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Register a new userThe Effects of Changes in Reaction Rates on Simulations of Nova Explosions
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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.
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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.
A review of the present status of nova modeling is made, with a special emphasis on some specific aspects. What are the main nucleosynthetic products of the explosion and how do they depend on the white dwarf properties (e.g. mass, chemical composition: CO or ONe)? Whats the imprint of nova nucleosynthesis on meteoritic presolar grains? How can gamma rays, if observed with present or future instruments onboard satellites, constrain nova models through their nucleosynthesis? What have we learned about the turnoff of classical novae from observation with past and present X-ray observatories? And last but not least, what are the most critical issues concerning nova modeling (e.g. ejected masses, mixing mechanism between core and envelope)?
Classical novae are thermonuclear explosions that take place in the envelopes of accreting white dwarfs in binary systems. The material piles up under degenerate conditions, driving a thermonuclear runaway. The energy released by the suite of nuclear processes operating at the envelope heats the material up to peak temperatures about 100 - 400 MK. During these events, about 10-3 - 10-7 Msun, enriched in CNO and, sometimes, other intermediate-mass elements (e.g., Ne, Na, Mg, Al) are ejected into the interstellar medium. To account for the gross observational properties of classical novae (in particular, the large concentrations of metals spectroscopically inferred in the ejecta), models require mixing between the (solar-like) material transferred from the secondary and the outermost layers (CO- or ONe-rich) of the underlying white dwarf. Recent multidimensional simulations have demonstrated that Kelvin-Helmholtz instabilities can naturally produce self-enrichment of the accreted envelope with material from the underlying white dwarf at levels that agree with observations. However, the feasibility of this mechanism has been explored in the framework of CO white dwarfs, while mixing with different substrates still needs to be properly addressed. Three-dimensional simulations of mixing at the core-envelope interface during nova outbursts have been performed with the multidimensional code FLASH, for two types of substrates: CO- and ONe-rich. We show that the presence of an ONe-rich substrate, as in neon novae, yields larger metallicity enhancements in the ejecta, compared to CO,rich substrates (i.e., non-neon novae). A number of requirements and constraints for such 3-D simulations (e.g., minimum resolution, size of the computational domain) are also outlined.
Deuterium represents the only bound isotope in the universe with atomic mass number $A=2$. Motivated by the possibility of other universes, where the strong force could be stronger, this paper considers the effects of bound diprotons and dineutrons on stars. We find that the existence of additional stable nuclei with $A=2$ has relatively modest effects on the universe. Previous work indicates that Big Bang Nucleosynthesis (BBN) produces more deuterium, but does not lead to catastrophic heavy element production. This paper revisits BBN considerations and confirms that the universe is left with an ample supply of hydrogen and other light nuclei for typical cosmological parameters. Using the $MESA$ numerical package, we carry out stellar evolution calculations for universes with stable diprotons, with nuclear cross sections enhanced by large factors $X$. This work focuses on $X=10^{15}-10^{18}$, but explores the wider range $X$ = $10^{-3}-10^{18}$. For a given stellar mass, the presence of stable diprotons leads to somewhat brighter stars, with the radii and photospheric temperatures roughly comparable to thoese of red giants. The central temperature decreases from the characteristic value of $T_capprox1.5times10^7$ K for hydrogen burning down to the value of $T_capprox10^6$ K characteristic of deuterium burning. The stellar lifetimes are smaller for a given mass, but with the extended possible mass range, the smallest stars live for trillions of years, far longer than the current cosmic age. Finally, the enhanced cross sections allow for small, partially degenerate objects with mass $M_ast=1-10M_J$ to produce significant steady-state luminosity and thereby function as stars.
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.
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