The ALMA-PILS survey: First detections of ethylene oxide, acetone and
  propanal toward the low-mass protostar IRAS 16293-2422
  One of the open questions in astrochemistry is how complex organic and
prebiotic molecules are formed. Aims. Our aim is to start the process of
compiling an inventory of oxygen-bearing complex organic molecules toward the
solar-type Class 0 protostellar binary IRAS16293-2422 from an unbiased spectral
survey with ALMA (PILS). Here we focus on the new detections of ethylene oxide
(c-C$_2$H$_4$O), acetone (CH$_3$COCH$_3$), and propanal (C$_2$H$_5$CHO).
Methods. With ALMA, we surveyed the spectral range from 329 to 363 GHz at
0.5$"$ (60 AU diameter) resolution. Using a simple model for the molecular
emission in LTE, the excitation temperatures and column densities of each
species were constrained. Results. We successfully detect propanal (44 lines),
ethylene oxide (20 lines) and acetone (186 lines) toward one component of the
protostellar binary, IRAS16293B. The high resolution maps demonstrate that the
emission for all investigated species originates from the compact central
region close to the protostar. This, along with a derived common excitation
temperature of $\sim$ 125 K, is consistent with a coexistence of these
molecules in the same gas. Conclusions. The observations mark the first
detections of acetone, propanal and ethylene oxide toward a low-mass protostar.
The relative abundance ratios of the two sets of isomers
(CH$_3$COCH$_3$/C$_2$H$_5$CHO $\sim$ 8 and CH$_3$CHO/c-C$_2$H$_4$O $\sim$ 12)
are comparable to previous observations toward high-mass protostars. The
majority of observed abundance ratios from these results as well as those
measured toward high-mass protostars are up to an order of magnitude above the
predictions from chemical models. This may reflect either missing reactions or
uncertain rates in the chemical networks. The physical conditions, such as
temperatures or densities, used in the models, may not be applicable to
solar-type protostars either.
