Modelling dead OH/IR stars

OH/IR stars present several OH maser emission lines which are formed in the outer regions of their circumstellar dust envelopes. These stars can be recognized at large distances, based only on radio and infrared observations. They can therefore be used as kinematic tracers in galaxies. The emission at 1612 MHz is the strongest line and presents a double profile (Delta v = 20-50 km/s), although in the planetary nebulae phase this line is sometimes no longer detected. In this work we calculate radiative transfer models to describe the dust envelopes of a sample of OH/IR stars. In this sample, the maser emission at 1612 MHz has disappeared. We also included some OH/IR stars in which this maser emission was recently detected. The results show that the physical parameters of the dust envelopes of almost the whole sample are those expected for normal oxygen-rich AGB stars. However, three sources have a different pattern: they present a double-peaked energy dustribution which may be due to an evolved dust envelope.