Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
K.-H. Hofmann, T. Blöcker, G. Weigelt,
and Y. Balega:
A multi-wavelength study of the oxygen-rich
AGB star CIT 3: Bispectrum speckle interferometry and dust-shell
modelling
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.121 (2003)
Abstract.
CIT 3 is an oxygen-rich
long-period variable evolving along the Asymptotic Giant Branch and
is one of the most extreme infrared AGB objects.
Due to substantial mass loss it is surrounded by an optically thick
dust shell
which absorbs almost all visible light radiated by the star and
finally re-emits it in the infrared regime.
We present the first near infrared bispectrum speckle-interferometry
observations of CIT 3 in the J-, H-, and K'-band
(resolution: 48mas, 56mas, and 73mas).
The interferograms were obtained with the Russian SAO 6m telescope.
While CIT 3 appears almost spherically symmetric in the
H- and K'-band, it is clearly elongated in the J-band along a symmetry
axis of position angle
-28o. Two structures can be identified: a compact
elliptical core (eccentricity ~0.8) and a fainter north-western
fan-like structure
(full opening angle ~40o).
The development of such asphericities close to the central
star suggests that CIT 3 is in the very end of its AGB evolution
or even in transition to the proto-planetary nebula phase where most
objects are observed in axi\-symmetric geometry.
Extensive radiative transfer calculations have been carried out and
confronted with the spectral energy distribution ranging from 1 µm to 1
mm, our 1.24µm, 1.65µm and 2.12µm visibility functions, as well as 11µm
ISI interferometry.
%
The best model found to match the observations refers to a cool central
star
with Teff=2250K which is surrounded by an optically thick dust shell
with tau(0.55µm) = 30. The central-star diameter is 10.9mas
and the inner dust shell diameter 71.9mas,
being in line with lunar occultation observations.
The inner dust-shell rim is located at r_1 = 6.6 Rstar and has a
temperature of T_1 = 900K. %
A two-component model existing of an inner uniform-outflow shell region
(rho ~ 1/r^2, r < 20.5 r_1)
and an outer region where the density declines more shallow as rho ~
1/r^1.5
proved to give the best overall match of the observations. Provided the
outflow velocity kept constant,
the more shallow density distribution in the outer shell
indicates that mass-loss has decreased with time in the past of CIT 3.
Adopting v_exp = 20 km/s, the termination of that mass-loss
decrease and the begin of the uniform-outflow phase took place 87 yr
ago. The present-day mass-loss rate can be determined to be Mdot =
(1.3-2.1) x 10^-5 Msol/yr for d=500-800pc.