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

Abstract

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 C III] 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 13C1/23P0o- 1/21S0 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 13C1/23P0o- 1/21S0 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 12C3P0o 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 C III] 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. © 1997 RAS.