Temporal Evolution of the Circumstellar Shell IRC+10216

A set of high-resolution 2 microns infrared spectral observations of the prototype obscured carbon star IRC+10216 have been modeled using a co-moving frame-of-reference formalism and monotonic spherically symmetric gas flow. The data set spans more than a decade and shows that the line profile shapes of the CO (0-2) overtone band transitions have changed during the period of observation. Preliminary results indicate that these lines are formed within 15 stellar radii (R_*) of the central star and suggest an evolution of this inner section of the circumstellar shell over a time span of a few years. This supports recent ideas which argue against constant outflow models for mass loss in evolved stars. The presented gas models required to fit the data show distinct phenomena: 1) An abrupt acceleration of the gas flow (Delta v ~ 10 km s(-1) ) is located in approximately the same region where the dust condensation occurs in the model (2.5 to 5 R_*). This observation is consistent with theories which predict that dust grains, accelerated by radiation pressure from the central star, drag the gas molecules outward. 2) A region of almost constant velocity located just past the dust condensation point (5 R_*) slowly accelerates from about 10.5 km s(-1) to the terminal expansion velocity of the shell (14.2 km s(-1) at 12 R_*). The evolution of the line profile shapes during the 12 years spanned by the data require this velocity plateau to expand radially by about 4 R_*. In contrast with the 1.75 year period of pulsation of the central star, these variations in the line profiles suggest a rate of evolution for the inner envelope of the circumstellar shell on the order of decades.