Temporal Variations in the Circumstellar Shell IRC+10216

The dynamical evolution of the inner regions of the circumstellar shell surrounding the prototypical evolved carbon star IRC + 10^circ216 is studied through the analysis and modeling of high-resolution (<0.02 cm^{-1} ) infrared CO vibrational-rotational fundamental (4.6 μm) and overtone (2.3 μm) band absorption line profiles. The overtone band lines sample a region of the circumstellar shell, near the Long-Period Variable (LPV) central star, where the mass ejection and dust formation and acceleration mechanisms dominate (2-10 R_*). Temporal changes in the line profiles observed over the last 12 years indicate the evolution of this region of the circumstellar shell. Modeling of the dust and gas components of the circumstellar shell in the observer's frame of reference is performed for each available spectroscopic data set in an effort to reproduce broadband photometric flux measurements and the observed molecular line profiles. Short-term (phase dependent) and long-term (years) systematic changes in line shapes have been successfully modeled, yielding further insight on the physical conditions governing this region of circumstellar shells. The dust condensation point in the model roughly coincides with the location of the largest gas velocity gradient. Radiation pressure on co-spatial momentum-coupled dust particles accelerates the gas by 0.23 +/- 0.06 km/sec/yr. Evolution of line absorption components suggests that the principal mass loss mechanism in cool evolved stars is stochastic in nature, and not related to the pulsation period of the central star. Broadband photometric data indicates that the episodic disturbance that modifies the line profile originated sometime between 1975 and 1978 and that it progressed outward through the CS. This analysis indicates that broadband IR monitoring of thermal dust emission from carbon stars can be used as an indicator of the onset of episodic mass loss events. Further attempts to derive mass loss from analysis of the H _2 1 - 0 S(1) quadrupole absorption line are hampered by unknown transmission of a photospheric component.