Bispectrum speckle interferometry of IRC+10216 : The dynamic evolution of the innermost circumstellar environment from 1995 to 2001

We present new near-infrared (JHK) bispectrum speckle- interferometry monitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with the SAO 6 m telescope. The J-, H-, and K-band resolutions are 50 mas, 56 mas, and 73 mas, respectively. 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 similar to191 mas in 1995 to similar to351 mas 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). The changes of the images can be related to changes of the optical depth caused, for instance, by mass-loss variations or new dust condensation in the wind. Our recent two-dimensional radiative transfer model of IRC +10216 suggests that the observed relative motion of components A and B is not consistent with the outflow of gas and dust at the well-known terminal wind velocity of 15 km s(-1). The apparent motion with a deprojected velocity of 19 km s(-1) on average and of recently 27 km s(-1) appears to be caused by a displacement of the dust density peak due to dust evaporation in the optically thicker and hotter environment. The present 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