Implications of Cool Bottom Processes in Thermally Pulsing Phases of AGB Stars (original) (raw)
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The new s-process in low mass TP-AGB stars
Nuclear Physics A, 1997
The s-process nucleosynthesis occurring in the Asymptotic Giant Branch phases has been computed through a post-process calculation, following the results of stellar evolutionary models obtained with the FRANEC code for low mass stars (1-3 M®) with metallicity Z = 0.02. Mass loss with the Reimers (1975)'s parameterization was taken into account (r/= 3). Since the development of the third dredge up, we assumed that a laC-pocket is formed at the H/He discontinuity when the H-shell is inactive. Neutrons are released by the 13C(a,n)160 source in radiative conditions in the interpulse period in a thin layer of a few 10 .4 M®. The 22Ne(a,n)2SMg reaction operates marginally in convective pulses, over a material that was previously s-processed in the radiative phase and mixed with fresh Fe and H-burning ashes. The conditions under which the solar main s-process component is reproduced are discussed.
The Astrophysical Journal, 2012
We present a new measurement of the α-spectroscopic factor (S α) and the asymptotic normalization coefficient (ANC) for the 6.356 MeV 1/2 + subthreshold state of 17 O through the 13 C(11 B, 7 Li) 17 O transfer reaction and we determine the α-width of this state. This is believed to have a strong effect on the rate of the 13 C(α, n) 16 O reaction, the main neutron source for slow neutron captures (the s-process) in asymptotic giant branch (AGB) stars. Based on the new width we derive the astrophysical S-factor and the stellar rate of the 13 C(α, n) 16 O reaction. At a temperature of 100 MK our rate is roughly two times larger than that by Caughlan & Fowler (1988) and two times smaller than that recommended by the NACRE compilation. We use the new rate and different rates available in the literature as input in simulations of AGB stars to study their influence on the abundances of selected s-process elements and isotopic ratios. There are no changes in the final results using the different rates for the 13 C(α, n) 16 O reaction when the 13 C burns completely in radiative conditions. When the 13 C burns in convective conditions, as in stars of initial mass lower than ∼2 M ⊙ and in post-AGB stars, some changes are to be expected, e.g., of up to 25% for Pb in our models. These variations will have to be carefully analyzed when more accurate stellar mixing models and more precise observational constraints are available.
Type Ia supernovae and the 12 C+ 12 C reaction rate
Astronomy & Astrophysics, 2011
Context. Even if the 12 C+ 12 C reaction plays a central role in the ignition of Type Ia supernovae (SNIa), the experimental determination of its cross-section at astrophysically relevant energies (E 2 MeV) has never been made. The profusion of resonances throughout the measured energy range has led to speculation that there is an unknown resonance at E 0 ∼ 1.5 MeV possibly as strong as the one measured for the resonance at 2.14 MeV, i.e. (ωγ) R = 0.13 meV. Aims. We study the implications that such a resonance would have for our knowledge of the physics of SNIa, paying special attention to the phases that go from the crossing of the ignition curve to the dynamical event.
s-Process nucleosynthesis in AGB stars: A Test for stellar evolution
We study the slow neutron capture process (s process) in Asymptotic Giant Branch (AGB) stars using three different stellar evolutionary models computed for a 3 M ⊙ and solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13 C(α,n) 16 O reaction that occurs in radiative conditions. A tiny region rich in 13 C (the 13 C pocket) is created by proton captures on the abundant 12 C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local protons over 12 C number ratio, p/ 12 C ∼ > 0.3, most of the 13 C nuclei produced are further converted by proton capture to 14 N. Besides, 14 N nuclei represent a major neutron poison. We find
Highlights of Astronomy
The evidence of an exponential distribution of neutron exposures for reproducing the solar-system s-isotopes between Zr and Pb, the main component, comes from a phenomenological analysis of the s-path in the σN versus A plot (see Käppeler, Beer and Wisshak, 1989). The resulting mean neutron exposure is τ0 = 0.30 ± 0.01 mb−1. The study of branchings with no or weak temperature dependence (95Zr; 147Nd, 147,148Pm; 185W, 186Re) allows one to derive an effective neutron density nn =(3.4±1.0)×108cm−3. On the other hand, an effective temperature is obtained from the branchings 134’135Cs, 151Sm, 154Eu and 176Lu: T8 = 3.4 ± 0.5 (Käppeler et al., 1990). Although the phenomenological approach is very useful because it is independent of stellar models, nonetheless it can only lead to effective physical conditions, symplifying the complexity of the astrophysical sites.
The s-process nucleosynthesis and the mass of carbon stars
Energy exchange inside SN ejecta and light curves of SNe Ia E.I. Sorokina, S.I. Blinnikov The 12 C(α, γ) 16 O Reaction Rate and the Rise Time of Type Ia Supernovae I. Domínguez, P. Höflich, O. Straniero Two key reactions in stellar nucleosynthesis : 12 C(α, γ) 16 O and 22 Ne(α, n) 25 Mg
2008
Type Ia supernovae (SNe Ia), the thermonuclear explosion of a white dwarf, were once considered standard candles. However, increased observations reveal inhomogeneities in chemical composition and luminosity behavior, roughly dividing SNe Ia into three luminosity classes; super-luminous, sub-luminous, and normally-luminous. After introducing the problem in the context of previous observations and modeling, this thesis explores the physical processes occurring in a SN Ia after explosion, and discusses observations of SN light curves. A simple model of the expanding ejecta calculates the energy deposition from the decay of radioactive 56 Ni as well as photon diffusion. It produces light curves that match early bolometric observations of normal SNe Ia. Variable chemical composition of the ejecta allows for testing a number of explosion scenarios. It becomes apparent that the shape of the light curve is sensitive to the amount and location of synthesized 56 Ni. Monitoring gamma ray transport through Compton scattering indicates that gamma rays escape at late times. At this epoch an assumption of instantaneous deposition of energy is inaccurate. It is unclear whether positrons escape the ejecta or are trapped at even later times. The photometry of SN2007ax proved it to be the dimmest and reddest SN Ia observed. SN2008D was serendipitously observed in X-rays before it was even visible in optical light, revealing that an early x-ray outburst may accompany every core collapse SN. Subsequent observations resulted in a well-sampled, multi-band early light curve. Observations of SN2006D, another SN Ia, in B, V, R, I up to ∼ 500 days after maximum light are also presented. The light curve may answer questions about the physics of SNe at late times, if more observations can be included. Future modifications of the simple model and strategies for useful observations are discussed.
s ‐Process Nucleosynthesis in Carbon Stars
The Astrophysical Journal, 2002
We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass selements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are derived using the spectral synthesis technique from high-resolution spectra. The N-stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study , and also smaller than those found in supergiant post-AGB stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M→MS→S→SC→C during the AGB phase. We compare the observational data with recent s-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low mass AGB star models (M 3M ⊙ ) and s-elements observations. In low mass AGB stars, the 13 C(α, n) 16 O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13 C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colours and the theoretical relations between stellar mass, metallicity and the final C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this work are intrinsic, thermally-pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.
Super and Massive AGB Stars: II- Nucleosynthesis and
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We have computed detailed evolution and nucleosynthesis models for super and massive AGB stars over the mass range 6.5-9.0 M ⊙ in divisions of 0.5 M ⊙ with metallicities Z=0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low and intermediate-mass AGB star models and high mass star models that become supernovae. For the considered metallicities, the composition of the yields is largely dominated by the thermodynamic conditions at the base of the convective envelope rather than by the pollution arising from third dredge up. We investigate the effects of various uncertainties, related to the mass-loss rate, mixing length parameter, and the treatment of evolution after the envelope instability that develops near the end of the (Super)AGB phase. Varying these parameters alter the yields mainly because of their impact on the amount of third dredge up enrichment, and to a lesser extent on the hot bottom burning conditions. Our models produce significant amounts of 4 He, 7 Li (depending on the mass-loss formulation) 13 C, 14 N, 17 O, 23 Na, 25 Mg, as well the radioactive isotope 26 Al in agreement with previous investigation. In addition our results show enrichment of 22 Ne, 26 Mg and 60 Fe, as well as a substantial increase in our proxy neutron capture species representing all species heavier than iron. These stars may provide important contributions to the Galaxy's inventory of the heavier Mg isotopes, 14 N, 7 Li and 27 Al.
Nucleosynthesis in massive stars and the 12C(α, γ)16O reaction rate
Physics Reports, 1993
The evolution of a grid of massive stars ranging from 12 1040 M 0 has been followed through all stages of nuclear burning up to the point ofiron core collapse. The critical and highly uncertain rate for the reaction 12C(~, ?)16O has been varied over a range from 0.5 to 3.0 times that given by Caughlan and Fowler and two different prescriptions for semiconvection have been explored. The nucleosynthesis resulting from integrating the yields of these models over plausible initial stellar mass distributions is found to be in excellent agreement with the observed solar abundances ofvirtually all the intermediate mass isotopes (16 A < 32) if, and only if, the rate of the '2C(a, y)iSO reaction is taken to be 1.7 ±0.5 times that given by Caughlan and Fowler. This range is a small subset ofwhat is allowed by current experimental measurements and can be taken as a nucleosynthetic "prediction" of the value that this rate needs to have in order to prevent 5-to 100-fold deviations from the observed relative abundances of key isotopes. These results are insensitive to the assumed slope of the initial stellar mass distribution within observational limits, and relatively insensitive to the theory of semiconveclion (except for the apparent excessive production of i8 0 when semiconvective mixing is suppressed). Three of the stars have been followed through simulated explosions to obtain the explosive modifications to their nucleosynthesis (including the "neutrino process"), which for most isotopes is relatively small. Isotopic yields of both stable and radioactive products are tabulated as are the calculated iron core masses of the presupernova stars.