Abstract
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.
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