4 Summary

We presented new near-infrared (JHK) bispectrum speckle-interferometry monitoring of the carbon star IRC +10216 obtained during 1999-2001 with the SAO 6 m telescope. The J-, H-, and K-band resolutions are 50 mas, 56 mas, and 73 mas, respectively. Together with the data of Paper II, the available K-band observations cover now 8 epochs from 1995 to 2001 and show the dynamic evolution of the inner dust shell. Our images show very good agreement with the images (1997-1999) reported by Tuthill et al. (2000). Four main components within a $0\hbox{$.\!\!^{\prime\prime}$ }2$ radius can be identified, which are surrounded by a fainter asymmetric nebula. The apparent separation of the two initially brightest components A and B increased from $\sim$191 mas in 1995 to $\sim$351 mas in 2001. At the same time, component B has been fading and almost disappeared in 2000 whereas the initially faint components C and D became brighter. There is weak evidence for an accelerated apparent motion of component B with respect to A. This might be related to the beginning development of a fast polar wind or, as favored by the radiative transfer calculations, to rapid dust evaporation due to backwarming effects. The changes of the images can be related to changes of the optical depth caused, for example, by mass-loss variations or new dust condensation in the wind. The observed relative motion of components A and B with a deprojected velocity of 19 km s^-1 is most likely due to dust evaporation in the optically thicker and hotter environment.

The present monitoring covers more than 3 pulsation periods and 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. We have demonstrated, that formation of new dust along the line of sight towards the star can explain the observed fading time scale of component B, for reasonable values of the parameters involved in our simple gas box model. In particular, this dust formation calculation lends independent support to the previous finding that the present-day mass loss rate of IRC +10216 should be of the order of $10^{-4}~M_{\odot}$/yr. Further high-resolution observations will be most important for testing different views and models, and for better understanding of the evolution of this complicated nebula.

Acknowledgements

The observations were made with the SAO 6 m telescope, operated by the Special Astrophysical Observatory, Russia. We thank Boris Yudin for providing near-infrared photometric data of IRC +10216. This research has made use of the SIMBAD database, operated by CDS in Strasbourg, and of NASA's Astrophysics Data System.