A dam around the Water Fountain Nebula?

The dust shell of IRAS16342-3814 spatially resolved with VISIR/VLT

¹ Insituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: tijl.verhoelst@ster.kuleuven.be
² University of Manchester, Jodrell Bank Centre for Astrophysics, Manchester, M13 9PL, UK
³ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁴ Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
⁵ Kapteyn Astronomical Institute, Landleven 12, 9747 AD Groningen, The Netherlands

Received: 4 March 2009
Accepted: 25 May 2009

Abstract

Context. Bipolar morphologies in planetary nebulae (PNe) are believed to be closely linked to binary central stars. Either by collimating a fast stellar wind or by driving a jet via accretion in the central system, dusty torii or stable disks may be crucial ingredients for the shaping of PNe.

Aims. We study the dust distribution in the very young proto-planetary nebula (PPN) IRAS16342-3814, also known as the Water Fountain Nebula, which is known to show strong bipolar characteristics in the shape of two reflection lobes, and high-velocity collimated molecular outlfows.

Methods. We use the new mid-infrared (MIR) instrument VISIR on the Very Large Telescope (VLT) both in imaging and spectroscopy mode 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 improved spatial resolution contradicts previous claims of an elliptical brightness distribution at the heart of IRAS16342: we find the waist region to be dark even in the MIR. We show that the filling angle of the obscuring dust lane, which is made mostly of amorphous silicates, 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, IRAS16342 does not show this extreme equatorial density enhancement, at least not on the scale of the dusty environment which lends the object its IR appearance. Rather, it appears that the observed precessing jets are shaping the bipolar nature in the remains of a spherically symmetric AGB superwind.