2002ApJ...568L..57P

We present high-resolution Hubble Space Telescope ultraviolet spectroscopy of NGC 3242, the only planetary nebula with a measured abundance of ³He. The Space Telescope Imaging Spectrograph has been used to observe the C III multiplet near 1908 Å. The presence of ¹²C and ¹³C lines at these wavelengths allows a direct estimate of the carbon isotopic ratio in the ionized gas of the nebula. We have detected the ¹²C doublet and obtained an upper limit on the ¹³C 1909.6 Å line, resulting in a carbon isotopic ratio of ¹²C/¹³C>38, in agreement with standard stellar models. The lack of the ¹³C line and the presence of the ³He line in the spectrum indicate that the progenitor star did not undergo a phase of deep mixing during the last stages of its evolution. The significance of this result for studies of stellar nucleosynthesis and Galactic chemical evolution is discussed.