IRAS 04368+2557 is a solar-type (low-mass) protostar embedded in a protostellar core (L1527) in the Taurus molecular cloud, which is only 140 parsecs away from Earth, making it the closest large star-forming region. The protostellar envelope has a flattened shape with a diameter of a thousand astronomical units (1 AU is the distance from Earth to the Sun), and is infalling and rotating. It also has a protostellar disk with a radius of 90 AU (ref. 6), from which a planetary system is expected to form. The interstellar gas, mainly consisting of hydrogen molecules, undergoes a change in density of about three orders of magnitude as it collapses from the envelope into the disk, while being heated from 10 kelvin to over 100 kelvin in the mid-plane, but it has hitherto not been possible to explore changes in chemical composition associated with this collapse. Here we report that the unsaturated hydrocarbon molecule cyclic-C3H2 resides in the infalling rotating envelope, whereas sulphur monoxide (SO) is enhanced in the transition zone at the radius of the centrifugal barrier (100 ± 20 AU), which is the radius at which the kinetic energy of the infalling gas is converted to rotational energy. Such a drastic change in chemistry at the centrifugal barrier was not anticipated, but is probably caused by the discontinuous infalling motion at the centrifugal barrier and local heating processes there.
Context. Complex organic molecules (COMs) have long been detected in the interstellar medium, especially in hot cores and in the hot corinos of low-mass protostars. Their formation routes however remain uncertain. Both warm gas-phase reactions and warm grain-surface reactions have been invoked to account for their presence in low-mass protostars. In this latter scheme, COMs result from radical-radical reactions on the grains as radicals become mobile when the nascent protostar warms up its surroundings and the resulting molecules are subsequently desorbed into the gas phase at higher temperatures. Aims. Prestellar cores are the direct precursors of low-mass protostars and offer a unique opportunity to study the formation of COMs before the warm-up phase. Their very low temperatures (≤10 K) and the absence of any heating source or outflow exclude any efficient warm gas phase or warm dust chemistry, so that the presence of COMs in prestellar cores would have to originate from non-thermal chemical processes. Methods. We used the IRAM 30 m telescope to look for four O-bearing COMs (acetaldehyde CH 3 CHO, dimethyl ether CH 3 OCH 3 , methyl formate CH 3 OCHO, and ketene CH 2 CO) in the prestellar core L1689B. Results. We report the unambiguous detection of all four molecules in the cold gas phase of L1689B. These detections support the role played by non-thermal (possibly photolytic) processes in COM formation and desorption, though the presence of dimethyl ether is so far unexplained by current grain formation scenarios. The data show univocally that COM synthesis has already started at the prestellar stage and suggests at least part of the COMs detected in hot corinos have a prestellar origin.
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