Methanol and deuterium fractionation in prestellar cores

p. 9Despite progress in the understanding of interstellar medium chemistry, the detailed mechanisms for forming molecules remain unclear. In particular it is not known whether molecules form in the gas phase or on grain surfaces. Since ion-neutral gas phase reactions do not produce methanol efficiently, it has been suggested that methanol forms via active grain chemistry by successive additions of H atoms on formaldehyde. Comparison of deuterium fractionation in deuterated isotopes of CH3OH and H2CO can help constrain models. In order to test the hypothesis of grain surface chemistry, we have carried out a study of methanol and formaldehyde deuteration in a sample of prestellar cores. We present here the first results of this study, which show that simple grain surface chemistry models cannot simultaneously reproduce the fractionations found in formaldehyde and methanol. The ratios we find suggest that formaldehyde may be formed additionally in the gas phase, or that other grain surface mechanisms like H atom substitutions may be at work