Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
G. Weigelt, T. Blöcker, K.-H. Hofmann,
A. Men'shchikov,
J.M. Winters, and Y. Balega:
Near-infrared monitoring of the carbon star
IRC+10216: A high spatial resolution time sequence of dust-shell
evolution
in
Planetary Nebulae: Their Evolution and Role in the Universe ,
IAU Symp. 209, Canberra, Australia, November 19-23, 2001,
M. Dopita, S. Kwok, R. Sutherland (eds.), Astron. Soc. Pac., v.209,
p.83 (2003)
Abstract.
The carbon star IRC+10216 is a long-period variable
evolving along the Asymptotic Giant Branch. Near-infrared imaging has
revealed that its dust shell is clumpy, changing on a time scale
of only ~1 yr, and that it is bipolar on sub-arcsecond scale
(Weigelt et al. 1998, Haniff & Buscher 1998, Osterbart et al.
2000).
Since most dust shells around AGB stars are known to be spherically
symmetric whereas most proto-planetary nebulae (PPN) appear in
axisymmetric geometry,
it is very likely that IRC+10216
has entered the transition phase into a PPN. This gives evidence
that the break of dust-shell symmetry already takes place at the end of
the AGB evolution.
Our recent two-dimensional radiative transfer modelling (Men'shchikov
et al., 2001)
has shown that the central star is not located at the brightest
dust-shell component A but at the position
of the northern component B. The central star is surrounded by an
optically
thick dust shell with polar cavities exhibiting a full opening angle of
36o. The bipolar structure
is inclined by 40o pointing with its southern lobe towards
the observer. Accordingly, the bright and compact
component A is the southern lobe of this bipolar structure dominated
by scattered light. We present near-infrared bispectrum
speckle-interferometry monitoring
of IRC+10216 in the J, H, and K band obtained with the SAO
6m telescope.
The J-, H-, and K-band resolutions are 50mas, 56mas, and 73mas, resp.
The K-band observations cover 8 different epochs from 1995 to 2001 and
show the dynamical evolution of the dust shell which consists
of several compact components within a 0.2'' radius and a fainter
asymmetric nebula.
For instance, the apparent separation of the two
initially brightest components A and B increased from 190 mas in 1995
to 340 mas in 2000. At the same time, component B is fading and has
almost diappeared in 2000
whereas the initially faint components C and D become brighter.
Now, component C is almost as bright as component A.
These changes of the dust-shell structure can be related to
corresponding changes of the optical depths and hence to
mass-loss changes. The present monitoring, covering more than 3
pulsational periods, show that the structural variations are not simply
related to the stellar pulsational cycle. This is consistent with the
predictions of hydrodynamical models that enhanced dust formation takes
place on a timescale
of several pulsational cycles (Fleischer et al. 1995).