The problematically short superwind of OH/IR stars

Probing the outflow with the 69 μm spectral band of forsterite

¹ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: bldevries.science@gmail.com
² Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
³ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

Received: 27 August 2013
Accepted: 31 October 2013

Abstract

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 μm.

Methods. The 69 μm 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 μm 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 μm bands can only be reproduced by models with a geometrically compact superwind (R_SW ≲ 2500 AU = 1400 R_∗). 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.