Temporal Variations in the Circumstellar Shell IRC+10216

Abstract

ABSTRACTThe dynamical evolution of the inner sections of the circumstellar shell (CS) surrounding the prototypical evolved carbon star IRC+10216 is studied through the analysis and modeling of high resolution (<0.02 cm-1) infrared 12CO and 13CO vibrational-rotational fundamental (4.6 microns) and overtone (2.3 microns) band absorption line profiles. The overtone band lines in particular sample a zone within 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 past 12 years indicates evolution of this region of the CS. 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. The dust density distribution and the gas velocity fields are adjusted in the model, yielding a time sequence of synthetic lines which implies evolution of the CS structure. Short-term (phase dependent) and long-term (years) systematic changes in line shapes are successfully reproduced by this method, yielding further insight on the physical conditions governing this region of the circumstellar shell. The dust condensation point in the model roughly coincides with the location of the largest gas velocity gradient, supporting the notion that the dust formation and gas mass loss mechanisms are associated. Radiation pressure, acting on co-spatial momentum-coupled dust particles, accelerates the ambient gas component by 7.3 ± 1.8 x 10-4 cm s-2 (0.23 ± 0.06 km s-1 yr-1) throughout the probed region of the CS. Evolution of the observed absorption line components suggests that the principal mass loss mechanism in cool evolved stars is not related to the pulsation period of the central star, but may be stochastic in nature. Broadband photometric observational data indicates that an episodic disturbance that modifies the line profiles between 1979 and 1991 may have 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.