A multi-wavelength study of the oxygen-rich AGB star CIT 3: Bispectrum speckle interferometry and dust-shell modelling

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 nally re-emits it in the infrared regime. We present the rst near infrared bispectrum speckle-interferometry observations of CIT 3 in the J-, H-, and K 0 -band. The J-, H-, and K 0 -band resolution is 48 mas, 56 mas, and 73 mas, resp. The interferograms were obtained with the Russian 6 m telescope at the Special Astrophysical Observatory. While CIT 3 appears almost spherically symmetric in the H -a ndK 0 -band it is clearly elongated in the J-band along a symmetry axis of position angle 28. Two structures can be identied: 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 = 123 mas/154 mas = 0.8. The full opening angle of the fan amounts to approximately 40 .E xtensive radiative transfer calculations have been carried out and confronted with the observations taking into account the spectral energy distribution ranging from 1 m to 1 mm, our near-infrared visibility functions at 1.24 m, 1.65 ma nd 2.12 m, as well as 11 m ISI interferometry. The best model found to match the observations refers to a cool central star with Te = 2250 K which is surrounded by an optically thick dust shell with (0:55 m) = 30. The models give a central-star diameter of = 10:9 mas and an inner dust shell diameter of 1 =7 1: 9m as being in line with lunar occultation observations. The inner rim of the dust-shell is located at r1 =6 :6 R and has a temperature of T1 = 900 K. 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 m a 0:25 m. Uniform outflow models, i.e. density distributions with 1=r 2 , turned out to underestimate the flux beyond 20 m. A two-component model existing of an inner uniform-outflow shell region ( 1=r 2 ) and an outer region where the density declines more shallow as 1=r 1:5 proved to remove this flux deciency and to give the best overall match of the observations. The transition between both density distributions is at r2 =2 0:5 r1 = 135:7 R where the dust-shell temperature has dropped to T2 = 163 K. 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 =2 0 km s 1 , 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 _ M =( 1:3 2:1) 10 5 M/yr for d = 500 800 pc.