Cool Bottom Processes on the Thermally Pulsing Asymptotic Giant Branch and the Isotopic Composition of Circumstellar Dust Grains

By K. Nollett, M. Busso, G. Wasserburg. 2002

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We examine the effects of cool bottom processing (CBP) on the isotopic ratios 18O/16O, 17O/16O, 14N/15N, 26Al/27Al, 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 (TP) 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 TP < 7.73. Using structures from several times on the TP-AGB, it was found that the results for all species except 26Al were essentially independent of the time chosen if log TP > 7.6. Abundant 26Al was produced by CBP for log TP > 7.65. While 26Al/27Al depends on TP, 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 18O/16O, 17O/16O, and 26Al/27Al observed for oxide grains formed at C/O < 1 are reasonably well understood. However, the 15N/14N, 12C/13C, and 26Al/27Al in carbide grains (C/O > 1) require that many of their stellar sources must have had 14N/15N 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 12C/13C < 10 cannot be produced by any red giant or AGB source, nor are they reconcilable with novae sources.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/148e7680ddab5c8d8916694efedc20c38c61395e

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Citations in the corpus

Short-Lived Nuclei in the Early Solar System: A Low Mass Stellar Source? (2003, unknown): “The solution shown in Table 4 would require 26Al/27Al ∼2 × 10−2 in the envelope, which could be readily reached by cool bottom processing (Nollett et al. 2003).”
STELLAR YIELDS FROM METAL-RICH ASYMPTOTIC GIANT BRANCH MODELS (2016, background): “Extra mixing also on the AGB has been invoked to explain the composition of roughly 10% of meteoritic stardust oxide grains (the Group 2 grains), which show depletions in 18O (Nollett et al. 2003; Palmerini et al. 2011).”
Aluminum-, Calcium- and Titanium-rich Oxide Stardust in Ordinary Chondrite Meteorites (2008, background): “We will discuss the composition of the 18O-rich grain KH2, which has the highest inferred 41Ca/40Ca ratio, further below in the context of a supernova origin.”
Characterization of Presolar Silicate and Oxide Grains in Primitive Carbonaceous Chondrites (2007, background): “…processing (CBP), extra mixing that occurs when material from the envelope is cycled to regions hot enough for H burning (Wasserburg et al. 1995; Nollett et al. 2003), or by hot bottom burning (HBB)when the convective envelope extends into the H-burning shell (Boothroyd et al. 1995; Cannon et…”
Stellar Models and Yields of Asymptotic Giant Branch Stars (2007, background): “Observations of AGB stars and pre-solar grain analysis indicate that a similar type of extra-mixing occurs in low-mass AGB stars (Boothroyd et al. 1995; Abia & Isern 1997; Busso et al. 1999; Nollett et al. 2003).”
NUGRID STELLAR DATA SET. I. STELLAR YIELDS FROM H TO BI FOR STARS WITH METALLICITIES Z = 0.02 and Z = 0.01 (2013, background): “Li may also be produced as a result of extra-mixing (the so-called cool bottom process) in AGB (and RGB) stars with lower initial masses (Sackmann & Boothroyd 1999; Nollett et al. 2003; Denissenkov & Merryfield 2011; Palmerini et al. 2011).”
Astrophysics with Extraterrestrial Materials (2016, background): “…26Al inferred in these grains from excesses of the decay product 26Mg, asserts that the grains originated in stars that underwent an additional mixing process, cool-bottom processing, that is not predicted by standard AGB evolutionary models (Wasserburg et al. 1995, Nollett et al. 2003).”
The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, methodology): “…calculations within a similar paradigm are provided by Boothroyd et al. (1994), Boothroyd et al. (1995), Wasserburg et al. (1995), Langer & Hoffman (1995), Sackmann & Boothroyd (1999), Nollett et al. (2003), Denissenkov & Tout (2000), Denissenkov et al. (2006), and Palmerini et al. (2009).”
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “A possibility to improve the C and N predictions is to hypothesise the occurrence of the Cool Bottom Processing (CBP, see Section 5.3), (Nollett et al. 2003; Busso et al. 2010).”
Astrophysics with Presolar Stardust (2004, background): “Moreover, O isotopes observed in some stars and, especially, in presolar oxide grains (see Section 5.2 below) indicate that this mixing occurs in AGB stars as well (Nollett et al. 2003, Wasserburg et al. 1995).”
Reaction Rate Uncertainties and the Production of 19F in Asymptotic Giant Branch Stars (2004, result): “This process occurs during the first red giant phase in stars with M 2:5 M (see, e.g., Charbonnel 1995), as well as possibly during the AGB phase (Nollett et al. 2003), and results in a lower 12C/13C ratio than the standard models, as required by the observations.”
Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars (2013, background): “To reproduce the nitrogen enhancement some extra mixing processes are required to burn part of the carbon in the CN cycle and modify the amount of nitrogen brought to the surface of the AGB star (see e.g. Nollett et al. 2003).”