  Glycolaldehyde is the simplest of the monosaccharide sugars and is directly
linked to the origin of life. We report on the detection of glycolaldehyde
(CH2OHCHO) towards the hot molecular core G31.41+0.31 through IRAM PdBI
observations at 1.4, 2.1, and 2.9 mm. The CH2OHCHO emission comes from the
hottest (> 300 K) and densest (>2E8 cm^-3) region closest (< 10^4 AU) to the
(proto)stars. The comparison of data with gas-grain chemical models of hot
cores suggests for G31.41+0.31 an age of a few 10^5 yr. We also show that only
small amounts of CO need to be processed on grains in order for existing hot
core gas-grain chemical models to reproduce the observed column densities of
glycolaldehyde, making surface reactions the most feasible route to its
formation.
