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

Arnold I. Boothroyd (CITA), and I.-Juliana Sackmann (Caltech)

Astrophysical Journal, 510, 232-250   (1999)

[Download below] [Data below]

Abstract: It is demonstrated 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.) Our models assume that extra mixing always reaches to the same distance in temperature from the H-burning shell, and that CBP begins when the H-burning shell erases the molecular weight discontinuity ("mu-barrier") established by first dredge-up. For Pop I stars, none of the other CNO isotopes except N-15 are expected to be altered by CBP. (If O-18 depletion occurs on the AGB, as some observations suggest, it would require that extra mixing reach closer to the H-burning shell on the AGB than on the RGB --- and should also result in a much lower C-12/C-13 ratio than is observed.) CBP increases as one reduces the stellar mass or metallicity --- roughly as 1/M^2 on the RGB, due to the longer RGB of low mass stars, and as 1/Z, due to higher H-shell burning temperatures at low metallicity. In low mass Pop II stars, all the CNO isotopes are expected to be altered by CBP. Field Pop II stars exhibit RGB abundances consistent with the predictions of our CBP models that have been normalized to reproduce the Pop I RGB abundances, but globular cluster stars are observed to encounter much more extensive processing; also, CBP is observed to start near the base of the globular cluster RGB (overcoming any "mu-barrier"). Standard first and second dredge-up are also presented, and enrichment of the interstellar medium, relative to SN.

See also our companion light-elements paper.

Download paper:

• PDF (553 kb: 36 pages, including figs); or
• gzip'd PostScript (309 kb, expands to 1.16 Mb: 36 pages, including figs); or
• gzip'd tar-file archive (234 kb, expands to 0.89 Mb), including LaTeX text, style files aaspp4.sty and epsf.sty, and 13 postscript figures;
• also available from the LANL preprint archives, as astro-ph/9512121

NOTE: The OPAL opacities make a slight difference to the depth of dredge-up, but less of an effect than the difference between the results of different investigators. "Second dredge-up" results for low mass cases ignore any effects of prior cool bottom processing on the RGB; also, below about 1.5 solar masses, second dredge-up becomes strongly dependent on the assumed mass loss rate.

Download data:

See also README.dr12 for a description of:

• the 63 dredge-up files in the gzip'd tar-file dr12.tar.gz (159 kb; expands to 0.8 Mb), which give the first and second dredge-up compositions as a function of stellar mass and metallicity (as discussed in the above two papers) for both OPAL and LAOL interior opacities
• the 12 dredge-up files in the gzip'd tar-file dr12opal.tar.gz (32 kb; expands to 160 kb) comprise only the subset with OPAL interior opacities.

The corresponding results for RGB cool bottom processing with our "evolving RGB" models are in the plain text files:

• cbpcnodrnnia.dat (9 kb) for OPAL interior opacities,
• cbpcnoallz.dat (9 kb), for LAOL interior opacities.
• See also he3cbpfac.dat (9 kb) for a table of the He-3 destruction factors.

Note that these files contain the corrected He3 abundances (and slightly improved C13 abundances), as compared to the versions prior to 29 May 1996.