A dam around the Water Fountain Nebula?The dust shell of IRAS16342-3814 spatially resolved with VISIRNLT

摘要

Context. Bipolar morphologies in planetary nebulae (PNe) are believed to be closely linked to binary central stars. Either by collimat-ing a fast stellar wind or by driving a jet via accretion in the central system, dusty torii or stable disks may be crucial ingredients forthe shaping of PNe.Aims. We study the dust distribution in the very young proto-planetary nebula (PPN) IRAS16342-3814, also known as the WaterFountain Nebula, which is known to show strong bipolar characteristics in the shape of two reflection lobes, and high-velocity colli-mated molecular outlfows.Methods. We use the new mid-infrared (MIR) instrument VISIR on the Very Large Telescope (VLT) both in imaging and spectroscopymode at wavelengths from 8 to 13 μm.Results. We present the first spatially resolved MIR observations of a dusty evolved star obtained with VISIR and find that the im-proved spatial resolution contradicts previous claims of an elliptical brightness distribution at the heart of IRAS16342: we find thewaist region to be dark even in the MIR. We show that the filling angle of the obscuring dust lane, which is made mostly of amorphoussilicates, is very large, possibly even close to a spherically symmetric superwind as seen in OH/IR stars.Conclusions. We conclude that, in contrast to the multitude of recent dusty-disk detections in post-AGB stars and PNe, IRAS16342does not show this extreme equatorial density enhancement, at least not on the scale of the dusty environment which lends the objectits IR appearance. Rather, it appears that the observed precessing jets are shaping the bipolar nature in the remains of a sphericallysymmetric AGB superwind.