SIMBAD references

In order to study the history of mass loss in extreme OH/IR stars, we observed a number of these objects using CO as a tracer of the density and temperature structure of their circumstellar envelopes. Combining CO observations from the Herschel Space Observatory with those from the ground, we trace mass loss rates as a function of radius in five extreme OH/IR stars. Using radiative transfer modelling, we modelled the dusty envelope as well as the CO emission. The high-rotational transitions of CO indicate that they originate in a dense superwind region close to the star while the lower transitions tend to come from a more tenuous outer wind which is a result of the mass loss since the early AGB phase. The models of the circumstellar envelopes around these stars suggest that they have entered a superwind phase in the past 200-500 years. The low ¹⁸O/¹⁷O (∼0.1 compared to the solar abundance ratio of ∼5) and ¹²C/¹³C (3-30 cf. the solar value of 89) ratios derived from our study support the idea that these objects have undergone hot-bottom burning and hence that they are massive M≥5M_☉ AGB stars.