Unusual C, N, O Isotopic Ratios in Planetary Nebulae

Rare isotopes such as ¹³C, ¹⁵N, and ¹⁷O have astrophysical significance, as their abundances with respect to the main isotope of each species (¹²C, ¹⁴N, and ¹⁶O, respectively) serve as effective tracers of stellar nucleosynthesis. While the Solar System values for these ratios are quite high (¹²C/¹³C ~ 89, ¹⁴N/¹⁵N ~ 435, ¹⁶O/¹⁷O ~ 2,632), observations of a plethora of chemical species and their ¹³C, ¹⁵N, and ¹⁷O-bearing variants in the planetary nebula (PN) K4-47 have yielded astonishingly high enrichment of the rare isotopes in this source, with ¹²C/¹³C = 2.0 ± 0.8; ¹⁴N/¹⁵N = 13.3 ± 6.5, and ¹⁶O/¹⁷O= 21.4 ± 10.3. Moreover, anomalously low ¹²C/¹³C ratios have now been detected in eight other PNe, including NGC 7293 (the Helix Nebula), M3-55, M3-28, M2-9 (the Butterfly Nebula), NGC 2440, and NGC 6720 (the Ring Nebula). The ¹²C/¹³C ratios for all eight sources were found to range between 1.0 ± 0.7 to 13.2 ± 4.9, with an average value of 3.7. These ratios are significantly lower than those seen in asymptotic giant branch (AGB) stars, which typically exhibit ¹²C/¹³C ~ 25–90. Together with the extreme bipolar/multipolar morphologies exhibited by all of these nebulae, the high enrichment of ¹³C suggests that an explosive process involving proton capture took place at some point in the transition between the AGB and PN phases. Details of the observations of these isotopically-substituted species and their analysis will be presented, along with the implications for our understanding of stellar nucleosynthesis in the late AGB stage.