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The goal of this study consisted of reducing the complete KPNO 4m telescope FTS spectral data archive of the CO molecule (2.3 $\mu$m and 4.6 $\mu$m) in a consistent fashion in order to extract distinct line profile shapes with enough signal-to-noise and resolution ($<$ 0.02 $cm^{-1}$) for different epochs which would reflect on changes in the structure of the inner regions (2 to 10 $R_{*}$) of the $IRC+10^{\circ}126$ circumstellar shell (CS). Existing models were applied to generate synthetic spectra, and these models were adjusted to best fit the observational data. The dust and gas components of the circumstellar shell were modeled separately assuming a spherically symmetric expanding shell with constant CO abundance and dust composition. The dust density distribution and the gas velocity fields were adjusted in the model, yielding a time sequence which implies evolution of the CS structure.
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 $\pm$ 0.06 $km/sec/yr$. Evolution of line absorption components suggests that the principal mass loss mechanism in cool evolved starts 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 profiles 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.
