The ALMA detection of CO rotational line emission in AGB stars in the
  Large Magellanic Cloud
  Context: Low- and intermediate-mass stars lose most of their stellar mass at
the end of their lives on the asymptotic giant branch (AGB). Determining gas
and dust mass-loss rates (MLRs) is important in quantifying the contribution of
evolved stars to the enrichment of the interstellar medium. Aims: Attempt to,
for the first time, spectrally resolve CO thermal line emission in a small
sample of AGB stars in the Large Magellanic Cloud. Methods: ALMA was used to
observe 2 OH/IR stars and 4 carbon stars in the LMC in the CO J= 2-1 line.
Results: We present the first measurement of expansion velocities in
extragalactic carbon stars. All four C-stars are detected and wind expansion
velocities and stellar velocities are directly measured. Mass-loss rates are
derived from modelling the spectral energy distribution and Spitzer/IRS
spectrum with the DUSTY code. Gas-to-dust ratios are derived that make the
predicted velocities agree with the observed ones. The expansion velocities and
MLRs are compared to a Galactic sample of well-studied relatively low MLRs
stars supplemented with "extreme" C-stars that have properties more similar to
the LMC targets. Gas MLRs derived from a simple formula are significantly
smaller than derived from the dust modelling, indicating an order of magnitude
underestimate of the estimated CO abundance, time-variable mass loss, or that
the CO intensities in LMC stars are lower than predicted by the formula derived
for Galactic objects. This could be related to a stronger interstellar
radiation field in the LMC. Conclusions: Although the LMC sample is small and
the comparison to Galactic stars is non-trivial because of uncertainties in
their distances it appears that for C stars the wind expansion velocities in
the LMC are lower than in the solar neighbourhood, while the MLRs appear
similar. This is in agreement with dynamical dust-driven wind models.
