CO Isotopes in Planetary Nebulae

Abstract

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 HII 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 12C/13C and 3He/H abundance ratios for stars with masses < 2 solar masses. 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, 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 PNe where both transitions have been detected. We detect both 12CO and 13CO in 9 PNe. If 12CO/13CO = 12C/13C, the range of 12C/13C is 2.2--31. Our results support theories which include some form of extra mixing.