  The recent unexpected detection of terrestrial complex organic molecules in
the cold (~ 10 K) gas has cast doubts on the commonly accepted formation
mechanisms of these species. Standard gas-phase mechanisms are inefficient and
tend to underproduce these molecules, and many of the key reactions involved
are unconstrained. Grain-surface mechanisms, which were presented as a viable
alternative, suffer from the fact that they rely on grain surface diffusion of
heavy radicals, which is not possible thermally at very low temperatures. One
of the simplest terrestrial complex organic molecules, methanol is believed to
form on cold grain surfaces following from successive H atom additions on CO.
Unlike heavier species, H atoms are very mobile on grain surfaces even at 10 K.
Intermediate species involved in grain surface methanol formation by CO
hydrogenation are the radicals HCO and CH3O, as well as the stable species
formaldehyde H2CO. These radicals are thought to be precursors of complex
organic molecules on grain surfaces. We present new observations of the HCO and
CH3O radicals in a sample of prestellar cores and carry out an analysis of the
abundances of the species HCO, H2CO, CH3O, and CH3OH, which represent the
various stages of grain-surface hydrogenation of CO to CH3OH. The abundance
ratios between the various intermediate species in the hydrogenation reaction
of CO on grains are similar in all sources of our sample, HCO:H2CO:CH3O:CH3OH ~
10:100:1:100. We argue that these ratios may not be representative of the
primordial abundances on the grains but, rather, suggest that the radicals HCO
and CH3O are gas-phase products of the precursors H2CO and CH3OH, respectively.
Gas-phase pathways are considered and simple estimates of HCO and CH3O
abundances are compared to the observations. Critical reaction rate constants,
branching ratios, and intermediate species are finally identified.
