¹²C/¹³C = 39.5, ¹⁴N/¹⁵N = 340).

A total of 30 distinct molecular species, plus 18 isotopologues, were securely detected. The inventory includes classic shock tracers (SiO, PN, NS), complex organic molecules (CH₃CN, CH₃OH, C₂H₅OH, CH₃OCH₃, HNCO, OCS), and several sulfur‑bearing species. For many molecules a two‑component fit (broad and narrow) was required, reflecting the coexistence of freshly shocked gas and more quiescent material.

The authors compared the molecular abundance ratios of the shock position with those measured toward the adjacent hot core (previously studied in Papers I–V of the GUAPOS series). The comparison reveals a striking dichotomy: nitrogen‑bearing species (e.g., CH₃CN, HNCO) show similar ratios in both environments, whereas oxygen‑ and sulfur‑bearing molecules differ by factors of several. This pattern supports the view that hot cores, heated over 10⁴–10⁶ yr, undergo extensive thermal processing and gas‑phase chemistry that reshapes the original ice mantle composition, while shock‑released gas reflects a much shorter timescale (10²–10³ yr) and thus preserves the pristine mantle chemistry.

To place the results in a broader Galactic context, the shock chemistry was cross‑matched with three well‑studied shock‑dominated sources: the Galactic‑center cloud G+0.693, and the protostellar outflows L1157‑B1 and L1157‑B2. Correlation analyses show that the G31.41 shock abundances align closely with these benchmark shocks, indicating that the composition of interstellar ice mantles is remarkably uniform across diverse Galactic environments.

Key implications of the study are: (1) Shock‑induced desorption provides a relatively unprocessed snapshot of the ice mantle, making it a valuable proxy for the material inherited by nascent planetary systems; (2) Nitrogen‑bearing complex organics appear to form early, before the onset of massive star formation, and survive both thermal and shock processing; (3) The similarity of shock chemistry across the Galaxy suggests that variations in metallicity or radiation field have limited impact on the bulk ice composition. Consequently, observations of shocked gas complement hot‑core studies and, together with upcoming JWST ice spectroscopy, will refine our understanding of the chemical inheritance from interstellar clouds to protoplanetary disks.