Measurement of the 12C/13C ratio in planetary nebulae

The 12C/13C isotopic ratio in the ionized gas in three planetary nebulae has been measured using a new method based on transitions caused by hyperfine effects. The Cm] multiplet near 1908 Å has an F= 1/2–1/2 transition at 1909.6 Å that is completely forbidden in 12C, but allowed as a result of the non-zero nuclear spin of 13C. The transition probability for the 13C31/2 Po0−11/2S0 transition has been calculated in a multi-configuration basis and found to be 6.87 × 10−4 s−1. The Goddard High Resolution Spectrograph aboard the Hubble Space Telescope has been used to observe this transition in three nebulae. The 13C line is detected in two C-rich nebulae: NGC 3918 and SMC N2. The wavelength of the 13C 31/2P−11/2S0 transition in NGC3918 is found to be displaced by 7.3 ±1.6 km s−1 from that determined from the experimental energies of the 12C3Po0 and 1S0 states; this displacement is attributed to an isotopic shift. The 12C/13C abundance ratio is determined to be 15 ± 3 in NGC 3918 and 21 ± 11 in SMC N2. In the Type I nebula LMC N122, which is found to have Cm] 3Po−1S velocity components over a range of 230 km s−1, a tentative detection of 13C is made, indicating a low value of the 12C/13C ratio. The 12C/ 13C ratio in these nebulae is lower than the typical range of values in carbon stars, with the exception of the very 13C-rich stars. The implications of these new 12C/13C determinations for models of AGB evolution and dredge-up are discussed.