PROBING THE COOL OUTER ENVELOPE OF NGC 6826 AND ITS PREVIOUS MASS-LOSS HISTORY

We made a direct, quantitative comparison between theoretical and observed density profiles of the planetary nebula NGC 6826. For this, we observed the optically thin (CO)-C-13 (J=1-0) and (CO)-C-13(J=2-1) rotational transition lines at a projected radial distance from the central star of 60 " and 75 ". The line strengths and ratios observed at the inner point, and the upper limits observed at the outer point, are consistent with density profiles predicted by mass-loss histories computed from our evolution models when non-LTE radiative transfer and the conditions of collisional excitation in the envelope are taken into account.