The problematically short superwind of OH/IR stars. Probing the outflow with the 69 micrometer spectral band of forsterite

Aims. Spectra of OH/IR stars show prominent spectral bands of crystalline olivine (Mg(2-2x)Fe(2x)SiO4). To learn more about the timescale of the outflows of OH/IR stars, we study the spectral band of crystalline olivine at 69 mircometer.

Methods. The 69 micrometer band is of interest because its width and peak wavelength position are sensitive to the grain temperature and to the exact composition of the crystalline olivine. With Herschel/PACS, we observed the 69 ?m band in the outflow of 14 OH/IR stars. By comparing the crystalline olivine features of our sample with those of model spectra, we determined the size of the outflow and its crystalline olivine abundance.

Results. The temperature indicated by the observed 69 micrometer bands can only be reproduced by models with a geometrically compact superwind (RSW <2500 AU = 1400 R_star). This means that the superwind started less than 1200 years ago (assuming an outflow velocity of 10 km/s). The small amount of mass lost in one superwind and the high progenitor mass of the OH/IR stars introduce a mass loss and thus evolutionary problem for these objects, which has not yet been understood.