Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.sao.ru/hq/balega/PUBL_BAL/PUB_2002/Hal_IAU209.ps Дата изменения: Wed Feb 24 16:01:48 2010 Дата индексирования: Tue Oct 2 11:08:05 2012 Кодировка: Поисковые слова: m 1

Planetary Nebulae and Their Role in the Universe
IAU Symposium 209, 2002
M. Dopita, S. Kwok and R.S. Sutherland, eds.
A multi-wavelength study of the oxygen-rich AGB star
CIT 3: Bispectrum speckle interferometry and dust-shell
modelling
K.-H. Hofmann, T. Blocker, G. Weigelt
Max{Planck{Institut fur Radioastronomie, 53121 Bonn, Germany
Y. Balega
Special Astrophysical Observatory, Nizhnij Arkhyz, 35147, Russia
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 substan-
tial mass loss it is surrounded by an optically thick dust shell which absorbs
almost all visible light radiated by the star and nally re-emits it in the infrared
regime. We present the rst near infrared bispectrum speckle-interferometry ob-
servations of CIT 3 in the J -, H-, and K 0 -band (resolution: 48 mas, 56 mas, and
73 mas). The interferograms were obtained with the Russian SAO 6 m telescope.
While CIT 3 appears almost spherically symmetric in the H- and K 0 -band it is
clearly elongated in the J-band along a symmetry axis of position angle 28 o .
Two structures can be identied: a compact elliptical core (eccentricity 0.8)
and a fainter north-western fan-like structure (full opening angle  40 o ). 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 axisymmetric geometry.
Extensive radiative transfer calculations have been carried out and con-
fronted with the spectral energy distribution ranging from 1 m to 1 mm, our
1.24 m, 1.65 m and 2.12 m visibility functions, as well as 11 m ISI inter-
ferometry. The best model found to match the observations refers to a cool
central star with T e = 2250 K which is surrounded by an optically thick dust
shell with (0:55m) = 30. The central-star diameter is 10.9 mas and the inner
dust shell diameter 71.9 mas, being in line with lunar occultation observations.
The inner dust-shell rim is located at r 1 = 6:6R  and has a temperature of
T 1 = 900 K. A two-component model existing of an inner uniform-out ow shell
region (  1=r 2 , r < 20:5r 1 ) and an outer region where the density declines
more shallow as   1=r 1:5 proved to give the best overall match of the obser-
vations. Provided the out ow 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-out ow phase took place 87 yr ago. The
present-day mass-loss rate can be determined to be _
M = (1:3 2:1)
10 5 M /yr
for d = 500 800 pc. A full description of these observations and models is given
in Hofmann et al. (2001, A&A 379, 529).
1