The origin of complex organic molecules in prestellar cores

TL;DR

This study presents new observations of complex organic molecules in the prestellar core L1544, suggesting they originate from an outer layer via non-thermal desorption, challenging previous assumptions about their formation sites.

Contribution

It provides the first full census of oxygen-bearing COMs in L1544 and proposes a new formation mechanism involving non-thermal desorption from the outer core layer.

Findings

Detected multiple COMs including methanol and acetaldehyde in L1544.

Non-LTE analysis indicates COM emission occurs at ~8000 AU from the core center.

Non-thermal desorption explains COM abundances without requiring inner core processes.

Abstract

Complex organic molecules (COMs) have been detected in a variety of environments, including cold prestellar cores. Given the low temperature of these objects, these last detections challenge existing models. We report here new observations towards the prestellar core L1544. They are based on an unbiased spectral survey of the 3mm band at the IRAM-30m telescope, as part of the Large Program ASAI. The observations allow us to provide the full census of the oxygen bearing COMs in this source. We detected tricarbon monoxide, methanol, acetaldehyde, formic acid, ketene, and propyne with abundances varying from 5e-11 to 6e-9. The non-LTE analysis of the methanol lines shows that they are likely emitted at the border of the core, at a radius of ~8000 AU where T~10 K and nH2~2e4 cm-3. Previous works have shown that water vapour is enhanced in the same region because of the photodesorption of water ices. We propose that a non-thermal desorption mechanism is also responsible for the observed emission of methanol and COMs from the same layer. The desorbed oxygen and a tiny amount of desorbed methanol and ethene are enough to reproduce the abundances of tricarbon monoxide, methanol, acetaldehyde and ketene measured in L1544. These new findings open the possibility that COMs in prestellar cores originate in a similar outer layer rather than in the dense inner cores, as previously assumed, and that their formation is driven by the non-thermally desorbed species.