  Glycolaldehyde is a key molecule in the formation of biologically relevant
molecules such as ribose. We report its detection with the Plateau de Bure
interferometer towards the Class 0 young stellar object NGC1333 IRAS2A, which
is only the second solar-type protostar for which this prebiotic molecule is
detected. Local thermodynamic equilibrium analyses of glycolaldehyde, ethylene
glycol (the reduced alcohol of glycolaldehyde) and methyl formate (the most
abundant isomer of glycolaldehyde) were carried out. The relative abundance of
ethylene glycol to glycolaldehyde is found to be ~5 -higher than in the Class 0
source IRAS 16293-2422 (~1), but comparable to the lower limits derived in
comets ($\geq$3-6). The different ethylene glycol-to-glycolaldehyde ratios in
the two protostars could be related to different CH3OH:CO compositions of the
icy grain mantles. In particular, a more efficient hydrogenation on the grains
in NGC 1333 IRAS2A would favor the formation of both methanol and ethylene
glycol. In conclusion, it is possible that, like NGC 1333 IRAS2A, other
low-mass protostars show high ethylene glycol-to-glycolaldehyde abundance
ratios. The cometary ratios could consequently be inherited from earlier stages
of star formation, if the young Sun experienced conditions similar to NGC1333
IRAS2A.
