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Register a new userBispectrum speckle interferometry of IRC+10216: the dynamic evolution of the innermost circumstellar environment from 1995 to 2001
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G. Weigelt
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We present new near-infrared (JHK) bispectrum speckle-interferometry monitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with the SAO 6m telescope. The J-, H-, and K-band resolutions are 50mas, 56mas, and 73mas, resp. The total sequence of K-band observations covers now 8 epochs from 1995 to 2001 and shows the dynamic evolution of the inner dust shell. The present observations show that the appearance of the dust shell has considerably changed compared to the epochs of 1995 to 1998. Four main components within a 0.2 radius can be identified in the K-band images. The apparent separation of the two initially brightest components A and B increased from ~191mas in 1995 to ~351mas in 2001. Simultaneously, component B has been fading and almost disappeared in 2000 whereas the initially faint components C and D became brighter (relative to peak intensity). These changes can be related to changes of the optical depth caused, e.g., by mass-loss variations or new dust condensation in the wind. Our 2D radiative transfer model suggests that the observed relative motion of A and B is not consistent with the known terminal wind velocity of 15 km/s. The apparent motion with a deprojected velocity of 19 km/s on average and of recently 27 km/s appears to be caused by adisplacement of the dust density peak due to dust evaporation in the optically thicker and hotter environment. Our monitoring, covering more than 3 pulsation periods, shows that the structural variations are not related to the stellar pulsation cycle in a simple way. This is consistent with the predictions of hydrodynamical models that enhanced dust formation takes place on a timescale of several pulsation periods. The timescale of the fading of component B can well be explained by the formation of new dust in the circumstellar envelope.
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Aims.Analysis of the innermost regions of the carbon-rich star IRC+10216 and of the outer layers of its circumstellar envelope have been performed in order to constrain its mass-loss history. Methods: .We analyzed the high dynamic range of near-infrared adaptive optics and the deep V-band images of the circumstellar envelope of IRC+10216 using high angular resolution, collected with the VLT/NACO and FORS1 instruments. Results: .From the near-infrared observations, we present maps of the sub-arcsecond structures, or clumps, in the innermost regions. The morphology of these clumps is found to strongly vary from J- to L-band. Their relative motion appears to be more complex than proposed in earlier works: they can be weakly accelerated, have a constant velocity, or even be motionless with respect to one another. From V-band imaging, we present a high spatial resolution map of the shell distribution in the outer layers of IRC+10216. Shells are resolved well up to a distance of about 90 to the core of the nebula and most of them appear to be composed of thinner elongated shells. Finally, by combining the NACO and FORS1 images, a global view is present to show both the extended layers and the bipolar core of the nebula together with the real size of the inner clumps. Conclusions: .This study confirms the rather complex nature of the IRC+10216 circumstellar environment. In particular, the coexistence at different spatial scales of structures with very different morphologies (clumps, bipolarity, and almost spherical external layers) is very puzzling. This confirms that the formation of AGB winds is far more complex than usually assumed in current models.
The hypergiant IRC+10420 is a unique object for the study of stellar evolution since it is the only object that is believed to be witnessed in its rapid transition from the red supergiant stage to the Wolf-Rayet phase. Its effective temperature has increased by 1000-2000K within only 20yr. We present the first speckle observations of IRC+10420 with 73mas resolution. A diffraction-limited 2.11 micron image was reconstructed from 6m telescope speckle data using the bispectrum speckle-interferometry method. The visibility function shows that the dust shell contributes 40% to the total flux and the unresolved central object 60%. Radiative transfer calculations have been performed to model both the spectral energy distribution and visibility function. The grain sizes, a, were found to be in accordance with a standard distribution function, n(a)~a^(-3.5), with 0.005 micron < a < 0.45 micron. The observed dust shell properties cannot be fitted by single-shell models but seem to require multiple components. At a certain distance we considered an enhancement over the assumed 1/r^x density distribution. The best model for both SED and visibility was found for a dust shell with a dust temperature of 1000K at its inner radius of 69Rstar. At a distance of 308Rstar the density was enhanced by a factor of 40 and and its density exponent was changed from x=2 to x=1.7. The shells intensity distribution was found to be ring-like.The ring diameter is equal to the inner diameter of the hot shell (69mas). The diameter of the central star is 1mas. The two-component model can be interpreted in terms of a termination of an enhanced mass-loss phase roughly 60 to 90 yr (for d=5kpc) ago.
H13CN J=8-7 sub-millimetre line emission produced in the circumstellar envelope around the extreme carbon star IRC+10216 has been imaged at sub-arcsecond angular resolution using the SMA. Supplemented by a detailed excitation analysis the average fractional abundance of H13CN in the inner wind (< 5E15 cm) is estimated to be about 4E-7, translating into a total HCN fractional abundance of 2E-5 using the isotopic ratio 12C/13C=50. Multi-transitional single-dish observations further requires the H13CN fractional abundance to remain more or less constant in the envelope out to a radius of about 4E16 cm, where the HCN molecules are effectively destroyed, most probably, by photodissociation. The large amount of HCN present in the inner wind provides effective line cooling that can dominate over that generated from CO line emission. It is also shown that great care needs to be taken in the radiative transfer modelling where non-local, and non-LTE, effects are important and where the radiation field from thermal dust grains plays a major role in exciting the HCN molecules. The amount of HCN present in the circumstellar envelope around IRC+10216 is consistent with predicted photospheric values based on equilibrium chemical models and indicates that any non-equilibrium chemistry occurring in the extended pulsating atmosphere has no drastic net effect on the fractional abundance of HCN molecules that enters the outer envelope. It further suggests that few HCN molecules are incorporated into dust grains.
We present high-resolution J-, H-, and K-band observations of the carbon star IRC+10216. The images were reconstructed from 6 m telescope speckle interferograms using the bispectrum speckle interferometry method. The H and K images consist of several compact components within a 0.2 radius and a fainter asymmetric nebula. The brightest four components are denoted with A to D in the order of decreasing brightness. A comparison of our images gives - almost like a movie of five frames - insight to the dynamical evolution of the inner nebula. For instance, the separation of the two brightest components A and B increased by almost 40% from 191 mas in 1995 to 265 mas in 1998. At the same time, component B is fading and the components C and D become brighter. The X-shaped bipolar structure of the nebula implies an asymmetric mass-loss suggesting that IRC+10216 is very advanced in its AGB evolution, shortly before turning into a protoplanetary nebula. The cometary shape of component A suggests that the core of A is not the central star, but the southern lobe of a bipolar structure. The position of the central star is probably at or near the position of component B.
New high-resolution far-infrared (FIR) observations of both ortho- and para-NH3 transitions toward IRC+10216 were obtained with Herschel, with the goal of determining the ammonia abundance and constraining the distribution of NH3 in the envelope of IRC+10216. We used the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel to observe all rotational transitions up to the J=3 level (three ortho- and six para-NH3 lines). We conducted non-LTE multilevel radiative transfer modelling, including the effects of near-infrared (NIR) radiative pumping through vibrational transitions. We found that NIR pumping is of key importance for understanding the excitation of rotational levels of NH3. The derived NH3 abundances relative to molecular hydrogen were (2.8+-0.5)x10^{-8} for ortho-NH3 and (3.2^{+0.7}_{-0.6})x10^{-8} for para-NH3, consistent with an ortho/para ratio of 1. These values are in a rough agreement with abundances derived from the inversion transitions, as well as with the total abundance of NH3 inferred from the MIR absorption lines. To explain the observed rotational transitions, ammonia must be formed near to the central star at a radius close to the end of the wind acceleration region, but no larger than about 20 stellar radii (1 sigma confidence level).
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