MODELING THE LIGHT-TRAVEL-TIME EFFECT ON THE FAR-INFRARED SIZE OF IRC+10216

Models of the far-infrared emission from the large circumstellar dust envelope surrounding the carbon star IRC +10216 are used to assess the importance of the light-travel-time effect (LTTE) on the observed siz e of the source. The central star is a long-period variable with an av erage period of 644 +/- 17 days and a peak-to-peak amplitude of two ma gnitudes, so a large light-travel-time effect is seen at 1' radius. An attempt is made to use the LTTE to reconcile the discrepancy between the observations of Fazio et al. and Lester et al. regarding the far-i nfrared source size. This discrepancy is reviewed in light of recent, high-spatial-resolution observations at 11 mu m by Danchi et al. We co nclude that IRC +10216 has been resolved on the arcminute scale by Faz io et al. Convolution of the model intensity profile at 61 mu m with t he 60'' x 90'' Gaussian beam of Fazio et al. yields an observed source size (FWHM) that ranges from similar to 67'' to 75'' depending on the phase of the star and the assumed distance to the source. Using a sim ple r(-2) dust distribution and the 106 degrees phase of the Fazio et al. observations, the LTTE model reaches a peak size of 74''.13 at a d istance of 300 pc. This agrees favorably with the 78'' x 6'' size meas ured by Fazio et al. Finally, a method is outlined for using the LTTE as a distance indicator to IRC +10216 and other stars with extended ma ss outflows.