A multi-wavelength study of the oxygen-rich AGB star CIT 3:
Bispectrum speckle interferometry and dust-shell modelling
K.-H. Hofmann, Y. Balega, T. Blöcker, and G. Weigelt
Astronomy & Astrophysics 379, 529-539 (2001)
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.
The J-, H-, and K'-band resolution is
48mas, 56mas, and 73mas, resp.
The interferograms were obtained with the Russian 6m telescope at the
Special Astrophysical Observatory.
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 and a fainter north-western fan-like structure.
The eccentricity of the elliptical core, given by the ratio of
minor to major axis, is approximately 123mas/154mas=0.8.
The full opening angle of the fan amounts to approximately 40o.
Extensive radiative transfer calculations have been carried out
and confronted with the observations taking into account
the spectral energy distribution ranging from 1µm to 1mm,
our near-infrared visibility functions at 1.24µm,
1.65µm and 2.12µm 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 models give a central-star diameter of Thetastar=10.9 mas
and an inner dust shell diameter of Theta1=71.9 mas
being in line with lunar occultation observations.
The inner rim of the dust-shell is located at
r1= 6.6 Rstar and has a
temperature of T1=900K. The grain sizes were found to comply with
a grain-size distribution according to Mathis et al. (1977) with
n(a) ~ a-3.5, and 0.005 µm < a < 0.25µm.
Uniform outflow models, i.e. density distributions with
rho ~ 1/r2 turned out to
underestimate the flux beyond 20µm.
A two-component model existing of an inner
uniform-outflow shell region (rho ~ 1/r2)
and an outer region where the density declines more shallow as
rho ~ 1/r1.5
proved to remove this flux deficiency and to give the best overall match of the
observations. The transition between both density distributions is at
r2 = 20.5 r1= 135.7 Rstar
where the dust-shell temperature has dropped
to T2 = 163K.
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 vexp=20km/s, the termination of that mass-loss
decrease and
the begin of the uniform-outflow phase
took place 87yr ago. The present-day mass-loss rate can be determined to be
Mdot = (1.3-2.1) 10-5Msol/yr for d=500-800pc.