1) K.-H. Hofmann, Y. Balega, T. Blöcker, and G. Weigelt

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

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.

2) Vinkovic, D., Bloecker, T., Hofmann, K.-H., Elitzur, M., Weigelt, G.

Bipolar outflow on the asymptotic giant branch - the case of IRC+10011

MNRAS, 352, 852-862 (2004)

Abstract
Near-infrared imaging of the asymptotic giant branch (AGB) star IRC+10011 (= CIT3) reveals the presence of a bipolar structure within the central ~0.1 arcsec of a spherical dusty wind. We show that the image asymmetries originate from ~10-4 MSun of swept-up wind material in an elongated cocoon whose expansion is driven by bipolar jets. We perform detailed 2D radiative transfer calculations with the cocoon modelled as two cones extending to ~1100 au within an opening angle of 30°, embedded in a wind with the standard r-2 density profile. The cocoon expansion started <~200 yr ago, while the total lifetime of the circumstellar shell is ~5500 yr. Similar bipolar expansion, at various stages of evolution, has been recently observed in a number of other AGB stars, culminating in jet breakout from the confining spherical wind. The bipolar outflow is triggered at a late stage in the evolution of AGB winds, and IRC+10011 provides its earliest example thus far. These new developments enable us to identify the first instance of symmetry breaking in the evolution from AGB to planetary nebula.

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3) A. Gauger, Y.Y.Balega, P. Irrgang, R. Osterbart and G. Weigelt

High-resolution speckle masking interferometry and radiative transfer modeling of the oxygen-rich AGB star AFGL 2290

Astronomy and Astrophysics 346, 505-519 (1999)

Abstract. We present the first diffraction-limited speckle masking observations of the oxygen-rich AGB star AFGL 2290. The speckle interferograms were recorded with the Russian 6m SAO telescope. At the wavelength 2.11µm a resolution of 75 milli-arcsec (mas) was obtained. The reconstructed diffraction-limited image reveals that the circumstellar dust shell (CDS) of AFGL 2290 is at least slightly non-spherical. The visibility function shows that the stellar contribution to the total 2.11µm flux is less than 40%, indicating a rather large optical depth of the circumstellar dust shell. The 2-dimensional Gaussian visibility fit yields a diameter of AFGL 2290 at 2.11µm of 43mas x 51mas, which corresponds to a diameter of 42AU x 50AU for an adopted distance of 0.98kpc.
Our new observational results provide additional constraints on the CDS of AFGL 2290, which supplement the information from the spectral energy distribution (SED). To determine the structure and the properties of the CDS we have performed radiative transfer calculations for spherically symmetric dust shell models. The observed SED approximately at phase 0.2 can be well reproduced at all wavelengths by a model with Teff=2000K, a dust temperature of 800K at the inner boundary, an optical depth tau_v=100 and a radius for the single-sized grains of a=0.1µm. However, the 2.11µm visibility of the model does not match the observation.
Exploring the parameter space, we found that grain size is the key parameter in achieving a fit of the observed visibility while retaining the match of the SED, at least partially. Both the slope and the curvature of the visibility strongly constrain the possible grain radii. On the other hand, the SED at longer wavelengths, the silicate feature in particular, determines the dust mass loss rate and, thereby, restricts the possible optical depths of the model. With a larger grain size of 0.16µm and a higher tau_v=150, the observed visibility can be reproduced preserving the match of the SED at longer wavelengths. Nevertheless, the model shows a deficiency of flux at short wavelengths, which is attributed to the model assumption of a spherically symmetric dust distribution, whereas the actual structure of the CDS around AFGL 2290 is in fact non-spherical. Our study demonstrates the possible limitations of dust shell models which are constrained solely by the spectral energy distribution, and emphasizes the importance of high spatial resolution observations for the determination of the structure and the properties of circumstellar dust shells around evolved stars.

4) Men'shchikov, A.B., Balega, Y.Y., Berger, M., Driebe, T., Hofmann, K.-H., Maximov, A.F., Schertl, D., Shenavrin V.I., and Weigelt, G.

Near-infrared speckle interferometry and radiative transfer modelling of the carbon star LP Andromedae

A&A, 448, pg.271-281 (2006)

Abstract
We present the near-infrared speckle interferometry for LP And in the H and $K^{\prime}$ bands with diffraction-limited resolutions of 56 and 72 mas, new $\it JHKLM$ photometry, and the results of our radiative transfer modelling of this carbon star. The reconstructed visibility reveals a spherically-symmetric envelope surrounding the central star. To determine the physical parameters of the latter and the properties of its dusty envelope, we performed extensive radiative transfer calculations. The well-defined spectral energy distribution of LP And in the entire range from the near-IR to millimeter wavelengths (including the absorption feature visible in the stellar continuum at 3 ${\mu}$m and the shapes of the dust emission bands at 11 and 27 ${\mu}$m), together with our H-band visibility can be reproduced by a spherical dust envelope with parameters that are very similar to those of CW Leo ( IRC $\!$+10 216 ), the best studied carbon star. For the newly estimated pulsation period $P = 617 \pm 6$ days and distance $D = 740 \pm 100$ pc, our model of LP And changes its luminosity $L_{\star}$ between 16 200 and 2900 $L_{\odot}$, its effective temperature $T_{\star}$ between 3550 and 2100 K, and its radius $R_{\star}$ between 340 and 410 $R_{\odot}$. The model estimates the star's mass-loss rate $\dot{M} \approx 1.9 \times 10^{-5}$ $M_{\odot}$ yr- 1, assuming a constant outflow velocity v = 14 km s-1. If the latter also applied to the innermost parts of the dusty envelope, then presently the star would be losing mass at a rate $\dot{M} \approx 6.0 \times 10^{-5}$ $M_{\odot}$ yr-1. However, we believe that the inner wind velocity must actually be closer to $v \approx 4$ km s-1 instead, as wind acceleration is expected in the dust-formation zone. The dusty envelope of LP And extends from $R_1 \approx 2$ $R_{\star}$ to distances of $R_2 \approx 3$ pc from the star. The total mass of the envelope lost by the central star is M = 3.2 $M_{\odot}$ assuming a dust-to-gas mass ratio of $\rho_{\rm d}/\rho$= 0.0039. The circumstellar optical depth towards the star is $\tau_{V} = 25$ in the visual. The dust model contains small silicon carbide grains, inhomogeneous grains made of a mixture of SiC and incompletely amorphous carbon, and thin mantles made of iron-magnesium sulfides. This dust mixture perfectly fits the infrared continuum and both the 11.3 ${\mu}$m and 27 ${\mu}$m emission bands. We find that our $K^{\prime}$-band visibility could not be fitted by our spherical model, so we discuss possible reasons for this interesting result. More observations are required in order to determine what causes this effect. If slight deviations from spherical geometry in its envelope are the reason, then the object's evolutionary stage would be even more similar to that of CW Leo . It appears that LP And is a highly-evolved intermediate-mass star (initial mass $M^0_{\star}$ $\approx$ 4 $M_{\odot}$) at the end of its AGB phase.

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5) Riechers, D., Balega, Y., Driebe, T., Hofmann, K.-H., Men'shchikov, A. B., and Weigelt, G.

A quasi-time-dependent radiative transfer model of OH 104.9+2.4

Astronomy and Astrophysics, 436, 925-931 (2005)

Abstract.

We investigate the pulsation-phase dependent properties of the circumstellar dust shell (CDS) of the OH/IR star OH 104.9+2.4 based on radiative transfer modeling (RTM) using the code DUSTY. Our previous study concerning simultaneous modeling of the spectral energy distribution (SED) and near-infrared (NIR) visibilities (Riechers et al. 2004) has been extended by means of a more detailed analysis of the pulsation phase dependence of the model parameters of OH 104.9+2.4. In order to investigate the temporal variation of the spatial structure of the CDS, additional NIR speckle interferometric observations in the K' band were carried out with the 6 m telescope of the Special Astrophysical Observatory (SAO). At a wavelength of  λ = 2.12 m the diffraction-limited resolution of 74 mas was attained. As regards our previous best-fitting model, several key parameters had to be adjusted in order to be consistent with the extended amount of observational data. It was found that a simple rescaling of the bolometric flux Fbol is not suffcient to take into account the variability of the source, as the change in optical depth τ over a full pulsation cycle is rather high. On the other hand, the impact of a change in effective temperature Teff on SED and visibility is rather small. However, observations as well as models for other AGB stars show the necessity to include a variation of Teff with pulsation phase in the radiative transfer models. Therefore, our new best-fitting model accounts for these changes.
Key words. radiative transfer -- stars: AGB and post AGB -- stars: mass loss -- stars: circumstellar matter -- infrared: stars -- stars: oscillations -- stars: individual: OH 104.9+2.4

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6) Riechers, D., Balega, Y., Driebe, T., Hofmann, K.-H., Men'shchikov, A. B., Shenavrin, V.I., and Weigelt, G.

High-Resolution Near-Infrared Speckle Interferometry and Radiative Transfer Modeling of the OH/IR star OH 104.9+2.4

Astronomy and Astrophysics, 424, 165-177 (2004)

Abstract.
We present near-infrared speckle interferometry of the OH/IR star OH 104.9+2.4 in the K' band obtained with the 6m telescope of the Special Astrophysical Observatory (SAO). At a wavelength of lambda = 2.12 micron the diffraction-limited resolution of 74 mas was attained. The reconstructed visibility reveals a spherically symmetric, circumstellar dust shell (CDS) surrounding the central star. The visibility function shows that the stellar contribution to the total flux at lambda = 2.12 micron is less than ~50%, indicating a rather large optical depth of the CDS. The azimuthally averaged 1-dimensional Gaussian visibility fit yields a diameter of 47 +/- 3mas (FHWM), which corresponds to 112 +/- 13 AU for an adopted distance of D = 2.38 +/- 0.24 kpc. To determine the structure and the properties of the CDS of OH 104.9+2.4, radiative transfer calculations using the code DUSTY were performed to simultaneously model its visibility and the spectral energy distribution (SED). We found that both the ISO spectrum and the visibility of OH 104.9+2.4 can be well reproduced by a radiative transfer model with an effective temperature T_eff = 2500 +/- 500 K of the central source, a dust temperature T_in = 1000 +/- 200 K at the inner shell boundary R_in = 9.1 R_star = 25.4 AU, an optical depth tau = 6.5 +/- 0.3 at 2.2 micron, and dust grain radii ranging from a_min = 0.005 +/- 0.003 micron to a_max = 0.2 +/- 0.02 micron with a power law with index -3.5. It was found that even minor changes in a_max have a major impact on both the slope and the curvature of the visibility function, while the SED shows only minor changes. Our detailed analysis demonstrates the potential of dust shell modeling constrained by both the SED and visibilities.

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