While warm dense gas is prevalent around low-mass protostars, the presence of complex saturated molecules-the chemical inventory characteristic of hot cores-has remained elusive in such environments. Here we report the results of an IRAM 30 m study of the molecular composition associated with the low-mass protostar IRAS 16293Ϫ2422. Our observations highlight an extremely rich organic inventory in this source with abundant amounts of complex O-and N-bearing molecules such as formic acid, HCOOH, acetaldehyde, CH 3 CHO, methyl formate, CH 3 OCHO, dimethyl ether, CH 3 OCH 3 , acetic acid, CH 3 COOH, methyl cyanide, CH 3 CN, ethyl cyanide, C 2 H 5 CN, and propyne, CH 3 CCH. We compare the composition of the hot core around this low-mass young stellar object with those around massive protostars and address the chemical processes involved in molecular complexity in regions of star formation.

Context. Despite the low cosmic abundance of deuterium (D/H ∼ 10 −5 ), high degrees of deuterium fractionation in molecules are observed in star-forming regions with enhancements that can reach 13 orders of magnitude, a level that current models have difficulty accounting for. Aims. Multi-isotopologue observations are a very powerful constraint for chemical models. The aim of our observations is to understand the processes that form the observed high abundances of methanol and formaldehyde in low-mass protostellar envelopes (gas-phase processes? chemistry on the grain surfaces?), as well as to better constrain the chemical models.Methods. With the IRAM 30 m single-dish telescope, we observed deuterated formaldehyde (HDCO and D 2 CO) and methanol (CH 2 DOH, CH 3 OD, and CHD 2 OH) towards a sample of seven low-mass class 0 protostars. Using population diagrams, we then derived the fractionation ratios of these species (abundance ratio between the deuterated molecule and its main isotopologue) and compared them to the predictions of grain chemistry models. Results. These protostars show a similar level of deuteration as in IRAS 16293−2422, where doubly-deuterated methanol -and even triply-deuterated methanol -were first detected. Our observations point to the formation of methanol on the grain surfaces, while formaldehyde formation cannot be fully pinned down. While none of the scenarii can be excluded (gas-phase or grain chemistry formation), they both seem to require abstraction reactions to reproduce the observed fractionations.

Abstract.We report the first detection of triply-deuterated methanol, with 12 observed transitions, towards the low-mass protostar IRAS 16293−2422, as well as multifrequency observations of 13 CH 3 OH, used to derive the column density of the main isotopomer CH 3 OH. The derived fractionation ratio [CD 3 OH]/[CH 3 OH] averaged on a 10 beam is 1.4%. Together with previous CH 2 DOH and CHD 2 OH observations, the present CD 3 OH observations are consistent with a formation of methanol on grain surfaces, if the atomic D/H ratio is 0.1 to 0.3 in the accreting gas. Such a high atomic ratio can be reached in the framework of gas-phase chemical models including all deuterated isotopomers of H + 3 .