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F. -K. Thielemann

Publications and source records attributed to F. -K. Thielemann.

At least 91 records · Page 5Linked to original sources
arXiv2000

Nucleosynthesis Calculations for the Ejecta of Neutron Star Coalescences

We present the results of fully dynamical r-process network calculations for the ejecta of neutron star mergers (NSMs). The late stages of the inspiral and the final violent coalescence of a neutron star binary have been calculated in detail using a 3D hydrodynamics code (Newtonian gravity plus backreaction forces emerging from the emission of gravitational waves) and a realistic nuclear equation of state. The found trajectories for the ejecta serve as input for dynamical r-process calculations where all relevant nuclear reactions (including beta-decays depositing nuclear energy in the expanding material) are followed. We find that all the ejected material undergoes r-process. For an initial Ye close to 0.1 the abundance distributions reproduce very accurately the solar r-process pattern for nuclei with A above 130. For lighter nuclei strongly underabundant (as compared to solar) distributions are encountered. We show that this behaviour is consistent with the latest observations of very old, metal-poor stars, despite simplistic arguments that have recently been raised against the possibility of NSM as possible sources of Galactic r-process material.

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arXiv2000

X-Ray Bursts and Proton Captures Close to the Dripline

The hydrogen-rich accreted envelopes of neutron stars in binary systems are the site for the rapid proton capture nucleosynthethic process (rp-process), which involves nuclei close to the proton dripline. An overview of the relevant reactions and nuclear properties for rp-process studies is given, along with motivations for further experimental nuclear physics studies.

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arXiv2000

Prediction of Astrophysical Reaction Rates: Methods, Data Needs, and Consequences for Nucleosynthesis Studies

The majority of nuclear reactions in astrophysics involve unstable nuclei which are not fully accessible by experiments yet. Therefore, there is high demand for reliable predictions of cross sections and reaction rates by theoretical means. The majority of reactions can be treated in the framework of the statistical model (Hauser-Feshbach). The global parametrizations of the nuclear properties needed for predictions far off stability probe our understanding of the strong force and take it to its limit. The sensitivity of astrophysical scenarios to nuclear inputs is illustrated in the framework of a detailed nucleosynthesis study in type II supernovae. Abundances resulting from calculations in the same explosion model with two different sets of reaction rates are compared. Key reactions and required nuclear information are identified.

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arXiv2000

Merging neutron stars: asymmetric systems

Results of 3D, Newtonian hydrodynamic calculations of the last stages of the inspiral and the final coalescence of neutron star binary systems are described. The focus is on sligthly asymmetric systems with the asymmetry stemming either from different spins or different masses of the binary components. Implications for nucleosynthesis and gamma ray bursts are discussed. Test calculations for the used viscosity scheme are provided in the appendix.

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arXiv2000

Capture of alpha Particles by Isospin-Symmetric Nuclei

The reaction rates for alpha capture processes on self-conjugate nuclei in the mass range A=20-40 have been investigated. The rates were calculated using the statistical model code NON-SMOKER taking into account isospin suppression rules. These theoretical predictions are compared with rates derived from the available experimental data about the alpha capture reactions taking also into account additional experimental information from different reaction channels populating the alpha unbound states of the self-conjugate compound nuclei.

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arXiv2000

Astrophysical Reaction Rates From Statistical Model Calculations

Theoretical reaction rates in the temperature range 0.01*10^9<=T[K]<=10.*10^9 are calculated in the statistical model (Hauser-Feshbach formalism) for targets with 10<=Z<=83 (Ne to Bi) and for a mass range reaching the neutron and proton driplines. Reactions considered are (n,gamma), (n,p), (n,alpha), (p,gamma), (p,alpha),(alpha,gamma), and their inverse reactions. Reaction rates as a function of temperature for thermally populated targets are given by analytic seven parameter fits. To facilitate comparison with experimental rates, the stellar enhancement factors are also tabulated. Two complete sets of rates have been calculated, one of which includes a phenomenological treatment of shell quenching for neutron-rich nuclei. These extensive datasets are provided on-line as electronic files, while a selected subset from one calculation is given as printed tables. A summary of the theoretical inputs and advice on the use of the provided tabulations is included.

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arXiv2000

Nucleosynthesis in Chandrasekhar Mass Models for Type Ia Supernovae and Constraints on Progenitor Systems and Burning-Front Propagation

We calculate explosive nucleosynthesis in Chandrasekhar mass models for Type Ia Supernovae(SNe Ia) to obtain new constraints on the rate of matter accretion onto the progenitor white dwarf and on the ignition density of central carbon deflagration. The calculated abundance of the Fe-group neutron-rich nuclei is highly sensitive to the electron captures taking place in the central layers. The yields obtained from a slow central deflagration, and from a fast deflagration or delayed detonation in the outer layers, are combined and put to comparison with solar isotopic abundances. We found that (1) to avoid too large ratios of $^{54}$Cr/$^{56}$Fe and $^{50}$Ti/$^{56}$Fe, the ignition density should be as low as \ltsim 2 \e9 \gmc, and that (2) to avoid the overproduction of $^{58}$Ni and $^{54}$Fe, either the flame speed should not exceed a few % of the sound speed in the central low $Y_e$ layers, or the progenitor star has to be metal-poor compared with solar. Such low central densities can be realized by a rapid accretion as fast as $\dot M$ \gtsim 1 $\times$ 10$^{-7}$M$_\odot$ yr$^{-1}$. In order to reproduce the solar abundance of $^{48}$Ca, one also needs progenitor systems that undergo ignition at higher densities. We also found that the total amount of $^{56}$Ni, the Si-Ca/Fe ratio, and the abundance of elements like Mn and Cr (incomplete Si-burning ashes), depend on the density of the deflagration-detonation transition in delayed detonations. Our nucleosynthesis results favor transition densities slightly below 2.2$\times 10^7$ g cm$^{-3}$.

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arXiv2000

Metal-poor halo stars as tracers of ISM mixing processes during halo formation

We introduce a stochastic halo formation model to compute the early chemical enrichment of the interstellar medium (ISM) of the halo. Contrary to 1-zone chemical evolution models, we are able to resolve local inhomogeneities in the ISM caused by single core-collapse supernovae. These inhomogeneities lead to different element abundance patterns in very metal-poor stars, which can be seen as scatter in the abundances of halo stars with metallicities [Fe/H] < 2.0. The early chemical evolution of the halo proceeds in different enrichment phases: At [Fe/H] < -3.0, the halo ISM is unmixed and dominated by local inhomogeneities caused by single core-collapse supernova (SN) events. For metallicities [Fe/H] >-2.0 the halo ISM is well mixed, showing an element abundance pattern integrated over the initial mass function. In the range -3.0 < [Fe/H] < -2.0 a continuous transition from the unmixed to the well mixed ISM occurs. For some elements (Si, Ca, Eu), the scatter in the element-to-iron ratio [El/Fe] seen in the observations of metal-poor halo stars can be reproduced. Stellar yields of other elements predict a scatter which, compared to the observations, is too large (O, Mg) or too small (Ni). Cr and Mn show a decreasing trend for lower metallicities, which can not be explained by metallicity independent yields, provided that the mixing of the ejecta with the interstellar medium does not depend on progenitor mass. This demonstrates the need for revised, self-consistent core-collapse SN yields.

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arXiv1999

Reaction rates for Neutron Capture Reactions to C-, N- and O-isotopes to the neutron rich side of stability

The reaction rates of neutron capture reactions on light nuclei are important for reliably simulating nucleosynthesis in a variety of stellar scenarios. Neutron capture reaction rates on neutron-rich C-, N-, and O-isotopes are calculated in the framework of a hybrid compound and direct capture model. The results are tabulated and compared with the results of previous calculations as well as with experimental results.

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arXiv1999

Nuclear Structure Studies at ISOLDE and their Impact on the Astrophysical r-Process

The focus of the present review is the production of the heaviest elements in nature via the r-process. A correct understanding and modeling requires the knowledge of nuclear properties far from stability and a detailed prescription of the astrophysical environment. Experiments at CERN/ISOLDE have played a pioneering role in exploring the characteristics of nuclear structure in terms of masses and beta-decay properties. Initial examinations paid attention to far unstable nuclei with magic neutron numbers related to r-process peaks, while present activities are centered on the evolution of shell effects with the distance from the valley of stability. We first show in site-independent applications the effect of both types of nuclear properties on r-process abundances. Then, we explore the results of calculations related to two different `realistic' astrophysical sites, (i) the supernova neutrino wind and (ii) neutron star mergers. We close with a list of remaining theoretical and experimental challenges needed to overcome for a full understanding of the nature of the r-process, and the role CERN/ISOLDE can play in this process.

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arXiv1999

Computational Methods for Nucleosynthesis and Nuclear Energy Generation

This review concentrates on the two principle methods used to evolve nuclear abundances within astrophysical simulations, evolution via rate equations and via equilibria. Because in general the rate equations in nucleosynthetic applications form an extraordinarily stiff system, implicit methods have proven mandatory, leading to the need to solve moderately sized matrix equations. Efforts to improve the performance of such rate equation methods are focused on efficient solution of these matrix equations, by making best use of the sparseness of these matrices. Recent work to produce hybrid schemes which use local equilibria to reduce the computational cost of the rate equations is also discussed. Such schemes offer significant improvements in the speed of reaction networks and are accurate under circumstances where calculations with complete equilibrium fail.

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arXiv1999

R-Process Abundances and Chronometers in Metal-Poor Stars

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (QRPA beta-decay rates and the ETFSI-Q nuclear mass model), have been made. These calculations are compared with ground-based and HST observations of neutron-capture elements in the metal poor halo stars CS 22892--052, HD 115444, HD 122563 and HD 126238. The elemental abundances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z >/= 56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Galaxy. This further suggests that there is one r-process site in the Galaxy, at least for elements Z >/= 56. Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, predictions for the zero decay-age abundances of the radioactive elements Th and U are made. We compare these (least-squares-fit ETFSI-Q) predictions with newly derived observational values in three very metal-poor halo stars: HD 115444, CS 22892--052 and HD 122563. Within the observational errors the observed ratio of [Th/Eu] is the same in both CS 22892--052 and HD 115444. Comparing with the theoretical ratio suggests an average age of these two very metal poor stars to be approximately 15.6 +/- 4.6 Gyr, consistent with earlier radioactive age estimates and recent globular and cosmological age estimates.

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arXiv1999

The Reaction Rate Sensitivity of Nucleosynthesis in Type II Supernovae

We explore the sensitivity of the nucleosynthesis of intermediate mass elements (28 < A < 80) in supernovae derived from massive stars to the nuclear reaction rates employed in the model. Two standard sources of reaction rate data (Woosley et al. 1978; and Thielemann et al. 1987) are employed in pairs of calculations that are otherwise identical. Both include as a common backbone the experimental reactions rates of Caughlan & Fowler (1988). Two stellar models are calculated for each of two main sequence masses: 15 and 25 solar masses. Each star is evolved from core hydrogen burning to a presupernova state carrying an appropriately large reaction network, then exploded using a piston near the edge of the iron core as described by Woosley & Weaver (1995). The final stellar yields from the models calculated with the two rate sets are compared and found to differ in most cases by less than a factor of two over the entire range of nuclei studied. Reasons for the major discrepancies are discussed in detail along with the physics underlying the two reaction rate sets employed. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed.

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arXiv1998

Mass ejection in neutron star mergers

We present the results of 3D Newtonian SPH simulations of the merger of a neutron star binary. The microscopic properties of matter are described by the physical equation of state of Lattimer and Swesty (LS-EOS). To test for the robustness of our results we check the sensitivity to the approximations of our model as well as to the binary system parameters. The main and new result is that for the realistic LS-EOS, depending on the initial spin, between 4e-3 and 4e-2 solar masses of material become unbound. If, as suggested, large parts of this matter consist of r-process nuclei, neutron star mergers could account for the whole observed r-process material in the Galaxy.

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arXiv1998

Coalescing Neutron Stars: A Solution to the R-Process Problem?

Recent calculations place questions on the ability of supernovae to produce r-process nuclei in the correct amounts. We present results from 3D Newtonian SPH calculations of mergers of equal mass neutron star binaries. We find the amounts of ejected mass to be very promising for an explanation of the observed r-process abundances. Preliminary abundance calculations show that practically all the material is subject to r-process conditions. The calculated abundance patterns reproduce the basic features of the solar r-process abundances very well. Especially the peak around A= 195 is easily reproduced without any tuning of the initial entropies. Thus, eventually all the observed r-process material could be explained by mass ejection during neutron star mergers.

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arXiv1998

The QSE-reduced αNetwork

Examination of the process of silicon burning, the burning stage that leads to the production of the iron peak nuclei, reveals that the nuclear evolution is dominated by large groups of nuclei in mutual equilibrium. These quasi-equilibrium (QSE) groups form well in advance of the global Nuclear Statistical Equilibrium (NSE). We present an improved ``minimal'' nuclear network, which takes advantage of quasi-equilibrium in order to calculate the nuclear evolution and energy generation while further reducing the computational cost compared to a conventional α-chain network. During silicon burning, the resultant \emph{QSE-reduced} αnetwork is twice as fast as the full αnetwork it replaces and requires the tracking of only half as many abundance variables, without significant loss of accuracy. When the QSE-reduced αnetwork is used in combination with a conventional αnetwork stretching from He to Si, the combined \alpha7 network provides an accurate approximation for all of the burning stages from He burning to NSE, while tracking only 7 abundances. These reductions in computational cost and the number of species evolved make the \alpha7 network well suited for inclusion within hydrodynamic simulations, particularly those in multi-dimension.

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arXiv1998

Global statistical model calculations and the role of isospin

An improved code for the calculation of astrophysical reaction rates in the statistical model is presented. It includes the possibility to study isospin effects. Such effects heavily affect capture rates involving self-conjugate nuclei and may also be found in reactions on other intermediate and heavy targets.

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