CO isotopes in planetary nebulae

Standard stellar evolution theory is inconsistent with the observed isotopic carbon ratio, C-12/C-13, in evolved stars. This theory is also inconsistent with the He-3/H abundance ratios observed in Galactic H II regions, when combined with chemical evolution theory. These discrepancies have been attributed to an extra, nonstandard mixing, which further processes material during the red giant branch and should lower both the C-12/C-13 and He-3/H abundance ratios for stars with masses less than or equal to2 M-.. Measurements of isotopic ratios in planetary nebulae probe material that escapes the star to be further processed by future generations of stars. We have measured the carbon isotopic abundance ratio, C-12/C-13, in 11 planetary nebulae (PNe) by observing the J = 2 --> 1 and J = 3 --> 2 millimeter transitions of (CO)-C-12 and (CO)-C-13 in molecular clouds associated with the PNe. A large velocity gradient (LVG) model has been used to determine the physical conditions for each PN for which both transitions have been detected. We detect both (CO)-C-12 and (CO)-C-13 in nine PNe. If (CO)-C-12/(CO)-C-13 = C-12/C-13, the range of C-12/C-13 is 2.2-31. Our results support theories that include some form of extra mixing.