2011A&A...533A..24W

It has been established that the classical gas-phase production of interstellar methanol (CH₃OH) cannot explain observed abundances. Instead it is now generally thought that the main formation path has to be by successive hydrogenation of solid CO on interstellar grain surfaces. While theoretical models and laboratory experiments show that methanol is efficiently formed from CO on cold grains, our aim is to test this scenario by astronomical observations of gas associated with young stellar objects (YSOs). We have observed the rotational transition quartets J=2_K-1_K of ¹²CH₃OH and ¹³CH₃OH at 96.7 and 94.4GHz, respectively, towards a sample of massive YSOs in different stages of evolution. In addition, the J=1-0 transitions of ¹²C¹⁸O and ¹³C¹⁸O were observed towards some of these sources. We use the ¹²C/¹³C ratio to discriminate between gas-phase and grain surface origin: If methanol is formed from CO on grains, the ratios should be similar in CH₃OH and CO. If not, the ratio should be higher in CH₃OH due to ¹³C fractionation in cold CO gas. We also estimate the abundance ratios between the nuclear spin types of methanol (E and A). If methanol is formed on grains, this ratio is likely to have been thermalized at the low physical temperature of the grain, and therefore show a relative over-abundance of A-methanol. We show that the ¹²C/¹³C isotopic ratio is very similar in gas-phase CH₃OH and C¹⁸O, on the spatial scale of about 40'', towards four YSOs. For two of our sources we find an overabundance of A-methanol as compared to E-methanol, corresponding to nuclear spin temperatures of 10 and 16 K. For the remaining five sources, the methanol E/A ratio is less than unity. While the ¹²C/¹³C ratio test is consistent with methanol formation from hydrogenation of CO on grain surfaces, the result of the E/A ratio test is inconclusive.