Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Blöcker, Y.Y. Balega, A.B.
Menshchikov, R. Osterbart,
and G. Weigelt:
The evolving dust envelope of IRC+10216
Abstract (contributed talk) for the
2nd Austrian ISO workshop,
"Atmospheres
of M, S and C Giants: Models and Observations"
May 27th - 29th, 1999. Vienna, Austria.
Abstract.
IRC+10216 (CW Leo) is the nearest (110-170pc), brightest and
best-studied carbon star.
It is without doubt in a very advanced stage of its
AGB evolution due to its long pulsational period (650d), high mass-loss
rate (2-5x10^-5 Msol/yr)
and carbon-rich chemistry of its dust shell The star's initial mass can
be estimated to be
4 Msol +/- 1 Msol due to the observed isotopic ratios of C, N and 0 in
the dust shell (Guelin et al. 1995) and the luminosity
of the central star (Weigelt et al. 1998). Accordingly, the core mass
should be ~0.7 to 0.8 Msol with corresponding thermal-pulse cycle times
of ~1-3x10^{4}yr (Blöcker 1995). The dust-shell chemistry indicates
that already a significant number of thermal pulses did take place.
However, the present mass-loss rate leads to a very effective erosion
of the envelope per thermal pulse cycle, possibly as high as ~1
Msol/cycle. Consequently, the whole envelope
may be lost during the next few thermal pulses leading to the
termination of the AGB evolution.
Thus, IRC+10216 is likely to have entered a phase immediately before
moving off the AGB. This seems to be supported by one of its most
striking features: the non-spherical appearence of its dust shell
(e.g. Weigelt et al. 1998, 1999; Haniff & Buscher 1998; Skinner et
al. 1998).
In contrast to their progenitors, AGB successors (as proto-planetary
nebulae) often expose prominent features of asphericities, mostly in
axisymmetric geometry. The dust shell of IRC+10216 is already
considerably elongated in NS direction probably even with a bipolar
structure.
We present high-resolution J-, H-, and K-band observations of IRC+10216
(Osterbart et al. 1999). The
images were reconstructed from 6 m telescope speckle interferograms
using the bispectrum speckle interferometry method. The H image has the
unprecedented resolution of 70 mas. The H and K images consist of
several compact components within a 0.2 arcsec radius and a fainter
asymmetric nebula. A series of K-band images from five epochs between
October 1995 and November 1998 reveals that the inner nebula is
dynamically evolving. For instance, the separation of the two brightest
components A and
B increased from 191 mas in 1995 to 265 mas in 1998 corresponding to a
relative
velocity within the plane of the sky of 23 mas/yr or 14 km/s.
At the same time component B is fading and the components C and D
become
brighter. The general geometry of the nebula seems to be bipolar.
Most likely, the brightest component A is not the central star, but the
southern lobe of a bipolar structure (Men'shchikow et al. 1999).
Instead, the position of the central star is probably at or close to
component B. Consistently component B is very red in the H-K color
(=4.2)
while A and the northern $J$--band components are relatively blue. The
polarization pattern as calculated from 1.06 µm HST archival data
supports this model showing strong polarization in the northern arms
and still a significant polarization in the peak A. The
apparent motions seem to be rather symmetric around
the position of B and the observed changes are consistent with the
assumption of an enhanced mass loss starting in 1997.
If the star is at B, then the components A, C, and D are
located at the inner boundary of the dust shell.
High-resolution imaging reveals the non-spherical and evolving
appearence
of the dust shell of IRC+10216 being at the very end of its AGB
evolution.
The clumpiness within the bipolar shape is probably due to small scale
fluctuations of the dust condensation radius which, in turn, might be
influenced by, e.g., giant surface convection cells. Mechanisms
responsible for bipolar structures include, e.g., stellar rotation and
binarity.