Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.284..348c&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 284, Issue 2, pp. 348-358.
Astronomy and Astrophysics
Astronomy
16
Stars: Abundances, Stars: Agb And Post-Agb, Planetary Nebulae: General, Planetary Nebulae: Individual: Ngc 3918, Planetary Nebulae: Individual: Smc N2, Planetary Nebulae: Individual: Lmc N122.
Scientific paper
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 1908A has an F=1/2-1/2 transition at 1909.6A that is completely forbidden in ^12C, but allowed as a result of the non-zero nuclear spin of ^13C. The transition probability for the ^13C^3_1/2P^o_0-^1_1/2S_0 transition has been calculated in a multi-configuration basis and found to be 6.87x10^-4s^-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^3_1/2P^o_0-^1_1/2S_0 transition in NGC 3918 is found to be displaced by 7.3+/-1.6kms^-1 from that determined from the experimental energies of the ^12C^3P^o_0 and ^1S_0 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[ ^3P^o-^1S velocity components over a range of 230kms^-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.
Clegg Robin E. S.
Neale Liesl
Storey Peter J.
Walsh Jonathan R.
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