Stellar Weak Interaction Rates Research Articles

Re-analysis of temperature dependent neutron capture rates and stellar β-decay rates of 95-98Mo

The neutron capture rates and temperature dependent stellar beta decay rates of Mo isotopes are investigated within the framework of the statistical code TALYS v1.96 and the proton neutron quasi particle random phase approximation (pn-QRPA) model. The Maxwellian average cross-section (MACS) and neutron capture rates for the Mo(n,γ) Mo radiative capture process are analyzed within the framework of the statistical code TALYS v1.96 based on the phenomenological nuclear level density model and gamma strength functions. The present model-based computations for the MACS are comparable to the existing measured data. The sensitivity of stellar weak interaction rates to various densities and temperatures is investigated within the framework of the pn-QRPA model. Particular attention is paid to the impact of thermally filled excited states in the decaying nuclei ( Mo) on electron emission and positron capture rates. Furthermore, we compare the neutron capture rates and stellar beta decay rates. It is found that neutron capture rates are higher than stellar beta decay rates at both lower and higher temperatures.

Stellar weak-interaction rates for rp-process waiting-point nuclei from projected shell model

We propose a projected shell model (PSM) for description of stellar weak-interaction rates between even-even and odd-odd nuclei with extended configuration space where up to six-quasiparticle (qp) configurations are included, and the stellar weak-interaction rates for eight rp-process waiting-point (WP) nuclei, 64Ge, 68Se, 72Kr, 76Sr, 80Zr, 84Mo, 88Ru and 92Pd, are calculated and analyzed for the first time within the model. Higher-order qp configurations are found to affect the underlying Gamow-Teller strength distributions and the corresponding stellar weak-interaction rates. Under rp-process environments with high temperatures and densities, on one hand, thermal population of excited states of parent nuclei tends to decrease the stellar β+ decay rates. On the other hand, the possibility of electron capture (EC) tends to provide increasing contribution to the rates with temperature and density. The effective half-lives of WP nuclei under the rp-process peak condition are predicted to be reduced as compared with the terrestrial case, especially for 64Ge and 68Se.

Impact of Se68 and Kr72 stellar weak interaction rates on rp -process nucleosynthesis and energetics

Impact of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Se</mml:mi><mml:mprescripts /><mml:none /><mml:mn>68</mml:mn></mml:mmultiscripts></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Kr</mml:mi><mml:mprescripts /><mml:none /><mml:mn>72</mml:mn></mml:mmultiscripts></mml:math> stellar weak interaction rates on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>r</mml:mi><mml:mi>p</mml:mi></mml:mrow></mml:math> -process nucleosynthesis and energetics

Stellar weak interaction rates and shape coexistence for 68Se and 72Kr waiting points

Beyond-mean-field results on stellar beta-decay and continuum electron capture rates for the rp-process waiting point nuclei 68Se and 72Kr at densities ρYe = 104−107 mol/cm3 and temperatures T = 108−1010 K are presented. The structure of the even-even parent and odd-odd daughter nuclei dominated by shape coexistence and mixing is self-consistently described within the complex Excited Vampir model using realistic effective interactions in rather large model spaces. The influence of the shape mixing in the structure of the low-lying parent states as well as in the independently calculated daughter states on the stellar weak interaction rates is discussed.

The effects of variations in nuclear interactions on nucleosynthesis in thermonuclear supernovae

The impact of nuclear physics uncertainties on nucleosynthesis in thermonuclear supernovae has not been fully explored using comprehensive and systematic studies with multiple models. To better constrain predictions of yields from these phenomena, we have performed a sensitivity study by post-processing thermodynamic histories from two different hydrodynamic, Chandrasekhar-mass explosion models. We have individually varied all input reaction and, for the first time, weak interaction rates by a factor of ten and compared the yields in each case to yields using standard rates. Of the 2305 nuclear reactions in our network, we find that the rates of only 53 reactions affect the yield of any species with an abundance of at least 10^-8 M_sun by at least a factor of two, in either model. The rates of the 12C(a,g), 12C+12C, 20Ne(a,p), 20Ne(a,g) and 30Si(p,g) reactions are among those that modify the most yields when varied by a factor of ten. From the individual variation of 658 weak interaction rates in our network by a factor of ten, only the stellar 28Si(b+)28Al, 32S(b+)32P and 36Ar(b+)36Cl rates significantly affect the yields of species in a model. Additional tests reveal that reaction rate changes over temperatures T > 1.5 GK have the greatest impact, and that ratios of radionuclides that may be used as explosion diagnostics change by a factor of less than two from the variation of individual rates by a factor of 10. Nucleosynthesis in the two adopted models is relatively robust to variations in individual nuclear reaction and weak interaction rates. Laboratory measurements of a limited number of reactions would help to further constrain predictions. As well, we confirm the need for a consistent treatment for relevant stellar weak interaction rates since simultaneous variation of these rates (as opposed to individual variation) has a significant effect on yields in our models.

Neutrino cooling rates due to 54,55,56Fe for presupernova evolution of massive stars

Accurate estimate of neutrino energy loss rates are needed for the study of the late stages of the stellar evolution, in particular for cooling of neutron stars and white dwarfs. The energy spectra of neutrinos and antineutrinos arriving at the Earth can also provide useful information on the primary neutrino fluxes as well as neutrino mixing scenario (it is to be noted that these supernova neutrinos are emitted from a much later stage in stellar evolution than that considered in this manuscript). Proton–neutron quasi-particle random phase approximation (pn-QRPA) theory has recently being used for a microscopic calculation of stellar weak interaction rates of iron isotopes with success. Here I present the detailed calculation of neutrino and antineutrino cooling rates due to key iron isotopes in stellar matter using the pn-QRPA theory. The rates are calculated on a fine grid of temperature-density scale suitable for core-collapse simulators. The calculated rates are compared against earlier calculations. The neutrino cooling rates due to isotopes of iron are in overall good agreement with the rates calculated using the large-scale shell model. During the presupernova evolution of massive stars, from oxygen shell burning till around end of convective core silicon burning phases, the calculated neutrino cooling rates due to 54Fe are three to four times larger than the corresponding shell model rates. The Brink’s hypothesis used in previous calculations can at times lead to erroneous results. The Brink’s hypothesis assumes that the Gamow-Teller strength distributions for all excited states are the same. It is, however, shown by the present calculation that both the centroid and total strength for excited states differ appreciably from the ground state distribution. These changes in the strength distributions of thermally populated excited states can alter the total weak interaction rates rather significantly. The calculated antineutrino cooling rates, due to positron capture and β-decay of iron isotopes, are orders of magnitude smaller than the corresponding neutrino cooling rates and can safely be neglected specially at low temperatures and high stellar densities.

Expanded calculation of weak-interaction-mediated neutrino cooling rates due to 56Ni in stellar matter

An accurate estimate of the neutrino cooling rates is required in order to study the various stages of stellar evolution of massive stars. Neutrino losses from proto-neutron stars play a crucial role in deciding whether these stars would be crushed into black holes or explode as supernovae. Both pure leptonic and weak-interaction processes contribute to the neutrino energy losses in stellar matter. At low temperatures and densities, the characteristics of the early phase of presupernova evolution, cooling through neutrinos produced via the weak interaction, are important. Proton–neutron quasi-particle random phase approximation (pn-QRPA) theory has recently been used with success for the calculation of stellar weak-interaction rates of fp-shell nuclide. The lepton-to-baryon ratio (Ye) during early phases of stellar evolution of massive stars changes substantially, mainly due to electron captures on 56Ni. The stellar matter is transparent to the neutrinos produced during the presupernova evolution of massive stars. These neutrinos escape the site and assist the stellar core in maintaining a lower entropy. Here, an expanded calculation of weak-interaction-mediated neutrino and antineutrino cooling rates due to 56Ni in stellar matter using the pn-QRPA theory is presented. This detailed scale is appropriate for interpolation purposes and is of greater utility for simulation codes. The calculated rates are compared with earlier calculations. During the relevant temperature and density regions of stellar matter the reported rates show few differences compared with the shell model rates and might contribute in fine-tuning of the lepton-to-baryon ratio during the presupernova phases of stellar evolution of massive stars.

Gamow-Teller strength distributions at finite temperatures and electron capture in stellar environments

We propose a new method to calculate stellar weak-interaction rates. It is based on the thermofield dynamics formalism and allows calculation of the weak-interaction response of nuclei at finite temperatures. The thermal evolution of the ${\mathrm{GT}}_{+}$ distributions is presented for the sample nuclei $^{54,56}\mathrm{Fe}$ and $^{76,78,80}\mathrm{Ge}$. For Ge we also calculate the strength distribution of first-forbidden transitions. We show that thermal effects shift the ${\mathrm{GT}}_{+}$ centroid to lower excitation energies and make possible negative- and low-energy transitions. In our model we demonstrate that the unblocking effect for ${\mathrm{GT}}_{+}$ transitions in neutron-rich nuclei is sensitive to increasing temperature. The results are used to calculate electron capture rates and are compared to those obtained from the shell model.

Neutrino energy loss rates and positron capture rates onCo55for presupernova and supernova physics

Proton-neutron quasi-particle random phase approximation (pn-QRPA) theory has recently being used for calculation of stellar weak interaction rates of $fp$-shell nuclide with success. Neutrino losses from proto-neutron stars play a pivotal role to decide if these stars would be crushed into black holes or explode as supernovae. The product of abundance and positron capture rates on $^{55}$Co is substantial and as such can play a role in fine tuning of input parameters of simulation codes specially in the presupernova evolution. Recently we introduced our calculation of capture rates on $^{55}$Co, in a luxurious model space of $7 \hbar \omega$, employing the pn-QRPA theory with a separable interaction. Simulators, however, may require these rates on a fine scale. Here we present for the first time an expanded calculation of the neutrino energy loss rates and positron capture rates on $^{55}$Co on an extensive temperature-density scale. These type of scale is appropriate for interpolation purposes and of greater utility for simulation codes. The pn-QRPA calculated neutrino energy loss rates are enhanced roughly up to two orders of magnitude compared with the large-scale shell model calculations and favor a lower entropy for the core of massive stars.

Microscopic calculations of stellar weak interaction rates and energy losses for fp- and fpg-shell nuclei

Nuclear weak interaction rates for fp- and fpg-shell nuclei in stellar matter and the associated energy losses are calculated using a modified form of proton–neutron quasiparticle random phase approximation model with separable Gamow–Teller forces. The stellar weak rates are calculated over a wide range of densities (10 ⩽ ρY e (g cm −3) ⩽ 10 11) and temperatures (10 7 ⩽ T (K) ⩽ 30 × 10 9). We also calculate the probability of particle emission processes in stellar environment. We present an extensive compilation of weak interaction rates in stellar matter calculated over a wide temperature–density grid and over a large mass range. The calculated capture and decay rates take into consideration the latest compilations of experimental energy levels and ft values. This paper is a follow up of our calculations introduced earlier [Eur. Phys. J. A 5 (1999) 337]. We have calculated stellar weak interaction rates for a total of 650 nuclei in the mass range A = 40–100. These also include many important neutron-rich nuclei which play an important role in the evolution process of stellar collapse.

Estimates of Stellar Weak Interaction Rates for Nuclei in the Mass Range A = 65–80

We estimate lepton capture and emission rates, as well as neutrino energy loss rates, for nuclei in the mass range A = 65-80. These rates are calculated on a temperature/density grid appropriate for a wide range of astrophysical applications including simulations of late time stellar evolution and X-ray bursts. The basic inputs in our single-particle and empirically inspired model are (i) experimentally measured level information, weak transition matrix elements, and lifetimes, (ii) estimates of matrix elements for allowed experimentally unmeasured transitions based on the systematics of experimentally observed allowed transitions, and (iii) estimates of the centroids of the GT resonances motivated by shell model calculations in the fp shell as well as by (n, p) and (p, n) experiments. Fermi resonances (isobaric analog states) are also included, and it is shown that Fermi transitions dominate the rates for most interesting proton-rich nuclei for which an experimentally determined ground state lifetime is unavailable. For the purposes of comparing our results with more detailed shell model based calculations we also calculate weak rates for nuclei in the mass range A = 60-65 for which Langanke & Martinez-Pinedo have provided rates. The typical deviation in the electron capture and β-decay rates for these ≈30 nuclei is less than a factor of 2 or 3 for a wide range of temperature and density appropriate for presupernova stellar evolution. We also discuss some subtleties associated with the partition functions used in calculations of stellar weak rates and show that the proper treatment of the partition functions is essential for estimating high-temperature β-decay rates. In particular, we show that partition functions based on unconverged Lanczos calculations can result in errors in estimates of high-temperature β-decay rates.

Neutrino spectra from stellar electron capture

Using the recent shell model evaluation of stellar weak interaction rates we have calculated the neutrino spectra arising from electron capture on pf-shell nuclei under presupernova conditions. We present a simple parametrization of the spectra which allows for an easy implementation into collapse simulations. We discuss that the explicit consideration of thermal ensembles in the parent nucleus broadens the neutrino spectra and results in larger average neutrino energies. The capture rates and neutrino spectra can be easily modified to account for phase space blocking by neutrinos which becomes increasingly important during the final stellar collapse.

Instrumentation for and first results on nuclear responses for supernova explosions

Our collaboration has set up a focal plane detection system and a focal plane polarimeter at the large acceptance Big-Bite Spectrometer at AGOR. The detector systems are equipped with a high performance readout and online data processing system, which allows polarization transfer and charge transfer measurements at extreme forward angles with high precision. Preliminary results on GT+ strength distributions obtained in (d,2He) measurements revealing the fine structure of the distributions are presented. Their relation to recent calculations of stellar weak interaction rates is discussed.

Presupernova collapse models with improved weak-interaction rates.

Improved values for stellar weak-interaction rates have been recently calculated based upon a large shell-model diagonalization. Using these new rates (for both beta decay and electron capture), we have examined the presupernova evolution of massive stars in the range (15--40)M(o). Comparing our new models with a standard set of presupernova models by Woosley and Weaver, we find significantly larger values for the electron-to-baryon ratio at the onset of collapse and smaller iron core masses. These changes may have important consequences for nucleosynthesis and the supernova explosion mechanism.

Shell-model calculations of stellar weak interaction rates: II. Weak rates for nuclei in the mass range [formula omitted] in supernovae environments

Based on large-scale shell-model calculations we have determined the electron capture, positron capture and beta-decay rates for more than 100 nuclei in the mass range A=45–65. The rates are given for densities ρY e=10 7 – 10 10 mol /cm 3 and temperatures T=10 9 – 10 10 K and hence are relevant for both types of supernovae (Type Ia and Type II). The shell-model electron capture rates are significantly smaller than currently assumed. For proton-to-baryon ratios Y e=0.42 –0.46 mol /g, the beta-decay rates are faster than the electron capture rates during the core collapse of a massive star.

Shell-model calculations of stellar weak interaction rates. I. Gamow-Teller distributions and spectra of nuclei in the mass range A = 45–65

Electron capture and beta-decay rates on nuclei in the mass range A = 45–65 play an important role in many astrophysical environments. The determination of these rates by large-scale shell-model calculations is desirable, but it requires to reproduce the Gamow-Teller strength distributions and spectra of the pf shell nuclei. We show in this paper that large-scale shell-model calculations, employing a slightly monopole-corrected version of the well-known KB3 interaction, fulfill these necessary requirements. In particular, our calculations reproduce the experimentally available GT + and GT − strength distributions and the nuclear halflives, and describe the nuclear spectra appropriately.

Stellar weak interaction rates for intermediate-mass nuclei. IV - Interpolation procedures for rapidly varying lepton capture rates using effective log (ft)-values

Simple expressions for continuum electron and positron capture phase space factors and the associated neutrino energy loss integrals are presented in terms of standard Fermi integrals. Continuous approximations to the relevant Fermi integrals and their first derivatives are made. These allow the computation of effective log (ft)--values, at each temperature and density point, for the continuum lepton capture rates considered in the earlier papers in this series. Since the effective log (ft)--values have most of the rapid temperature and density dependence associated with the phase space integrals removed, interpolation in temperature and density to obtain stellar rates is greatly facilitated in speed and accuracy. Computer simulations of stellar evolution will be able to implement more accurately our calculations of the stellar nuclear weak interaction rates of intermediate-mass nuclei. Generalization of the Fermi integral expressions for the lepton continuum capture phase space factors are given for astrophysical environments where there exists an equilibrium distribution of electron-type neutrinos. These allow rough estimates of the effect of neutrino blocking on our tabulated rates and estimates of total neutrino capture rates.

Stellar weak interaction rates for intermediate mass nuclei. III - Rate tables for the free nucleons and nuclei with A = 21 to A = 60

view Abstract Citations (432) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar weak interaction rates for intermediate mass nuclei. III - Rate tables for the free nucleons and nuclei with A = 21 to A = 60 Fuller, G. M. ; Fowler, W. A. ; Newman, M. J. Abstract Stellar electron and positron emission rates and continuum electron and positron capture rates, as well as the associated neutrino energy loss rates, are tabulated for the free nucleons and 226 nuclei with masses between A =21 and 60. These rates were calculated in accordance with the procedure described in Papers I and II of this series and are presented here in tabular form on an abbreviated temperature and density grid. Results of these calculations on a detailed temperature and density grid are available in computer readable form on magnetic tape upon request to MJN. The stellar weak rate calculation procedure is reviewed, and the results are discussed. Comparison of the stellar weak rates to terrestrial decay rates are made where possible. Publication: The Astrophysical Journal Supplement Series Pub Date: March 1982 DOI: 10.1086/190779 Bibcode: 1982ApJS...48..279F Keywords: Astrophysics; Nuclear Astrophysics; Nuclei (Nuclear Physics); Nucleons; Stellar Evolution; Weak Energy Interactions; Electron Capture; Energy Dissipation; Neutrinos; Positrons; Tables (Data); Astrophysics full text sources ADS |

Stellar weak interaction rates for intermediate-mass nuclei. II - A = 21 to A = 60

Astrophysical electron and positron emission, continuum electron and positron capture rates, as well as the associated neutrino energy loss rates are calculated for free nucleons and 226 nuclei with masses between A = 21 and 60. Measured nuclear level information and matrix elements are used where available. Unmeasured matrix elements for allowed transitions are assigned as in Paper I. Simple shell model arguments are used to estimate Gamow-Teller sum rules and collective state resonance excitation energies. The discrete state contribution to the rates, dominated by experimental information and the Fermi transitions, determines the nuclear rates in the regime of temperatures and densities characteristic of the hydrostatic phases of presupernova stellar evolution. At the higher temperatures and densities characteristic of the supernova collapse phase, the nuclear rates are dominated by the Fermi and the Gamow-Teller collective resonance contributions. Also included is the important effect of neutron shell closure blocking of electron capture on neutron-rich nuclei. Uncertainties in the rate calculation are discussed. Reference is made to other treatments of the problem. Results of the calculations on a detailed temperature-density grid are available in computer readable form on magnetic tape upon request to M. J. N.

Stellar weak-interaction rates for sd-shell nuclei. I - Nuclear matrix element systematics with application to Al-26 and selected nuclei of importance to the supernova problem

Stellar weak-interaction rates for sd-shell nuclei. I - Nuclear matrix element systematics with application to Al-26 and selected nuclei of importance to the supernova problem