CO Isotopes in Planetary Nebulae

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.