Chemical modelling of glycolaldehyde and ethylene glycol in star-forming regions

TL;DR

This study uses chemical modeling to investigate the formation and abundance ratios of glycolaldehyde and ethylene glycol in star-forming regions, comparing predictions with observations and exploring various formation pathways.

Contribution

It evaluates different chemical formation scenarios for these molecules and introduces a model incorporating decreasing H$_2$ density with luminosity for better agreement.

Findings

Recombination of HCO radicals best explains observed ratios.

HCO + CH$_2$OH reaction may also contribute.

Including decreasing H$_2$ density improves model predictions.

Abstract

Glycolaldehyde (HOCH$_2$CHO) and ethylene glycol ((CH$_2$OH)$_2$) are two complex organic molecules detected in the hot cores and hot corinos of several star-forming regions. The ethylene glycol/glycolaldehyde abundance ratio seems to show an increase with the source luminosity. In the literature, several surface-chemistry formation mechanisms have been proposed for these two species. With the UCLCHEM chemical code, we explored the different scenarios and compared the predictions for a range of sources of different luminosities with the observations. None of the scenarios reproduce perfectly the trend. A better agreement is, however, found for a formation through recombination of two HCO radicals followed by successive hydrogenations. The reaction between HCO and CH$_2$OH could also contribute to the formation of glycolaldehyde in addition to the hydrogenation pathway. The predictions are improved when a trend of decreasing H$_2$ density within the core region with T $\geq$ 100 K as a function of luminosity, is included in the model. Destruction reactions of complex organic molecules in the gas phase would also need to be investigated, since they can affect the abundance ratios once the species have desorbed in the warm inner regions of the star-forming regions.