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

摘要

We present new near-infrared (JHK) bispectrum speckle-interferometrymonitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with theSAO 6m telescope. The J-, H-, and K-band resolutions are 50mas, 56mas, and73mas, resp. The total sequence of K-band observations covers now 8 epochs from1995 to 2001 and shows the dynamic evolution of the inner dust shell. Thepresent observations show that the appearance of the dust shell hasconsiderably changed compared to the epochs of 1995 to 1998. Four maincomponents within a 0.2" radius can be identified in the K-band images. Theapparent separation of the two initially brightest components A and B increasedfrom ~191mas in 1995 to ~351mas in 2001. Simultaneously, component B has beenfading and almost disappeared in 2000 whereas the initially faint components Cand D became brighter (relative to peak intensity). These changes can berelated to changes of the optical depth caused, e.g., by mass-loss variationsor new dust condensation in the wind. Our 2D radiative transfer model suggeststhat the observed relative motion of A and B is not consistent with the knownterminal wind velocity of 15 km/s. The apparent motion with a deprojectedvelocity of 19 km/s on average and of recently 27 km/s appears to be caused byadisplacement of the dust density peak due to dust evaporation in the opticallythicker and hotter environment. Our monitoring, covering more than 3 pulsationperiods, shows that the structural variations are not related to the stellarpulsation cycle in a simple way. This is consistent with the predictions ofhydrodynamical models that enhanced dust formation takes place on a timescaleof several pulsation periods. The timescale of the fading of component B canwell be explained by the formation of new dust in the circumstellar envelope.