2015A&A...581A..60D

Asymptotic giant branch (AGB) stars are in one of the latest evolutionary stages of low to intermediate-mass stars. Their vigorous mass loss has a significant effect on the stellar evolution, and is a significant source of heavy elements and dust grains for the interstellar medium. The mass-loss rate can be well traced by carbon monoxide (CO) line emission. We present new Herschel/HIFI and IRAM 30 m telescope CO line data for a sample of 53 galactic AGB stars. The lines cover a fairly large range of excitation energy from the J=1-0 line to the J=9-8 line, and even the J=14-13 line in a few cases. We perform radiative transfer modelling for 38 of these sources to estimate their mass-loss rates. We used a radiative transfer code based on the Monte Carlo method to model the CO line emission. We assume spherically symmetric circumstellar envelopes that are formed by a constant mass-loss rate through a smoothly accelerating wind. We find models that are consistent across a broad range of CO lines for most of the stars in our sample, i.e., a large number of the circumstellar envelopes can be described with a constant mass-loss rate. We also find that an accelerating wind is required to fit, in particular, the higher-J lines and that a velocity law will have a significant effect on the model line intensities. The results cover a wide range of mass-loss rates (∼10^–8 to 2x10^–5M_☉/yr) and gas expansion velocities (2 to 21.5km/s), and include M-, S-, and C-type AGB stars. Our results generally agree with those of earlier studies, although we tend to find slightly lower mass-loss rates by about 40%, on average. We also present ``bonus'' lines detected during our CO observations.