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

The C-12/C-13 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 C III] multiplet near 1908 Angstrom has an F = 1/2-1/2 transition at 1909.6 Angstrom, that is completely forbidden in C-12, but allowed as a result of the non-zero nuclear spin of C-13. The transition probability for the (C1/2P0o)-C-13-P-3-S-1(1/2)0 transition has been calculated in a multi-configuration basis and found to be 6.87 x 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 C-13 line is detected in two C-rich nebulae: NGC 3918 and SMC N2. The wavelength of the (C1/2P0o)-C-13-P-3-S-1(1/2)0 transition in NGC 3918 is found to be displaced by 7.3 +/- 1.6 km s(-1) from that determined from the experimental energies of the (CP0o)-C-12-P-3 and S-1(0) states; this displacement is attributed to an isotopic shift. The C-12/C-13 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 C III] P-3(o)-S-1 velocity components over a range of 230 km s(-1), a tentative detection of C-13 is made, indicating a low value of the C-12/C-13 ratio. The C-12/C-13 ratio in these nebulae is lower than the typical range of values in carbon stars, with the exception of the very C-13-rich stars. The implications of these new C-12/C-13 determinations for models of AGE evolution and dredge-up are discussed.