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