2010ApJ...716..825O

Gas-phase complex organic molecules have been detected toward a range of high- and low-mass star-forming regions at abundances which cannot be explained by any known gas-phase chemistry. Recent laboratory experiments show that UV irradiation of CH₃OH-rich ices may be an important mechanism for producing complex molecules and releasing them into the gas phase. To test this ice formation scenario, we mapped the B1-b dust core and nearby protostar in CH₃ OH gas using the IRAM 30 m telescope to identify locations of efficient non-thermal ice desorption. We find three CH₃ OH abundance peaks tracing two outflows and a quiescent region on the side of the core facing the protostar. The CH₃ OH gas has a rotational temperature of ∼10 K at all locations. The quiescent CH₃ OH abundance peak and one outflow position were searched for complex molecules. Narrow, 0.6-0.8 km/s wide, HCOOCH₃ and CH₃ CHO lines originating in cold gas are clearly detected, CH₃OCH₃ is tentatively detected, and C₂H₅ OH and HOCH₂CHO are undetected toward the quiescent core, while no complex molecular lines were found toward the outflow. The core abundances with respect to CH₃ OH are ∼2.3% and 1.1% for HCOOCH₃ and CH₃CHO, respectively, and the upper limits are 0.7%-1.1%, which is similar to most other low-mass sources. The observed complex molecule characteristics toward B1-b and the pre-dominance of HCO-bearing species suggests a cold ice (below 25 K, the sublimation temperature of CO) formation pathway followed by non-thermal desorption through, e.g., UV photons traveling through outflow cavities. The observed complex gas composition together with the lack of any evidence of warm gas-phase chemistry provides clear evidence of efficient complex molecule formation in cold interstellar ices.