The GUAPOS Project Reveals Isomers of C2H4O2 in the G31.41+0.31 Region

Massive Star forming region. Credit: ESA/Herschel/PACS, SPIRE/Hi-GAL Project. Acknowledgement: UNIMAP / L. Piazzo, La Sapienza – Università di Roma; E. Schisano / G. Li Causi, IAPS/INAF, Italy

June 10, 2023

Astrochemistry aims to unravel the chemical intricacies within star-forming regions and identify the precursors of life's building blocks in the interstellar medium. To achieve this, unbiased spectral surveys with large bandwidth and high resolution are crucial. Such surveys help resolve line blending in chemically rich sources and enable the identification of each molecule, especially complex organic molecules (COMs). Previous successful observations have primarily focused on the Galactic Center, which exhibits unique environmental conditions.


This article presents the findings of an unbiased spectral survey conducted on G31.41+0.31, one of the most chemically rich hot molecular cores located outside the Galactic Center. The survey, part of the GUAPOS project, utilizes the Atacama Large Millimeter Array (ALMA) to identify and characterize the gas emission of various molecular species, with a specific emphasis on complex organic molecules.

Methods and Results:

The ALMA interferometer was employed to observe G31.41+0.31, covering the 84-116 GHz band (32 GHz bandwidth) with an angular resolution of 1.2'' × 1.2'' (~4400 au × 4400 au) and a spectral resolution of ~0.488 MHz (~1.3-1.7 km/s). The survey focused on the three isomers of C2H4O2: methyl formate, glycolaldehyde, and acetic acid. Using the XCLASS software, the transitions of these molecules were analyzed to determine the physical parameters of the emitted gas. All three isomers were detected in G31.41+0.31, with abundances of (2 ± 0.6) × 10^-7, (4.3-8) × 10^-8, and (5.0 ± 1.4) × 10^-9 for methyl formate, acetic acid, and glycolaldehyde, respectively. These abundances represent the highest detections thus far when compared to other sources in the literature. The size of the emission varied among the three isomers, with acetic acid exhibiting the most compact emission and methyl formate displaying the most extended emission. Different chemical pathways involving grain-surface and gas-phase reactions have been proposed for the formation of these molecules. However, due to the limited number of detections, particularly of acetic acid and glycolaldehyde, confirming or discarding these models has been challenging. Comparisons with existing chemical models indicate the necessity of grain-surface routes for methyl formate formation in G31.41+0.31, while both scenarios may be feasible for glycolaldehyde. The proposed grain-surface reaction for acetic acid fails to reproduce the observed abundance, warranting further testing of the gas-phase scenario.

Discussion and Conclusion:

The GUAPOS project's spectral survey of G31.41+0.31 has unveiled valuable insights into the chemical complexity of this high-mass star-forming region. The detections and relative abundances of the three C2H4O2 isomers shed light on the formation mechanisms and pathways involved. These findings contribute to a deeper understanding of chemical evolution in star-forming regions and the potential for the emergence of prebiotic molecules.