2002ApJ...572..326B

Standard stellar evolution theory is inconsistent with the observed isotopic carbon ratio, ¹²C/¹³C, in evolved stars. This theory is also inconsistent with the ³He/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 ¹²C/¹³C and ³He/H abundance ratios for stars with masses ≤2 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, ¹²C/¹³C, in 11 planetary nebulae (PNe) by observing the J=2⟶1 and J=3⟶2 millimeter transitions of ¹²CO and ¹³CO 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 ¹²CO and ¹³CO in nine PNe. If ¹²CO/¹³CO=¹²C/¹³C, the range of ¹²C/¹³C is 2.2-31. Our results support theories that include some form of extra mixing.