The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud
van Loon, J. Th.; Olofsson, H.; Sahai, R.; Wood, P. R.; Justtanont, K.; Decin, L.; Zijlstra, A. A.; Boyer, M. L.; Meixner, M.; Groenewegen, M. A. T.; Marigo, P.; Bernard-Salas, J.; McDonald, I.; Lagadec, E.; Matsuura, M.; Kerschbaum, F.; Jones, O. C.; Sloan, G. C.; Guzman-Ramirez, L.; Rawlings, M. G.; Srinivasan, S.; Vlemmings, W. H. T.; Feast, M. W.; Goldman, S. R.
Belgium, Sweden, Italy, United States, South Africa, United Kingdom, Austria, Australia, Chile, Netherlands, France, Taiwan
Abstract
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: This study attempts to spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud (LMC).
Methods: The Atacama Large Millimeter Array was used to observe two OH/IR stars and four 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. The derived gas-to-dust ratios allow the predicted velocities to agree with the observed gas-to-dust ratios. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with extreme C stars with properties that are more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than those derived from 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 (hence luminosities), it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear to be similar. This is in agreement with dynamical dust-driven wind models.