2026A&A...706A.108R

Context. Interstellar complex organic molecules, also known as iCOMs, are species of special interest in astrochemistry because of their potential role in the emergence of life. The discovery of iCOMs in the interstellar medium has sparked a decades-long debate about how they are formed. In principle, two main routes are possible: on the surfaces of dust grains and in the gas phase. A powerful way to discriminate between and constrain the two routes is to observe iCOMs along protostellar outflow-shocked regions, provided their ages are well constrained. In this way, the chemical evolution over time can be probed.
Aims. We focused on glycolaldehyde (CH₂OHCHO) and ethanol (C₂H₅OH), and their possible daughter-mother relationship, as suggested by previous studies. More precisely, our objective was to verify whether the gas-phase reactions derived in these studies dominate the formation of glycolaldehyde, so whether this gas-phase formation pathway can account for the abundance of glycolaldehyde observed in star-forming regions. We targeted the well-known southern outflow of L1157, which hosts three shocked regions, B0, B1 and B2, of increasing ages: approximately 900, 1500 and 2300 years, respectively.
Methods. We obtained high-resolution (∼4″) IRAM NOEMA maps of three glycolaldehyde lines and one ethanol line toward the entire southern outflow lobe of L1157 and used them to derive the abundances of the two species in B0, B1 and B2, as well as their abundance ratio. We then used a pseudo time-dependent astrochemical code to model the post-shock gas-phase chemistry of the two molecules under study, in which glycolaldehyde is formed via gas-phase reactions starting from ethanol, via the so-called ethanol-tree scheme, or on the grain surfaces. On the contrary, ethanol is assumed to be form on grain surfaces and to be released into the gas phase by the passage of the shocks. Once in the gas, C₂H₅OH is gradually consumed to form, among other iCOMs, glycolaldehyde. Ethanol is mainly destroyed through reactions with the OH radical, which in turn is primarily formed by injected (previously frozen) water (H₂O).
Results. We present the first spatially resolved maps of ethanol and glycolaldehyde toward the L1157 southern outflow and, more generally, toward solar-like star-forming regions. From these maps, we computed their column densities in B0, B1 and B2. Assuming an excitation temperature of 30 K for both species, we find column densities ranging between 1 and 3 ×10¹³ cm^–2 for glycolaldehyde and between 4 and 6 ×10¹³ for ethanol. Their relative abundance ratio [CH₂OHCHO] / [C₂H₅OH], equal to 0.25-0.4, increases between B1 and B2. The measured abundance ratios in B1 and B2 are relatively well reproduced by the astrochemical model. However, our model cannot simultaneously reproduce the observations toward B0, and toward B1 and B2, whether we assume that glycolaldehyde is primarily formed in the gas phase or on the grain-surfaces. This likely indicates that one of the assumptions in our model is incorrect. Possible candidates include the excitation temperature and grain mantle composition, assumed to be the same in B0, B1 and B2; the age of B0; and the gas temperature, assumed to be constant after the shock passage. Nonetheless, our modeling rules out the possibility that all the observed gaseous glycolaldehyde is a grain-surface product.
Conclusions. The study of resolved iCOM emission in the direction of protostellar molecular outflows proves to be an efficient way to constrain their formation routes. In the future, it will be important to carry out similar studies in regions other than L1157 outflow. In addition, improved modeling of shock chemistry should be developed, in which physical properties vary with time along with chemistry.