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On the Evolution of Stars That Form Electron-degenerate Cores Processed by Carbon Burning. II. Isotope Abundances and Thermal Pulses in a 10 Msun Model with an ONe Core and Applications to Long-Period Variables, Classical Novae, and Accretion-induced Collapse
Ritossa, Claudio; Garcia-Berro, Enrique; Iben, Icko, Jr.
Astrophysical Journal v.460, p.489 (ApJ Homepage)
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A 10 Msun model of Population I composition is evolved from the hydrogen-burning main sequence to the thermally pulsing `super' asymptotic giant branch (TPSAGB) stage, where it has an oxygen-neon (ONe) core of mass 1.196 Msun and is experiencing thermal pulses driven by helium-burning thermonuclear flashes. Interior abundance characteristics are relevant to an understanding of the core collapse of massive acereting white dwarfs in close binary star systems. At mass point 0.2 Msun., abundances by mass are X(16O) = 0.656, X(20Ne) = 0.215, X(23Na) = 0.0467, X(24Mg) = 0.0325, X(25Mg) = 0.0115, X(12 C)= 0.0112, X(22Ne) = 0.00893, X(21Ne) = 0.00689, X(26Mg) = 0.00560, and X(27Al) = 0.00528. Abundances near the surface of the core are relevant to an understanding of nova outbursts in cataclysmic variables. At mass point 1.17 Msun, abundances by mass are X(16O) = 0.511, X(20Ne) = 0.313, X(23Na) = 0.0644, X(24Mg) = 0.0548, X(25Mg) = 0.0158, X(27 Al)= 0.0108, X(12C) = 0.00916, X(26Mg) = 0.00989, X(21Ne) = 0.00598, and X(22Ne) = 0.00431.
Carbon burning is quenched at the beginning of the thermally pulsing phase, and a layer of CO matter of mass 0.015 Msun separates the ONe core from overlying helium- and hydrogen-rich layers. The outer 0.01 M of the CO layer contains essentially no neon: very little new 20Ne has been made, and most of the 22Ne which has been made from the original CNO elements has been converted into 25Mg and neutrons which have been captured to form neutron-rich isotopes. If the observational counterpart of the model is in a close binary and fills its Roche lobe near the end of the carbon-burning phase, and if the binary evolves into a cataclysmic variable, one expects that the ejecta of approximately 1000 nova outbursts will exhibit an under- abundance of neon and overabundance of carbon, oxygen, and magnesium.

During the TPSAGB phase, characteristics of a pulse cycle approach local limit-cycle values after ˜30 pulses. Helium-shell flashes are of about the same strength (LmaxHe ˜3 × 106 Lsun, LminHe ˜ 100 Lsun) as in AGB models with CO cores of mass ˜1 Msun, but the time between flashes (˜200 yr) and the mass of helium fuel built up between flashes (˜1.3 x 10-4 Msun) are much smaller. The amount of energy released in a flash is not enough to propel matter at the hydrogen-helium discontinuity far enough outward that associated cooling extinguishes hydrogen burning LminH ˜ 6 × 104 Lsun, (LmaxH ˜ 6 × 104 Lsun). The temperature at the base of the convective shell forced by helium burning attains a maximum of TmaxCSB ˜ 360 × 106 K. Depending on the choice of cross section for the 22Ne(alpha, n)25Mg reaction, 50%-80% the 22Ne initially in the convective shell is converted into 25Mg, providing 20-30 neutrons for every 56Fe seed nucleus. The neutron density (˜6 x 1012 cm-3) is presumably much larger than is appropriate for producing s-process isotopes in the solar system distribution at critical branch points. During pulse powerdown, at least 7% and perhaps as much as 30% of the matter which has been in the convective shell is dredged up into the convective envelope. Thus, an observational counterpart of the model may exhibit an enhancement of heavy s-process isotopes in a nonsolar distribution and Mg isotopes in a distinctly nonsolar distribution, but because of the large mass of the convective envelope, these anomalies may not be detectable in a typical TPSAGB star. The abundance of Li relative to H in a model may be much larger or much smaller than Li/H ˜ 10-10, depending on the treatment of convection and on where the model is in the TPSAGB phase. At the beginning of the TPSAGB phase, the surface abundances by number of CNO elements are in the ratio (C:N:0) = (2.4:4.3:6.3), compared with the initial ratios (C:N:0) = (3.6:1.0:8.0).

During the TPSAGB phase, the ratio of C to N decreases, and the ratio of 12C to 13C decreases from ˜25 to ˜4. A test of these predictions involves abundance estimates of the brightest long-period variables in the Galaxy and in the Magellanic Clouds. Perhaps the major signature of a TPSAGB star is a brightness greater than the "classical limit" of Mbol = -7.1. Betelgeuse in our Galaxy and four stars in the Magellanic Clouds are brighter than the supposed limit, but they exhibit abundance characteristics which can be accounted for in the framework of TPSAGB theory. Assuming that a superwind removes mass from the surface at a rate of ˜10-4 Msun yr-1, the final mass of the ONe white dwarf formed by our TPSAGB model is ˜1.26 Msun, the outer 0.06 Msun of which is composed primarily of carbon and oxygen.

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