Res. Clegg et al., MEASUREMENT OF THE C-12 C-13 RATIO IN PLANETARY-NEBULAE/, Monthly Notices of the Royal Astronomical Society, 284(2), 1997, pp. 348-358
The C-12/C-13 isotopic ratio in the ionized gas in three planetary neb
ulae 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 tran
sition has been calculated in a multi-configuration basis and found to
be 6.87 x 10(-4) s(-1). The Goddard High Resolution Spectrograph aboa
rd 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: NG
C 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 isotop
ic shift. The C-12/C-13 abundance ratio is determined to be 15 +/- 3 i
n NGC 3918 and 21 +/- 11 in SMC N2. In the Type I nebula LMC N122, whi
ch 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 e
xception 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 d
iscussed.