The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge‐up

It is demonstrated for the first time that deep circulation mixing below the base of the standard convective envelope, and the consequent "cool bottom processing" (CBP) of the CNO isotopes, can reproduce the trend with stellar mass of the C-12/C-13 observations in low mass red giants. (This trend is opposite to what is expected from standard first dredge-up.) We investigated where on the red giant branch (RGB) or asymptotic giant branch (AGB) CBP is expected. CBP increases dramatically as one reduces the stellar mass or metallicity (roughly as 1/M^2 on the RGB, and 1/Z). For low mass stars (< 2.3 Msun for solar metallicity), CBP is expected both on the upper RGB and on the entire AGB; much more CBP is expected in Population II stars than in Population I stars. For intermediate mass stars (2.3 - 4 Msun) CBP is only expected on the AGB; the effect is probably minor for Population I stars, but may be significant for Population II stars. Population I stars of > 4 Msun do not encounter significant CBP anywhere on the RGB or AGB, although Population II stars of > 4 Msun may encounter some CBP on the early AGB. Note that, on the thermally pulsing AGB, stars of M > 4 Msun encounter hot bottom burning instead of CBP. Standard first and second dredge-up yields are also presented, and consequent enrichment of the interstellar medium compared to the supernova contribution.