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Register a new userThe Effects of the pep Nuclear Reaction and Other Improvements in the Nuclear Reaction Rate Library on Simulations of the Classical Nova Outburst
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We have continued our studies of the Classical Nova outburst by evolving TNRs on 1.25Msun and 1.35Msun WDs (ONeMg composition) under conditions which produce mass ejection and a rapid increase in the emitted light, by examining the effects of changes in the nuclear reaction rates on both the observable features and the nucleosynthesis during the outburst. In order to improve our calculations over previous work, we have incorporated a modern nuclear reaction network into our hydrodynamic computer code. We find that the updates in the nuclear reaction rate libraries change the amount of ejected mass, peak luminosity, and the resulting nucleosynthesis. In addition, as a result of our improvements, we discovered that the pep reaction was not included in our previous studies of CN explosions. Although the energy production from this reaction is not important in the Sun, the densities in WD envelopes can exceed $10^4$ gm cm$^{-3}$ and the presence of this reaction increases the energy generation during the time that the p-p chain is operating. The effect of the increased energy generation is to reduce the evolution time to the peak of the TNR and, thereby, the accreted mass as compared to the evolutionary sequences done without this reaction included. As expected from our previous work, the reduction in accreted mass has important consequences on the characteristics of the resulting TNR and is discussed in this paper.
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The nucleosynthesis and other observable consequences of a nova outburst depend sensitively on the details of the thermonuclear runaway which initiates the outburst. One important source of uncertainty in our current models is the nuclear reaction data used as input for the evolutionary calculations. We present preliminary results of the first analyses of the impact on nova nucleosynthesis of all reaction rate uncertainties considered simultaneously.
Nuclear reaction rates are quantities of fundamental importance in astrophysics. Substantial efforts have been devoted in the last decades to measure or calculate them. The present paper presents for the first time a detailed description of the Brussels nuclear reaction rate library BRUSLIB and of the nuclear network generator NETGEN so as to make these nuclear data packages easily accessible to astrophysicists for a large variety of applications. BRUSLIB is made of two parts. The first one contains the 1999 NACRE compilation based on experimental data for 86 reactions with (mainly) stable targets up to Si. The second part of BRUSLIB concerns nuclear reaction rate predictions calculated within a statistical Hauser-Feshbach approximation, which limits the reliability of the rates to reactions producing compound nuclei with a high enough level density. These calculations make use of global and coherent microscopic nuclear models for the quantities entering the rate calculations. The use of such models is utterly important, and makes the BRUSLIB rate library unique. A description of the Nuclear Network Generator NETGEN that complements the BRUSLIB package is also presented. NETGEN is a tool to generate nuclear reaction rates for temperature grids specified by the user. The information it provides can be used for a large variety of applications, including Big Bang nucleosynthesis, the energy generation and nucleosynthesis associated with the non-explosive and explosive hydrogen to silicon burning stages, or the synthesis of the heavy nuclides through the s-, alpha- and r-, rp- or p-processes.
Classical novae participate in the cycle of Galactic chemical evolution in which grains and metal enriched gas in their ejecta, supplementing those of supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for the ISM. Once in the diffuse gas, this material is mixed with the existing gases and then incorporated into young stars and planetary systems during star formation. Infrared observations have confirmed the presence of carbon, SiC, hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that some fraction of the pre-solar grains identified in meteoritic material come from novae. The mean mass returned by a nova outburst to the ISM probably exceeds ~2 x 10^{-4} Solar Masses. Using the observed nova rate of 35 per year in our Galaxy, it follows that novae introduce more than ~7 x 10^{-3} Solar Masses per year of processed matter into the ISM. Novae are expected to be the major source of 15N and 17O in the Galaxy and to contribute to the abundances of other isotopes in this atomic mass range. Here, we report on how changes in the nuclear reaction rates affect the properties of the outburst and alter the predictions of the contributions of novae to Galactic chemical evolution. We also discuss the necessity of including the pep reaction in studies of thermonuclear runaways in material accreted onto white dwarfs.
Nuclear reaction rate ($lambda$) is a significant factor in the process of nucleosynthesis. A multi-layer directed-weighted nuclear reaction network in which the reaction rate as the weight, and neutron, proton, $^4$He and the remainder nuclei as the criterion for different reaction-layers is for the first time built based on all thermonuclear reactions in the JINA REACLIB database. Our results show that with the increase of the stellar temperature ($T_{9}$), the distribution of nuclear reaction rates on the $R$-layer network demonstrates a transition from unimodal to bimodal distributions. Nuclei on the $R$-layer in the region of $lambda = [1,2.5times10^{1}]$ have a more complicated out-going degree distribution than the one in the region of $lambda = [10^{11},10^{13}]$, and the number of involved nuclei at $T_{9} = 1$ is very different from the one at $T_{9} = 3$. The redundant nuclei in the region of $lambda = [1, 2.5times10^{1}]$ at $T_{9} = 3$ prefer $(gamma,p)$ and $({gamma,alpha})$ reactions to the ones at $T_{9}=1$, which produce nuclei around the $beta$ stable line. This work offers a novel way to the big-data analysis on nuclear reaction network at stellar temperatures.
We explore properties of core-collapse supernova progenitors with respect to the composite uncertainties in the thermonuclear reaction rates by coupling the reaction rate probability density functions provided by the STARLIB reaction rate library with $texttt{MESA}$ stellar models. We evolve 1000 15 $M_{odot}$ models from the pre main-sequence to core O-depletion at solar and subsolar metallicities for a total of 2000 Monte Carlo stellar models. For each stellar model, we independently and simultaneously sample 665 thermonuclear reaction rates and use them in a $texttt{MESA}$ in situ reaction network that follows 127 isotopes from $^{1}$H to $^{64}$Zn. With this framework we survey the core mass, burning lifetime, composition, and structural properties at five different evolutionary epochs. At each epoch we measure the probability distribution function of the variations of each property and calculate Spearman Rank-Order Correlation coefficients for each sampled reaction rate to identify which reaction rate has the largest impact on the variations on each property. We find that uncertainties in $^{14}$N$(p,gamma)^{15}$O, triple-$alpha$, $^{12}$C$(alpha,gamma)^{16}$O, $^{12}$C($^{12}$C,$p$)$^{23}$Na, $^{12}$C($^{16}$O,$p$)$^{27}$Al, $^{16}$O($^{16}$O,$n$)$^{31}$S, $^{16}$O($^{16}$O,$p$)$^{31}$P, and $^{16}$O($^{16}$O,$alpha$)$^{28}$Si reaction rates dominate the variations of the properties surveyed. We find that variations induced by uncertainties in nuclear reaction rates grow with each passing phase of evolution, and at core H-, He-depletion are of comparable magnitude to the variations induced by choices of mass resolution and network resolution. However, at core C-, Ne-, and O-depletion, the reaction rate uncertainties can dominate the variation causing uncertainty in various properties of the stellar model in the evolution towards iron core-collapse.
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