Complex organic molecules in low-mass protostars on solar system scales
  -- I. Oxygen-bearing species
  Complex organic molecules (COMs) are thought to form on icy dust grains in
the earliest phase of star formation. The evolution of these COMs from the
youngest Class 0/I protostellar phases toward the more evolved Class II phase
is still not fully understood. Since planet formation seems to start early, and
mature disks are too cold for characteristic COM emission lines, studying the
inventory of COMs on solar system scales in the Class 0/I stage is relevant.
ALMA Band 3 (3 mm) and Band 6 (1 mm) observations are obtained of seven Class 0
protostars in the Perseus and Serpens star-forming regions. By modeling the
inner protostellar region using 'LTE' models, the excitation temperature and
column densities are determined for several O-bearing COMs. B1-c, B1-bS, and
Serpens S68N show COM emission, i.e, three out of the seven sources. No clear
correlation seems to exist between the occurrence of COMs and source
luminosity. The abundances of several COMs with respect to CH3OH are remarkably
similar for the three COM-rich sources, and to IRAS 16293-2422B and HH 212. For
other COMs the abundances differ by up to an order of magnitude, indicating
that local source conditions are case determining. B1-c hosts a cold
($T_{ex}\approx60$ K), more extended component of COM emission with a column
density of typically a few % of the warm/hot ($T_{ex}\sim 200$ K), central
component. A D/H ratio of 1-3 % is derived based on the CH2DOH/CH3OH ratio
suggesting a temperature of $\sim$15~K during the formation of methanol. This
ratio is consistent with other low-mass protostars. Future mid-infrared
facilities such as JWST/MIRI will be essential to directly observe COM ices.
Combining this with a larger sample of COM-rich sources with ALMA will allow
for directly linking ice and gas-phase abundances in order to constrain the
routes that produce and maintain chemical complexity during the star formation
process.
