2002/11/13 by Kenneth M. Nollett, M. Busso, G. J. Wasserburg · 3 citations
Physics and Astronomy · #Astro and Planetary Science #Astrophysics #Astrophysics and Star Formation Studies #Asymptotic giant branch #Convection #Formalism (music) #Optics #Physics #Radiative transfer #Stars #Stellar evolution #Stellar, planetary, and galactic studies #Thermodynamics #astro-ph
paper · pdf · doi:10.1086/344817
published as Astrophys.J. 582 (2003) 1036-1058 · 35 pages, plus 18 included figures. Scheduled for January 10, 2003 issue of ApJ
arxiv created 2002/11/13 · openalex publication_date 2003/01/10 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
We examine the effects of cool bottom processing (CBP) on the isotopic ratios 18 O/ 16 O, 17 O/ 16 O, 14 N/ 15 N, 26 Al/ 27 Al, C/O, and N/O in the convective envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase of evolution in a 1.5 M ☉ initial mass star of solar initial composition. We use a parametric model that treats extra mixing by introducing mass flow between the convective envelope and the underlying radiative zone. The parameters of this model are the mass circulation rate ( ) and the maximum temperature ( T P ) experienced by the circulating material. The effects of nuclear reactions in the flowing matter were calculated using a set of static structures of the radiative zone selected from particular times in a complete stellar evolution calculation. The compositions of the flowing material were obtained, and the resulting changes in the envelope determined. No major shifts in the star's energy budget occur from the imposed CBP if log T P < 7.73. Using structures from several times on the TP-AGB, it was found that the results for all species except 26 Al were essentially independent of the time chosen if log T P > 7.6. Abundant 26 Al was produced by CBP for log T P > 7.65. While 26 Al/ 27 Al depends on T P , the other isotopic ratios depend dominantly on the circulation rate. The relationship is shown between models of CBP as parameterized by a diffusion formalism within the stellar evolution model and those using the mass-flow formalism employed here. They are shown to be effectively equivalent. In general, the CBP treatment readily permits calculation of envelope compositions as affected by different degrees of extra mixing, based on stellar structures computed by normal stellar evolution models. Using these results, the isotopic ratios under conditions of C/O < 1 and C/O > 1 are compared with the data on circumstellar dust grains. It is found that the 18 O/ 16 O, 17 O/ 16 O, and 26 Al/ 27 Al observed for oxide grains formed at C/O < 1 are reasonably well understood. However, the 15 N/ 14 N, 12 C/ 13 C, and 26 Al/ 27 Al in carbide grains (C/O > 1) require that many of their stellar sources must have had 14 N/ 15 N at least a factor of 4 lower than the solar value. This allows a self-consistent description of all these isotopes in most SiC grains. The rare grains with 12 C/ 13 C < 10 cannot be produced by any red giant or AGB source, nor are they reconcilable with novae sources.