No abstract

Context. High methanol (CH 3 OH) deuteration has been revealed in Class 0 protostars with the detection of singly, doubly, and even triply D-substituted forms. Methanol is believed to form during the pre-collapse phase via gas-grain chemistry and then eventually injected into the gas when the heating produced by the newly formed protostar sublimates the grain mantles. The molecular deuterium fraction of the warm gas is thus a relic of the cold pre-stellar era and provides hints of the past history of the protostars. Aims. Pre-stellar cores represent the preceding stages in the process of star formation. We aim at measuring methanol deuteration in L1544, a prototypical dense and cold core on the verge of gravitational collapse. The aim is to probe the deuterium fractionation process while the "frozen" molecular reservoir is accumulated onto dust grains. Methods. Using the IRAM 30 m telescope, we mapped the methanol emission in the pre-stellar core L1544 and observed singly deuterated methanol (CH 2 DOH and CH 3 OD) towards the dust peak of L1544. Non-LTE radiative transfer modelling was performed on three CH 3 OH emissions lines at 96.7 GHz, using a Bonnor-Ebert sphere as a model for the source. We have also assumed a centrally decreasing abundance profile to take the molecule freeze-out in the inner core into account. The column density of CH 2 DOH was derived assuming LTE excitation and optically thin emission. Results. The CH 3 OH emission has a highly asymmetric morphology, resembling a non-uniform ring surrounding the dust peak, where CO is mainly frozen onto dust grains. The observations provide an accurate measure of methanol deuteration in the cold pre-stellar gas. The derived abundance ratio is [CH 2 DOH]/[CH 3 OH] = 0.10 ± 0.03, which is significantly smaller than the ones found in lowmass Class 0 protostars and smaller than the deuterium fraction measured in other molecules towards L1544. The singly-deuterated form CH 3 OD was not detected at 3σ sensitivity of 7 mK km s −1 , yielding a lower limit of [CH 2 DOH]/[CH 3 OD] ≥ 10, consistent with previous measurements towards Class 0 protostars. Conclusions. The low deuterium fractionation observed in L1544 and the morphology of the CH 3 OH emission suggest that we are mainly tracing the outer parts of the core, where CO just started to freeze-out onto dust grains.

Dense cloud cores present chemical differentiation because C-and N-bearing molecules are distributed differently, the latter being less affected by freeze-out onto dust grains. In this letter we show that two C-bearing molecules, CH 3 OH and c-C 3 H 2 , present a strikingly different (complementary) morphology while showing the same kinematics towards the prestellar core L1544. After comparing their distribution with the large-scale H 2 column density N(H 2 ) map from the Herschel satellite, we find that these two molecules trace different environmental conditions in the surrounding of L1544: the c-C 3 H 2 distribution peaks close to the southern part of the core, where the surrounding molecular cloud has an N(H 2 ) sharp edge, while CH 3 OH mainly traces the northern part of the core, where N(H 2 ) presents a shallower tail. We conclude that this is evidence of chemical differentiation driven by different amounts of illumination from the interstellar radiation field: in the south, photochemistry maintains more C atoms in the gas phase, allowing carbon-chain (such as c-C 3 H 2 ) production; in the north, C is mainly locked in CO, and methanol traces the zone where CO starts to freeze out significantly. During the process of cloud contraction, different gas and ice compositions are thus expected to mix towards the central regions of the core, where a potential solar-type system will form. An alternative view on carbon-chain chemistry in star-forming regions is also provided.

Context. Samples of pristine solar system material found in meteorites and interplanetary dust particles are highly enriched in 15 N. Conspicuous nitrogen isotopic anomalies have also been measured in comets, and the 14 N/ 15 N abundance ratio of the Earth is itself higher than the recognised presolar value by almost a factor of two. Low-temperature ion/molecule reactions in the proto-solar nebula have been repeatedly indicated as being responsible for these 15 N-enhancements.