The puzzling deuteration of methanol in low- to high-mass protostars

Abstract

Context. The current theory of methanol deuteration on interstellar grains predicts that the abundance ratio of the singly deuterated isotopologues [CH2DOH]/[CH3OD] should always be ∼3. In warm regions where grain mantles have sublimated, gaseous methanol is detectable via its rotational transitions. In previous observational studies, the gas-phase [CH2DOH]/[CH3OD] ratio was measured and found to be significantly larger than 3 in low-mass protostars and close to 1 in the Orion IRc2 massive hot core. Aims. We present new measurements of the gas-phase [CH2DOH]/[CH3OD] ratio in two additional high-mass protostars, as well as in two intermediate-mass protostars, to either confirm or exclude the dependence of this ratio on the mass of the protostar. Methods. The observations were carried out using the IRAM-30  m telescope. Several rotational lines of each isotopologue were detected toward the intermediate-mass protostars, while only CH3OD lines were detected in the massive hot cores. The ratio [CH2DOH]/[CH 3OD] (or its upper limit) was computed from both the averaged column densities and directly from line flux ratios. Results. Our results confirm that the [CH2DOH]/[CH3OD] ratio is substantially lower in massive hot cores than in (low-mass) hot-corinos, by typically one order of magnitude. Furthermore, they suggest that intermediate-mass protostars have similar properties to low-mass protostars. Conclusions. The measured [CH 2DOH]/[CH3OD] ratios are inconsistent with the current theory of methanol deuteration, independently of the mass of the source. While the large ratios measured in low- and intermediate-mass sources can be explained qualitatively by various selective depletion mechanisms, the small ratios (<2) measured toward massive hot cores are puzzling. A revision of the deuterium chemistry in hot cores is suggested. © 2011 ESO.