The mineralogy, geometry and mass-loss history of IRAS 16342-3814
Dijkstra C., Waters L.B.F.M., Kemper F., Min M., Matsuura M., Zijlstra A.A.,
de Koter A., Hony S., Dominik C., 2003, A&A, 399, 1037
Abstract
We present the 2-200 micron Infrared Space Observatory (ISO) spectrum
and 3.8-20 micron ISAAC and TIMMI2 images of the extreme OH/IR star IRAS
16342-3814. Amorphous silicate absorption features are seen at 10 and
20 micron, together with crystalline silicate absorption features up to
almost 45 micron. No other OH/IR star is known to have crystalline
silicate features in absorption up to these wavelengths. This suggests
that IRAS 16342-3814 must have, or recently had, an extremely high
mass-loss rate. Indeed, preliminary radiative transfer calculations
suggest that the mass-loss rate may be as large as 10-3
Msun yr-1. The
3.8 micron ISAAC image shows a bipolar reflection nebula with a dark
equatorial waist or torus, similar to that seen in optical images
taken with the Hubble Space Telescope (HST). The position angle of the
nebula decreases significantly with increasing wavelength, suggesting
that the dominant source of emission changes from scattering to
thermal emission. Still, even up to 20 micron the nebula is oriented
approximately along the major axis of the nebula seen in the HST and
ISAAC images, suggesting that the torus must be very cold, in
agreement with the very red ISO spectrum. The 20 micron image shows a
roughly spherically symmetric extended halo, approximately 6 arcsec in
diameter, which is probably due to a previous phase of mass-loss on
the AGB, suggesting a transition from a (more) spherically symmetric
to a (more) axial symmetric form of mass-loss at the end of the
AGB. Using a simple model, we estimate the maximum dust particle sizes
in the torus and in the reflection nebula to be 1.3 and 0.09 micron
respectively. The size of the particles in the torus is large compared
to typical ISM values, but in agreement with high mass-loss rate
objects like AFGL 4106 and HD161796. We discuss the possible reason
for the difference in particle size between the torus and the
reflection nebula.
paper (pdf 500 kb)
Last update: June 10, 2003