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

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 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- and K'-band it is clearly elongated in the J-band along a symmetry axis of position angle -28 degrees. Two structures can be identified: a compact elliptical core and a fainter north-western fanlike structure. The eccentricity of the elliptical core, given by the ratio of minor to major axis, is approximately epsilon = 123 mas/154 mas = 0.8. The full opening angle of the fan amounts to approximately 40 degrees. Extensive radiative transfer calculations have been carried out and confronted with the observations taking into account the spectral energy distribution ranging from 1 mum to 1 mm, our near-infrared visibility functions at 1.24 mum, 1.65 mum and 2.12 mum, as well as 11 mum ISI interferometry. The best model found to match the observations refers to a cool central star with T-eff = 2250 K which is surrounded by an optically thick dust shell with tau (0.55 mum) = 30. The models give a central-star diameter of Theta (*) = 10.9 mas and an inner dust shell diameter of Theta (1) = 71.9 mas being in line with lunar occultation observations. The inner rim of the dust-shell is located at r(1) = 6.6 R-* and has a temperature of T-1 = 900 K. The grain sizes were found to comply with a grain-size distribution according to Mathis et al. (1977) with n(a) similar to a(-3.5), and 0.005 mum less than or equal to a less than or equal to 0.25 mum. Uniform out ow models, i.e. density distributions with rho similar to 1/r(2), turned out to underestimate the flux beyond 20 mum. A two-component model existing of an inner uniform-out ow shell region (rho similar to 1/r(2)) and an outer region where the density declines more shallow as rho similar to 1/r(1.5) proved to remove this flux deficiency and to give the best overall match of the observations. The transition between both density distributions is at r(2) = 20.5 r(1) = 135.7 R-* where the dust-shell temperature has dropped to T-2 = 163 K. 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(-1), 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) x 10(-5) M-./yr for d = 500-800 pc.