Stellar Nucleosynthesis

Nearly sixty years after BBFH [1], it is very important to re-investigate neutron capture processes. Since 1957 everybody has spoken about s-process and r-process, but how are these processes realized in fact? Do s-path and r-path exist, or these are only two useful approximations?
It is the best to investigate the beginnings. How does the formation of nuclei occur starting from seed nucleus Fe-26 ? It is possible that other nuclei can capture a neutron as well, but because of the high abundance of iron, this process is the most important.
The possibility of experimental investigation is very limited, so it is very useful to investigate it through a model. The computational model is almost the only way to look into the details.
There is a simple full network computer model for neutron capture nucleosynthesis [2,3] which was made strictly following Käppeler [4]. In the early 2000s it was possible to run such a simple network model on a computer. In case of this model it was not necessary to exclude any nuclei from the neutron capture process arbitrarily. It is possible to get the order of formation of nuclei by the model step by step.  Surprisingly the Fe-60 is formed before the Ni-60  and before any other nuclei above  Ni-60 along the s-path.

We model the abundance gradients in the disk of the Milky Way for several chemical elements (O, Mg, Si, S, Ca, Sc, Ti, Co, V, Fe, Ni, Zn, Cu, Mn, Cr, Ba, La and Eu), and compare our results with the most recent and homogeneous observational data. We adopt a chemical evolution model able to well reproduce the main properties of the solar vicinity. We compute, for the first time, the abundance gradients for all the above mentioned elements in the galactocentric distance range 4 - 22 kpc. The comparison with the observed data on Cepheids in the galactocentric distance range 5-17 kpc gives a very good agreement for many of the studied elements. In addition, we fit very well the data for the evolution of Lanthanum in the solar vicinity for which we present results here for the first time. We explore, also for the first time, the behaviour of the abundance gradients at large galactocentric distances by comparing our results with data relative to distant open clusters and red giants and select the best chemical evolution model model on the basis of that. We find a very good fit to the observed abundance gradients, as traced by Cepheids, for most of the elements, thus confirming the validity of the inside-out scenario for the formation of the Milky Way disk as well as the adopted nucleosynthesis prescriptions.

Heavy elements are formed in nucleosynthesis processes. Abundances of these elements can be classified as elemental abundance, isotopic abundance, and abundance of nuclei. In this work the nuclei are identified by (Z,N), which allows reading out new information from the measured abundances. We are interested in the neutron density required to reproduce the measured abundance of nuclei assuming equilibrium processes. This is only possible when two stable nuclei are separated by an unstable nucleus. At these places we investigated the neutron density required for equilibrium nucleosynthesis both isotopically and isotonically at temperatures of AGB interpulse and thermal pulse phases. We obtained an estimate for equilibrium nucleosynthesis neutron density in most of the cases. Next we investigated the possibility of partial formation of nuclei. We analyzed the meaning of the branching factor. We found a mathematical definition for the unified interpretation of a branching point closed at isotonic case and open at isotopic case. We introduce a more expressive variant of branching ratio called partial formation rate. With these we are capable of determining the characteristic neutron density values. We found that all experienced isotope ratios can be obtained both at

System dynamics is a methodology to model and simulate complexity in science and engineering. In this approach there are limited mathematical descriptions due to a large time in the behavior and a non-mathematical definition of the relationships between variables of the phenomena. It proposes causality and feedbacks to explain relationships between components. In spite that there are many examples of complex systems in Astrophysics, the uses of System Dynamics in this area are not documented. However, we used this methodology to develop models of the stellar nucleosynthesis on the main sequence and galactic evolution to exploit the possibility to treat important aspects as complexity, scale factors, unknown elements and known variables in the same context. For nucleosynthesis reactions, feedback loops keep the solar-type stars on the main sequence and production of chemical elements. The galactic evolution model has two feedback loops connected through density of molecular gas, generating the dynamic of stellar formation. Our results for energy emission rate, nuclear luminosity and central and effective temperatures for a solar-type star are similar to those found in the literature. As well as the qualitative behavior of the stellar mass, molecular gas density and intergalactic gas density for the galactic evolution model. Therefore we concluded that System Dynamics is a powerful tool for modelling complex phenomena with feedback in Astrophysics.

Type Ia supernovae (SNe Ia) are manifestations of helium-deficient stars disrupting in a thermonuclear runaway. While explosions of carbon-oxygen white dwarfs are thought to account for the majority of events, part of the observed diversity may be due to varied progenitor channels. We demonstrate that helium stars with masses between ∼1.8 and 2.5 M may evolve into highly degenerate, near-Chandrasekhar mass cores with helium-free envelopes that subsequently ignite carbon and oxygen explosively at densities ∼ (1.8−5.9)×10 9 g cm −3. This happens either due to core growth from shell burning (when the core has a hybrid CO/NeO composition), or following ignition of residual carbon triggered by exothermic electron captures on 24 Mg (for a NeOMg-dominated composition). We argue that the resulting thermonuclear runaways are likely to prevent core collapse, leading to the complete disruption of the star. The available nuclear energy at the onset of explosive oxygen burning suffices to create ejecta with a kinetic energy of ∼10 51 erg, as in typical SNe Ia. Conversely, if these runaways result in failed detonations, the resulting transients would resemble faint SN Iax events similar to SN 2002cx. If helium stars in this mass range indeed explode as SNe Ia, then the frequency of events would be comparable to the observed SN Ib/c rates, thereby sufficing to account for the majority of SNe Ia in star-forming galaxies.

There is a preponderance of evidence, both physical and scientific, that Space and Matter are two Phases of the same thermodynamic System with fluid-like properties within a universe structured in the model of spacetime. This conclusion is drawn by means of both physical evidence and abductive reasoning - a form of logical inference. Abduction is inference arriving at the best explanation. The basis of this theory rests mainly upon Einstein's Special Theory of Relativity and on one aspect of Einstein's General Theory of Relativity - Einstein;s Equivalence Principle.

Gravity is posed to be the radial convergence of compressible spatial flow - spaceflow - upon elemental matter.  Spaceflow is an endothermic process which serves several purposes. 1) It cools the universe and 2) it is responsible for the creation of heavier atoms within stellar nucleosynthesis, and 3)  it stabilizes baryonic matter by sustaining the nucleon-nucleon interaction - the residual strong force.

It is posed that, while gravity is known to be the weakest of all  fundamental interactions, its presence over cosmic spans of time is critical in that it supports the existence and stability of the residual strong force binding nucleons within the nucleus. Gravity is the dynamic energy source (flow) that, over time, sustains the potential energy reservoir (pool) responsible for the strong force.

Gravitational collapse - At such a time - when space ceases to flow into a system, all matter within the system will revert into another state. Supernova and new stars are born in this process; and whole universes die in a similar fashion. When the supply of spaceflow is insufficient to sustain all matter, all matter within the universe will revert to its primordial state of energy. When this occurs  it will do so in a  in another Big Bang event. This quasi-time independent (quantum) process is what suggests that the universe has a finite and cyclic  lifespan.

Heavy elements (beyond iron) are formed in neutron capture nucleosynthesis processes. A simple unified model is proposed to investigate the neutron capture nucleosynthesis in arbitrary neutron density environment. Neutron density required to reproduce the measured abundance of nuclei assuming equilibrium processes is investigated as well. Medium neutron density was found to play a particularly important role in neutron capture nucleosynthesis. Using these findings most of the nuclei can be formed in a medium neutron capture density environment e.g. in certain AGB stars. Besides these observations the proposed model suits educational purposes as well. INTRODUCTION Nearly sixty years after BBFH [1], it is possible and necessary to review and rethink our knowledge about the neutron capture nucleosynthesis. The result of the formation of the nuclei is shown in the various abundances. It is important to mention that the unstable nuclei decayed into stable nuclei and we are only able to observe the abundance of the remaining stable nuclei. "The success of any theory of nucleosynthesis has to be measured by comparison with the abundance patterns observed in nature." – say Käppeler, Beer and Wisshak [2], that is, we need to create such model that gives back the observed abundances.

We present evidence supporting a model in which the cosmological dark matter corresponds to a scalar field, which can be “seen”, however indirectly, in the oscillation of the effective gravitational constant, and manifests itself in the periodicity of the number density distribution of galaxies observed by Broadhurst et al. [T. Broadhurst, R. Ellis, D. Koo and A. Szalay, Nature 343 (1990) 726]. A numerical analysis shows that the model successfully passes all cosmological tests, a fact that lends support to the conclusion that ≈ 98% of the energy density of the Universe is stored in this scalar field. This suggests that, if the observations of the galactic periodicity are not a statistical fluke, the cosmological component of dark matter might already have been observed.

We present the first results from a survey of SDSS quasars selected for strong H i damped Lyα (DLA) absorption with corresponding low equivalent width absorption from strong low-ion transitions (e.g., C ii λ1334 and Si ii λ1260). These metal-poor DLA candidates were selected from the SDSS fifth release quasar spectroscopic database, and comprise a large new sample for probing low-metallicity galaxies. Medium-resolution echellette spectra from the Keck Echellette Spectrograph and Imager spectrograph for an initial sample of 35 systems were obtained to explore the metal-poor tail of the DLA distribution and to investigate the nucleosynthetic patterns at these metallicities. We have estimated saturation corrections for the moderately underresolved spectra, and systems with very narrow Doppler parameters (b 􏰎 5 km s−1) will likely have underestimated abundances. For those systems with Doppler parameters b > 5 km s−1, we have measured low-metallicity DLA gas with [X/H] < −2.4 for at least one of C, O, Si, or Fe. Assuming non-saturated components, we estimate that several DLA systems have [X/H] < −2.8, including five DLA systems with both low equivalent widths and low metallicity in transitions of both C ii and O i. All of the measured DLA metallicities, however, exceed or are consistent with a metallicity of at least 1/1000 of solar, regardless of the effects of saturation in our spectra. Our results indicate that the metal-poor tail of galaxies at z ∼ 3 drops exponentially at [X/H] 􏰏 −3. If the distribution of metallicity is Gaussian, the probability of identifying interstellar medium gas with lower abundance is extremely small, and our results suggest that DLA systems with [X/H] < −4.0 are extremely rare, and could comprise only 8 × 10−7 of DLA systems. The relative abundances of species within these low-metallicity DLA systems are compared with stellar nucleosynthesis models, and are consistent with stars having masses of 30M⊙ < M∗ < 100M⊙. The observed ratio of [C/O] for values of [O/H] < −2.5 exceeds values seen in moderate metallicity DLA systems, and also exceeds theoretical nucleosynthesis predictions for higher mass Population III stars. We also have observed a correlation between the column density N(C iv) with [Si/H] metallicity, suggestive of a trend between mass of the DLA system and its metallicity.

Heavy elements (beyond iron) are formed in neutron capture nucleosynthesis processes. We have proposed a simple unified model to investigate the neutron capture nucleosynthesis in arbitrary neutron density environment. We have also investigated what neutron density is required to reproduce the measured abundance of nuclei assuming equilibrium processes. We found both of these that the medium neutron density has a particularly important role at neutron capture nucleosynthesis. About these results most of the nuclei can formed at medium neutron capture density environment e.g. in some kind of AGB stars. Besides these observations our model is capable to use educational purpose.

We prospose a unified model for the nucleosynthesis of heavy (A > 57) elements in stars. The neutron flux can be set to describe neutron capture in arbitrary neutron flux. Our approach solves the coupled differential equations, that describe the neutron capture and decays of 2696 nuclei, numerically without truncating those to include only either capture or decay as traditionally assumed in weak neutron flux (s process). As a result the synthesis of heavy nuclei always evolves along a wide band in the valley of stable nuclei. The observed abundances in the Solar system are reproduced reasonably already in the simplest version of the model. The model predicts that the nucleosynthesis in weak or modest neutron flux produces elements that are traditionally assumed to result in the high neutron flux of supernovae explosions (r process).

The final stages of the evolution of electron--degenerate ONe cores, resulting from carbon burning in ``heavy weight'' intermediate--mass stars ($8 M_{\sun}\la M \la 11 M_{\sun}$) and growing in mass, either from carbon burning in a shell or from accretion of matter in a close binary system, are examined in the light of their detailed chemical composition. In particular, we have modelled the evolution taking into account the abundances of the following minor nuclear species, which result from the previous evolutionary history: $^{12}$C, $^{23}$Na, $^{24}$Mg, and $^{25}$Mg. Both $^{23}$Na and $^{25}$Mg give rise to Urca processes, which are found to be unimportant for the final outcome of the evolution. $^{24}$Mg was formerly considered a major component of ONe cores (hence called ONeMg cores), but updated evolutionary calculations in this mass range have severely reduced its abundance. Nevertheless, we have parameterized it and we have found that the minimum amount of $^{24}$Mg required to produce NeO burning at moderate densities is $\sim 23%$, a value exceedingly high in the light of recent evolutionary models. Finally, we have determined that models with relatively small abundances of unburnt carbon ($X(^{12}$C)$\sim 0.015$) could be a channel to explosion at low to moderate density ($\sim 1\times 10^9$ g cm$^{-3}$). This is clearly below the current estimate for the explosion/collapse threshold and would have interesting consequences.

We study the finite temperature and density effects on beta decay rates to compute their contributions to nucleosynthesis in the early universe and compact stars. We express nucleosynthesis parameters as a function of temperature and density in different astronomical systems of interest. It is explicitly shown that the chemical potential in the core of supermassive and superdense stars affect beta decay and their helium abundance but the background contributions is still dependent on relative temperature. We calculate this contribution for temperature below the chemical potential. It has been noticed that the acceptable background contribution are obtained for comparatively larger values of T as temperature plays a role of regulating parameter in an extremely dense system.

We have observed the X-ray brightest planetary nebula BD +30° 3639 with the new Japanese astronomical satellite Suzaku. With its superior spectral resolution back-illuminated CCD camera, a blend of K-lines from highly ionized C, N, O elements are successfully resolved into individual species, and an extremely high relative abundance ratio of C/O $\sim$ 40 is obtained. This strongly suggests the direct measurement of the helium shell-burning products.

The study fits the elemental abundances observed in MS and S giants using self-consistent models of nucleosynthesis and dredge-up for the TP-AGB evolutionary phases of low-mass stars. The initial envelope abundances of C-12, C-13, and N-14 are taken from observations of red giants that experienced the first dredge-up and are not yet on the AGB. It is found that the

We calculate collision strengths and their thermally-averaged Maxwellian values for electron excitation and de-excitation between the fifteen lowest levels of singly-ionised cobalt, Co+, which give rise to emission lines in the near- and mid-infrared. Transition probabilities are also calculated and relative line intensities predicted for conditions typical of supernova ejecta. The diagnostic potential of the 10.52, 15.46 and 14.74 micro-metre transition lines is briefly discussed.