DEATHSTAR: Nearby AGB stars with the Atacama Compact Array I. CO
  envelope sizes and asymmetries: A new hope for accurate mass-loss-rate
  estimates
  This is the first publication of the DEATHSTAR project. The goal of the
project is to reduce the uncertainties of observational estimates of mass-loss
rates from Asymptotic Giant Branch (AGB) stars. Line emission from 12CO J=2-1
and 3-2 were mapped using the ACA. In this initial analysis, the emission
distribution was fit to a Gaussian distribution in the uv-plane. Detailed
radiative transfer analysis will be presented in the future. The axes of the
best-fit Gaussian at the line center of the 12CO J=2-1 emission gives a first
indication of the size of the emitting region. Furthermore, the fitting
results, such as the major and minor axis, center position, and the goodness of
fit across both lines, constrain the symmetry of the emission distribution. We
find that the CO envelope sizes are, in general, larger for C-type than for
M-type AGB stars, which is expected if the CO/H2 ratio is larger in C-type
stars. Furthermore, a relation between the 12CO J=2-1 size and circumstellar
density is shown that, while in broad agreement with photodissociation
calculations, reveals large scatter and systematic differences between the
stellar types. The majority of the sources have CO envelopes that are
consistent with a spherically symmetric, smooth outflow. For about a third of
the sources, indications of strong asymmetries are found. This is consistent
with previous interferometric investigations of northern sources. Smaller scale
asymmetries are found in a larger fraction of sources. These results for CO
envelope radii and shapes can be used to constrain detailed radiative transfer
modeling of the same stars so as to determine mass-loss rates that are
independent of photodissociation models. For a large fraction of the sources,
observations at higher spatial resolution will be necessary to further
investigate the complex circumstellar dynamics revealed by our ACA
observations.
