Cold and Yet Complex: Detection of Ethylene Oxide in a Prestellar Core

Complex, near-saturated, organic molecules have been detected in a wide range of environments, in harsh photon dominated regions, in protoplanetary disks, in the hot cores of star forming regions, and also in cold prestellar cores. In the latter case, the formation scenarios remain debated. While both gas phase or grain surface mechanisms have been considered, both types of scenarios have difficulties accounting for observed gas-phase abundances but none can be ruled out. The observation of structural isomers can help bring further constraints on molecular formation mechanisms, because their relative ratios should be ruled by kinetics, and not thermodynamics, in these low-density environments. Unfortunately so far no structural isomers of complex organic molecules have been detected. In this paper, we present the detection of ethylene oxide (c-C2H4O) in a prestellar core, using the IRAM 30 m telescope. While this species has previously been detected in hot cores of star forming regions, this represents the first detection in the cold (10 K) interstellar medium. Its more stable structural isomer acetaldehyde (CH3CHO) is also detected but not vinyl alcohol (CH2CHOH). We derive the abundances and abundance ratio of both detected species and compare them with predictions from the literature. We find that neither the previously proposed ion-neutral gas-phase nor the thermal grain surface scenarios can account for the observed abundances, while the radical-radical reaction between C2H5 and O remains a possible formation mechanism for both isomers. In addition, experiments have shown that cosmic ray induced electrons in ices containing CO, CO2, and C2H4 can explain the presence of both acetaldehyde and ethylene oxide in cold regions, but more quantitative data are needed to conclude whether the produced abundances can account for the observations.