CO Isotopes in Planetary Nebulae

Standard stellar evolution theory is inconsistent with the observed isotopic carbon ratio, C/C, in evolved stars. This theory is also inconsistent with the He/H abundance ratios observed in Galactic H II regions, when combined with chemical evolution theory. These discrepancies have been attributed to an extra, non-standard mixing which further processes material during the RGB and should lower both the C/C and He/H abundance ratios for stars with masses ≤ 2 M . Measurements of isotopic ratios in planetary nebulae probe material which escapes the star to be further processed by future generations of stars. We have measured the carbon isotopic abundance ratio, C/C, in 11 planetary nebulae (PNe) by observing the J=2→1 and J=3→2 millimeter transitions of CO and CO in molecular clouds associated with the PNe. A large velocity gradient (LVG) model has been used to determine the physical conditions for each PNe where both transitions have been detected. We detect both CO and CO in 9 PNe. If CO/CO = C/C, the range of C/C is 2.2–31. Our results support theories which include some form of extra mixing. Subject headings: ISM: abundances — ISM: molecules — planetary nebulae: general — radio lines: ISM