Circumstellar (CS) Envelopes
A-M.Lagrange (June 12, 2002)
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A. Context
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 Evolved stars in the post-AGB phase eject a large fraction of their
mass in the form of an envelope of gas and dust which play an
essential role in the enrichment of the interstellar medium.

In many cases, the envelope has a bipolar appearance, with the shape
of an hourglass. The bipolarity and the dark gap found in many
post-AGB envelopes indicate that an equatorial density enhancement is
chanelling the outflow. Dynamical models are successful in explaining
the envelope shape, but the cause of the axial symmetry is not clear
and could be due to a binary companion, rotation, or stellar magnetic
field.

At 30-50 mas, the structure of the innermost part of the envelope is
accessible, especially the region where dust condenses and new
constraints could be set.

At shorter wavelengths, the envelope is detected by light scattered on
grains : the brightness profile gives thus the grain density ; any
departure from the r^-2 behaviour (steady-state mass-loss) is a clue
on the recent history of the mass-loss (for instance pulses), or on
the distribution of grains sizes with respect to radial distance.
Polarimetric imaging will complete the constraints on the grains
characteristics, especially the size.


C. NACO observations
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It is proposed to perform broad-band imaging from J to L plus Br gamma,
on a small set of newly discovered bipolar or multipolar post-AGB
circumstellar envelopes. 

Polarimetric imaging will be done at short wavelength (J, H, K).

All objects are pretty red and fuzzy, so that the IRWFS is well
appropriate for  servoing  NAOS.

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Exp. time: elementary exposure as long as possible, without saturation
on the central star .  Integration time =  typically one hour per filter
to reach the low brightness of the outer part of the envelope.  
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Atmospheric conditions: being photometric is not required as there will be
the central star in the FoV, so relative photometry is ok.  Median seeing
conditions (750 mas) is OK. 
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