Rapid Proton Capture Research Articles

Thermal conductivity and phase separation of the crust of accreting neutron stars

Recently, crust cooling times have been measured for neutron stars after extended outbursts. These observations are very sensitive to the thermal conductivity kappa of the crust and strongly suggest that kappa is large. We perform molecular dynamics simulations of the structure of the crust of an accreting neutron star using a complex composition that includes many impurities. The composition comes from simulations of rapid proton capture nucleosynthesis followed by electron captures. We find that the thermal conductivity is reduced by impurity scattering. In addition, we find phase separation. Some impurities with low atomic number Z are concentrated in a subregion of the simulation volume. For our composition, the solid crust must separate into regions of different compositions. This could lead to an asymmetric star with a quadrupole deformation. Observations of crust cooling can constrain impurity concentrations.

β-decay half-life of therp-process waiting-point nuclideMo84

A half-life of 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 s has been deduced for the ground-state $\ensuremath{\beta}$ decay of $^{84}\mathrm{Mo}$, more than $1\ensuremath{\sigma}$ shorter than the previously adopted value. $^{84}\mathrm{Mo}$ is an even-even $N=Z$ nucleus lying on the proton dripline, created during explosive hydrogen burning in type I x-ray bursts in the rapid proton capture ($\mathit{rp}$) process. The effect of the measured half-life on $\mathit{rp}$-process reaction flow is explored. Implications on theoretical treatments of nuclear deformation in $^{84}\mathrm{Mo}$ are also discussed.

The15O(α,γ)19Ne Reaction Rate and the Stability of Thermonuclear Burning on Accreting Neutron Stars

Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of theaccretedmaterialintheatmosphereof theneutronstarleadstoathermonuclearexplosion,whichisobservedas an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized bya sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction 15 O(�; � ) 19 Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the 15 O(�; � ) 19 Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations. Subject headingg nuclear reactions, nucleosynthesis, abundances — stars: neutron — X-rays: bursts

Astrophysical nuclear reactions and the break-out from the hot CNO cycles

Thermonuclear stellar explosions depend critically on the available fuel and the ignition through nuclear reactions. In this paper we intend to demonstrate the sensitive interplay between nuclear fuel supply and depletion by nuclear reaction processes at high temperature and densities. We demonstrate this on the example of X-ray bursts which are explained as thermonuclear explosions in the atmosphere of an accreting neutron star. These bursts occur periodically between hours and days depending on the accretion rate. The burst itself is triggered by the 15O(α,γ) 19Ne reaction regulating the flow between the β -limited hot CNO cycle and the rapid proton capture process. The 18Ne(α,p) 21Na reaction on the other hand provides a continuous processing flow of light 4He fuel towards heavier elements. The reaction rates of both α capture processes carry significant uncertainties. In this paper we report on the measurements of the critical nuclear parameters for the determination of the rate of the 15O(α,γ) 19Ne and the 18Ne(α,p) 21Na reactions. We demonstrate and analyze the impact of these experimental results on the burst pattern and burst periodicity for a range of accretion rates.

Mass measurements in the endpoint region of the rp-process at SHIPTRAP

The Penning-trap mass spectrometer SHIPTRAP was designed for precision mass measurements of radionuclides produced in fusion-evaporation reactions. The latest measurement campaign covered heavy nuclei (A > 90) related to the astrophysical rapid proton capture process. The masses of 34 neutron-deficient radionuclides have been measured since February 2006 with relative uncertainties between 5 × 10-8 and 1 × 10-7. Furthermore, the use of an octupolar RF excitation for the time-of-flight ion-cyclotron-resonance technique was investigated and an increase of the resolving power by a factor of ten was observed in agreement with simulations. This will allow to resolve isomeric states with excitation energies of a few 10 keV only.

The rp‐Process in Neutrino‐driven Winds

Recent hydrodynamic simulations of core-collapse supernovae with accurate neutrino transport suggest that the bulk of the early neutrino-heated ejecta is proton rich, in which the production of some interesting proton-rich nuclei is expected. As suggested in recent nucleosynthesis studies, the rapid proton-capture (rp) process takes place in such proton-rich environments by bypassing the waiting point nuclei with β+-lives of a few minutes via the faster capture of neutrons continuously supplied from the neutrino absorption by protons. In this study, the nucleosynthesis calculations are performed with a wide range of neutrino luminosities and electron fractions (Ye), using semianalytic models of proto-neutron-star winds. The masses of proto-neutron stars are taken to be 1.4 and 2.0 M☉, where the latter is regarded as the test for somewhat high-entropy winds (about a factor of 2). For Ye > 0.52, the neutrino-induced rp-process takes place in many wind trajectories, and p-nuclei up to A ~ 130 are synthesized in interesting amounts. However, 92Mo is somewhat underproduced compared to other p-nuclei with similar mass numbers. For 0.46 < Ye < 0.49, on the other hand, 92Mo is significantly enhanced by the nuclear flows in the vicinity of the abundant 90Zr that originates from the α-process at higher temperature. The nucleosynthetic yields are averaged over the ejected masses of winds, and further, over the Ye distribution predicted by a recent hydrodynamic simulation of a core-collapse supernova. Comparison of the Ye- and mass-averaged yields to the solar compositions implies that the neutrino-driven winds can potentially be the origin of light p-nuclei up to A ~ 110, including 92,94Mo and 96,98Ru, that cannot be explained by other astrophysical sites.

Nuclear structure of the exotic mass region along the rp process path

Isomeric states in the nuclei along the rapid proton capture process path are studied by the projected shell model. Emphasis is given to two waiting point nuclei 68Se and 72Kr that are characterized by shape coexistence. Energy surface calculations indicate that the ground state of these nuclei corresponds to an oblate-deformed minimum, while the lowest state at the prolate-deformed minimum can be considered as a shape isomer. The impact of these isomer states on isotopic abundance in x-ray bursts is studied in a multi-mass-zone x-ray burst model by assuming an upper-lower limit approach.

New approach for measuring properties of rp-process nuclei.

A new experimental approach was developed that can reduce the uncertainties in astrophysical rapid proton capture (rp) process calculations due to nuclear data. This approach utilizes neutron removal from a radioactive ion beam to populate the nuclear states of interest. Excited states were deduced by the gamma-decay spectra measured in a semiconductor Ge-detector array. In the first case studied, 33Ar, excited states were measured with uncertainties of several keV. The 2 orders of magnitude improvement in the uncertainty of the level energies resulted in a 3 orders of magnitude improvement in the uncertainty of the calculated 32Cl(p,gamma)33Ar rate that is critical to the modeling of the rp process. This approach has the potential to measure key properties of almost all interesting nuclei on the rp-process path.

Branching ratioΓα/Γγof the 4.033 MeV3/2+state in19Ne

The branching ratio ${\ensuremath{\Gamma}}_{\ensuremath{\alpha}}/{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$ of the 4.033 MeV ${3/2}^{+}$ state in ${}^{19}\mathrm{Ne}$ plays a crucial role in the breakout from the hot CNO cycle into the rapid proton capture process. This ratio has been studied by making use of the advantages of inverse kinematics. The state was populated via the ${}^{3}\mathrm{He}{(}^{20}\mathrm{Ne},\ensuremath{\alpha}{)}^{19}{\mathrm{Ne}}^{*}$ reaction and its decay via $\ensuremath{\gamma}$ or $\ensuremath{\alpha}$ emission was measured by detecting the heavy reaction products ${(}^{19}\mathrm{Ne}$ or ${}^{15}\mathrm{O})$ in coincidence in a magnetic spectrograph. An upper limit ${\ensuremath{\Gamma}}_{\ensuremath{\alpha}}/{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}l~6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ has been obtained. With these results, the astrophysical reaction rate for the ${}^{15}\mathrm{O}$ $(\ensuremath{\alpha},\ensuremath{\gamma})$ ${}^{19}\mathrm{Ne}$ reaction has been calculated. Its influence on the breakout at various astrophysical sites, novas, x-ray bursts, and supermassive stars, is discussed.

Nuclear structure theory for the astrophysical rp-process and r-process

The astrophysical processes of rapid-proton capture and rapid-neutron capture require the knowledge of many nuclear properties which are not known from experiment. I will describe two examples of how theoretical models are used to provide this input. The first of these uses the Hartree-Fock method for displacement energies to obtain the masses of proton-rich nuclei needed for the rp-process. The second uses a model for configuration mixing near 132Sn to provide Q values and beta-decay lifetimes for the r-process.

Mirror decay of 75Sr

The β-decay of 75Sr to its mirror nucleus 75Rb was studied at the ISOLDE PSB facility at CERN by means of β-delayed γ and proton spectroscopy. The decay Q-value and β-delayed γ intensity were measured for the first time. These results, 10.60±0.22 MeV and 4.5+1.9 -0.7%, together with accurate measurements of the β-decay half-life and β-delayed proton branching ratio yielded the Gamow-Teller strength 0.35±0.05 for the mirror transition. Implications of the results on studies of deformation effects and on the path of the rapid proton capture process are discussed.

End point of the rp process on accreting neutron stars.

We calculate the rapid proton ( rp) capture process of hydrogen burning on the surface of an accreting neutron star with an updated reaction network that extends up to Xe, far beyond previous work. In both steady-state nuclear burning appropriate for rapidly accreting neutron stars (such as the magnetic polar caps of accreting x-ray pulsars) and unstable burning of type I x-ray bursts, we find that the rp process ends in a closed SnSbTe cycle. This prevents the synthesis of elements heavier than Te and has important consequences for x-ray burst profiles, the composition of accreting neutron stars, and potentially galactic nucleosynthesis of light p nuclei.

Mass measurements of70Se,71Se,72Br,and73Br

The path and termination point for the rapid proton-capture (rp) process above Ni-56 is uncertain due to a lack of knowledge of nuclear properties, especially masses, near the proton drip line. To address this need we have begun a program to measure masses along the rp-process path in the A similar to 60-80 region using beta-gamma coincidence spectroscopy, Q(EC) values derived from beta (+) spectrum end points were used to calculate mass excess values for Se-70, Se-71, Br-72, Br-73. The results are compared to predictions of mass models and to estimates based on systematic trends in this region.

Mapping the proton drip line from [formula omitted] to [formula omitted

The structure of proton drip line nuclei in the 60<A<100 mass range is studied with the relativistic Hartree–Bogoliubov (RHB) model. For the elements which determine the astrophysical rapid proton capture process path, the RHB model predicts the location of the proton drip-line, the ground-state quadrupole deformations and one-proton separation energies at and beyond the drip-line. The results of the present theoretical investigation are compared with available experimental data. For possible odd- Z ground-state proton emitters, the calculated deformed single-particle orbitals occupied by the odd valence proton and the corresponding spectroscopic factors are compared with predictions of the macroscopic–microscopic mass model.

The Rapid Proton Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star

The temperature and nuclear composition of the crust and ocean of an accreting neutron star depend on the mix of material (the ashes) that is produced at lower densities by fusion of the accreting hydrogen and helium. The hydrogen/helium burning is thermally stable at high accretion rates, a situation encountered in weakly magnetic (B 1011 G) neutron stars accreting at rates > 10-8 M☉ yr-1 and in most accreting X-ray pulsars, where the focusing of matter onto the magnetic poles results in local accretion rates high enough for stable burning. For a neutron star accreting at these high rates, we calculate the steady state burning of hydrogen and helium in the upper atmosphere (ρ < 2 × 106 g cm-3), where T ≈ (5-15) × 108 K. Since the breakout from the hot CNO cycle occurs at a temperature comparable to that of stable helium burning (T 5 × 108 K), the hydrogen is always burned via the rapid proton capture (rp) process of Wallace & Woosley. The rp-process makes nuclei far beyond the iron group, always leading to a mixture of elements with masses A ~ 60-100. The average nuclear mass of the ashes is set by the extent of helium burning via (α, p) reactions and, because these reactions are temperature sensitive, depends on the local accretion rate. Nuclear statistical equilibrium, leading to a composition of mostly iron, occurs only for very high local accretion rates in excess of 50 times the Eddington rate. We briefly discuss the consequences of our results for the properties of the neutron star. The wide range of nuclei made at a fixed accretion rate and the sensitivity of the ash composition to the local accretion rate makes it inevitable that accreting neutron stars have an ocean and crust made up of a large variety of nuclei. This has repercussions for the thermal and electrical properties and structural properties (the shear modulus and viscosity) of the neutron star crust. A crustal lattice as impure as implied by our results will have the conductivity throughout most of its mass set by impurity scattering, allowing for more rapid Ohmic diffusion of magnetic fields than previously estimated for mononuclear mixes.

Is U Aquarii a Thorne‐Żytkow Object?

The R Coronae Borealis (RCB) star U Aqr was known to show an unusual abundance pattern with respect to the post-iron peak elements. The abundances of elements in the light s-process peak—Rb, Sr, Y, and Zr—are greatly enhanced, whereas elements representative of the heavy s-process peak, such as Ba, do not exhibit large abundance enhancements. Recent calculations of the abundances of elements formed by rapid proton capture in Thorne-Żytkow objects indicate that qualitatively the abundance pattern of post-iron peak elements in Thorne-Żytkow objects appeared to match that observed in U Aqr. New high-resolution spectra of U Aqr and two other stars are used to explore the possibility that U Aqr is a Thorne-Żytkow object. Our spectra show that on the basis of the light s-process elements, in particular the absence of an enhancement of Mo, Cu, and Zn, it does not appear that U Aqr is a good candidate for a Thorne-Żytkow object. However, the light s-process abundance pattern resembles that of Sakurai's star that has recently begun to show RCB characteristics. We suggest that these stars represent a possible source for the weak component of the solar system s-process distribution of heavy elements.

19Ne(p,γ)20Naand19Ne(d,n)20Nareactions and its astrophysical implications for the transition of the hot CNO cycle to therpprocess

The population of the Na-20 resonances at 448, 661, 797, and 887 keV above the proton threshold has been studied in measurements of the Ne-19(p,gamma)Na-20 reaction with radioactive Ne-19 beams. The data have been analyzed fully in terms of resonance strengths, enabling a comparison with theoretical estimates and the extraction of upper and lower limits on the resonant part of the Ne-19(p, gamma)Na-20 astrophysical reaction rate. The total cross section of the Ne-19(d,n)Na-20 reaction has been measured from 0.6 to 1.8 MeV (c.m.). On the basis of the results of a subsequent distorted-wave Born approximation analysis the direct component of the Ne-19(p,gamma)Na-20 astrophysical reaction rate has been calculated in a potential model. The astrophysical implications of the resulting total reaction rate have been investigated: the conditions for the breakout from the hot CNO cycle into the rapid-proton capture process are determined by the preceding O-15(alpha, gamma)Ne-19 reaction and the photodisintegration of the produced Na-20 nuclei will not impede the breakout. [S0556-2813(98)05305-9].

Expressions for the14O(α,p)17F and17F(p,γ)18Ne astrophysical reaction rates

The ${}^{14}\mathrm{O}(\ensuremath{\alpha}{,p)}^{17}$F and ${}^{17}\mathrm{F}(p,\ensuremath{\gamma}{)}^{18}$Ne astrophysical reaction rates are needed to determine the breakout conditions from the hot CNO cycle into the rapid proton capture process in hot stellar environments. We have examined recent indirect experimental measurements of these reactions and generated analytical expressions for the reaction rates as a function of temperature. We also present a correction to the previously reported calculation of the ${}^{17}\mathrm{F}(p,\ensuremath{\gamma})$ rate.

An approximation scheme for the rp-process

The Rapid Proton Capture Process ( rp-process) describes the sequence of responsible nuclear reactions for the thermonuclear energy generation during explosive hydrogen burning on accreting compact objects in binary stellar systems. In principle, all unstable proton-rich nuclei must be taken into account, but treating each nucleus separately makes numerical simulations which include hydrodynamics inefficient and expensive. Therefore, an accurate approximation scheme for the rp-process is highly desirable. In the present investigation we show how a general understanding of the nuclear reaction flows leads automatically to such a scheme and provides information about the reactions where experimental information from radioactive beam facilities is required.

An Approximation for the rp‐Process

Hot (explosive) hydrogen burning or the Rapid Proton Capture Process (rp-process) occurs in a number of astrophysical environments. Novae and X-ray bursts are the most prominent ones, but accretion disks around black holes and other sites are candidates as well. The expensive and often multidimensional hydro calculations for such events require an accurate prediction of the thermonuclear energy generation, while avoiding full nucleosynthesis network calculations. In the present investigation we present an approximation scheme applicable in a temperature range which covers the whole range of all presently known astrophysical sites. It is based on the concept of slowly varying hydrogen and helium abundances and assumes a kind of local steady flow by requiring that all reactions entering and leaving a nucleus add up to a zero flux. This scheme can adapt itself automatically and covers situations at low temperatures, characterized by a steady flow of reactions, as well as high temperature regimes where a $(p,\gamma)-(\gamma,p)$-equilibrium is established. In addition to a gain of a factor of 15 in computational speed over a full network calculation, and an energy generation accurate to more than 15 %, this scheme also allows to predict correctly individual isotopic abundances. Thus, it delivers all features of a full network at a highly reduced cost and can easily be implemented in hydro calculations.