A cold complex chemistry toward the low-mass protostar B1-b: evidence
  for complex molecule production in ices
  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 CH3OH
gas using the IRAM 30m telescope to identify locations of efficient non-thermal
ice desorption. We find three CH3OH abundance peaks tracing two outflows and a
quiescent region on the side of the core facing the protostar. The CH3OH gas
has a rotational temperature of ~10 K at all locations. The quiescent CH3OH
abundance peak and one outflow position were searched for complex molecules.
Narrow, 0.6-0.8 km s-1 wide, HCOOCH3 and CH3CHO lines originating in cold gas
are clearly detected, CH3OCH3 is tentatively detected and C2H5OH and HOCH2CHO
are undetected toward the quiescent core, while no complex molecular lines were
found toward the outflow. The core abundances with respect to CH3OH 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 suggest 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 provide
clear evidence of efficient complex molecule formation in cold interstellar
ices.
