Multiple shells in IRC+10216: shell properties

We report on the properties of the multiple shells in the circumstellar env elope of IRC+10216, using deep optical imaging, including data from the Hub ble Space Telescope. The intensity profiles confirm the presence of thin (s imilar to 0." 5-3 "), limb-brightened shells in the envelope, seen in stell ar and ambient Galactic light scattered by dust. The shells are spaced at i rregular intervals of similar to 5 "-20 ", corresponding to time scales of 200-800 yr, although intervals as short as similar to 1 " (40 yr) are seen close to the star. The location of the main shells shows a good correlation with high-resolution, molecular line maps of the inner envelope, indicatin g that the dust and gas are well coupled. The shell/intershell density cont rast is typically similar to 3, and we find that the shells form the domina nt mass component of the circumstellar envelope. The shells exhibit importa nt evolutionary effects: the thickness increases with increasing radius, wi th an effective dispersion velocity of 0.7 km s(-1) and there is evidence f or shell interactions. Despite the presence of bipolar structure close to t he star, the global shell pattern favors a roughly isotropic, episodic mass loss mechanism, with a range of time scales.