Explosive Hydrogen Burning Research Articles

Thermonuclear 28P(p, γ)29S reaction rate and astrophysical implication in ONe nova explosion

Context. An accurate 28P(p, γ)29S reaction rate is crucial to defining the nucleosynthesis products of explosive hydrogen burning in ONe novae. Using the recently released nuclear mass of 29S, together with a shell model and a direct capture calculation, we reanalyzed the 28P(p, γ)29S thermonuclear reaction rate and its astrophysical implication. Aims. We focus on improving the astrophysical rate for 28P(p, γ)29S based on the newest nuclear mass data. Our goal is to explore the impact of the new rate and associated uncertainties on the nova nucleosynthesis. Methods. We evaluated this reaction rate via the sum of the isolated resonance contribution instead of the previously used Hauser-Feshbach statistical model. The corresponding rate uncertainty at different energies was derived using a Monte Carlo method. Nova nucleosynthesis is computed with the 1D hydrodynamic code SHIVA. Results. The contribution from the capture on the first excited state at 105.64 keV in 28P is taken into account for the first time. We find that the capture rate on the first excited state in28 P is up to more than 12 times larger than the ground-state capture rate in the temperature region of 2.5 × 107 K to 4 × 108 K, resulting in the total 28P(p, γ)29S reaction rate being enhanced by a factor of up to 1.4 at ~1 × 109 K. In addition, the rate uncertainty has been quantified for the first time. It is found that the new rate is smaller than the previous statistical model rates, but it still agrees with them within uncertainties for nova temperatures. The statistical model appears to be roughly valid for the rate estimation of this reaction in the nova nucleosynthesis scenario. Using the 1D hydrodynamic code SHIVA, we performed the nucleosynthesis calculations in a nova explosion to investigate the impact of the new rates of 28P(p, γ)29S. Our calculations show that the nova abundance pattern is only marginally affected if we use our new rates with respect to the same simulations but statistical model rates. Finally, the isotopes whose abundance is most influenced by the present 28P(p, γ)29S uncertainty are 28Si, 33,34S, 35,37Cl, and 36Ar, with relative abundance changes at the level of only 3% to 4%.

The challenging direct measurement of the 65 keV resonance strength of the 17O(p, γ)18F reaction at LUNA

The 17O(p, γ)18F reaction plays a crucial role in AGB nucleosynthesis as well as in explosive hydrogen burning occurring in type Ia novae. At the temperatures of interest for the former scenario ( 20MK < T < 80MK) the main contribution to the astrophysical reaction rate comes from the poorly constrained ER = 65 keV resonance. The strength of this resonance is presently determined only through indirect measurements, with an adopted value ωγ =(16 ± 3) peV. A new high sensitivity setup was installed at LUNA, located at LNGS. The underground location of the LUNA 400kV accelerator guarantees a reduction of the cosmic ray background by several orders of magnitude. The residual background was further reduced installing a devoted shielding. On the other hand, to increase the efficiency, the 4π BGO detector was coupled with Al target chamber and holder. With more than 400C accumulated on Ta2O5 targets, nominal 17O enrichment of 90%, the LUNA collaboration has performed the first direct measurement of the 65 keV resonance strength.

The challenging direct measurement of the 65 keV resonance strength of the 17O(p,γ)18F reaction at LUNA

A precise determination of proton capture rates on oxygen is mandatory to predict the abundance ratios of the oxygen isotopes in a stellar environment where hydrogen burning is active. The 17O(p,γ)18F reaction, specifically, plays a crucial role in AGB nucleosynthesis as well as in explosive hydrogen burning occurring in type Ia novae. At temperatures of interest for the former scenario (20 MK ≤ T ≤ 80 MK) the main contribution to the astrophysical reaction rate comes from the Ec.m. = 65 keV resonance. The strength of this resonance is presently determined only through indirect measurements, with an adopted value of ωγ = (1.6 ± 0.3) × 10−11 eV. Thanks to the low background environment of the Laboratori Nazionali del Gran Sasso, the intense and stable beam provided by the LUNA 400 kV accelerator and the experience in oxygen target production, the LUNA collaboration is aiming the first direct measurement of the above mentioned resonance strength. In the present work details of challenging direct measurement planned at LUNA will be described.

Level structure of S31 via S32(p,d)S31

Background: Properties of proton-unbound $^{31}\mathrm{S}$ states determine the $^{30}\mathrm{P}(p,\ensuremath{\gamma})^{31}\mathrm{S}$ reaction rate, which has a significant impact on explosive hydrogen burning in classical novae and type-I x-ray bursts. Despite several previous studies, uncertainties still remain with respect to the nuclear structure of $^{31}\mathrm{S}$ near the proton threshold.Purpose: The level structure of $^{31}\mathrm{S}$ has been presently investigated via a charged-particle spectroscopy experiment using the $^{32}\mathrm{S}(p,d)^{31}\mathrm{S}$ reaction.Method: Deuterons corresponding to $^{31}\mathrm{S}$ excited states with $3.285\ensuremath{\le}\phantom{\rule{4pt}{0ex}}{E}_{x}\phantom{\rule{4pt}{0ex}}\ensuremath{\le}10.8$ MeV were momentum analyzed via an Enge split-pole spectrograph at six laboratory angles between ${10}^{\ensuremath{\circ}}$ and ${62}^{\ensuremath{\circ}}$. Differential cross sections of the $^{32}\mathrm{S}(p,d)^{31}\mathrm{S}$ reaction were measured at ${E}_{p}\phantom{\rule{4pt}{0ex}}=34.5$ MeV. Distorted-wave Born approximation calculations were performed to constrain the spin-parity assignments of several of the observed levels.Results: We have detected 72 excited states of $^{31}\mathrm{S}$, out of which 17 are within the astrophysical region of interest corresponding to the temperature range of 0.1--1.5 GK. We have resolved the discrepancy in the spin and parity of an excited state with ${E}_{x}\phantom{\rule{4pt}{0ex}}=6542$ keV, showing that is it not ${J}^{\ensuremath{\pi}}\phantom{\rule{4pt}{0ex}}=3/{2}^{\ensuremath{-}}$, and therefore the contribution of this state to the $^{30}\mathrm{P}(p,\ensuremath{\gamma})$ reaction rate is likely much less significant than previously thought owing to the larger angular-momentum transfer required to populate this excited state. Moreover, our measurement results help consolidate the spin-parity assignments for the 6377 and 6636 keV states in $^{31}\mathrm{S}$.Conclusions: This work presents the most comprehensive spin-parity assignments to date from a single-neutron transfer reaction on $^{32}\mathrm{S}$ to $^{31}\mathrm{S}$ excited states in the region between 6 to 7 MeV excitation energy. This region is significant for the determination of the $^{30}\mathrm{P}(p,\ensuremath{\gamma})^{30}\mathrm{S}$ reaction rate over the temperatures characteristic of explosive hydrogen burning in novae.

Simultaneous measurement of β -delayed proton and γ emission of P26 for the Al25(p,γ)Si26 reaction rate

$\beta$ decay of $^{26}$P was used to populate the astrophysically important $E_x=$5929.4(8) keV $J^{\pi}=3{^+}$ state of $^{26}$Si. Both $\beta$-delayed proton at 418(8) keV and gamma ray at 1742(2) keV emitted from this state were measured simultaneously for the first time with corresponding absolute intensities of 11.1(12)\% and 0.59(44)\%, respectively. Besides, shell model calculations with weakly bound effects were performed to investigate the decay properties of other resonant states and a spin-parity of $4^+$ rather than $0^+$ was favored for the $E_x=$5945.9(40) keV state. Combining the experimental results and theoretical calculations, $^{25}$Al($p,\gamma$)$^{26}$Si reaction rate in explosive hydrogen burning environments was calculated and compared with previous studies.

Experimental Challenge to Heavy Element Synthesis under Explosive Burning on Neutron Stars

Binary stellar systems that involve a neutron star or two neutron stars make interesting phenomena, X-ray bursts and kilo-novae, respectively, which involve explosive burning either in the proton-rich environment or in the neutron-rich environment. These are very important problems for nuclear astrophysics. First, the current effort for the explosive hydrogen burning, the rapid proton capture (rp) process which would take place on a neutron star surface in X-ray burst is discussed together with a new X-ray observation that suggests the rp-process termination at around A = 100. The observation of the afterglow of the binary neutron star merger appears to be the kilo-nova predicted in the last decade in nuclear astrophysics, and to be the great success of the field. However, the detailed study of the kilo-nova by the r-process should be a great challenge for full understanding heavy element synthesis and the neutron star merger. Nuclear physics problems are discussed for the kilonova.

Screening effects on resonant thermonuclear reaction rates

The reaction rate of several astrophysically important nuclear reactions is dominated by the contribution of narrow resonances at the astrophysically most effective energies. In the stellar plasma the partial width of the resonances in the entrance channel is modified due to screening corrections. This effect, so far ignored in stellar reaction network calculations, reduces the conventional Salpeterscreening enhancement of the reaction rate. We derive analytical screening correction factors for the contributions of narrow resonances to the stellar reaction rate and discuss the effects for UC (a, j)lsO, 150(a,7)19 Ne and other reactions of relevance to explosive hydrogen burning

Common Occurrence of Explosive Hydrogen Burning in Type II Supernovae

Abstract We report Mo isotopic data for 16 15N-rich presolar SiC grains of type AB (14N/15N < solar, AB1) and their correlated Sr and Ba isotope ratios when available. Of the 16 AB1 grains, 8 show s-process Mo isotopic compositions, together with s-process Ba and/or Sr isotopic compositions. We found that a higher percentage of AB1 grains show anomalous isotopic compositions than that of AB2 grains (14N/15N > solar), thus providing further support to the division of the two AB subgroups recently proposed by Liu et al., who showed that AB1 grains most likely originated from Type II supernovae (SNe) with explosive H burning. Comparison of the Sr, Mo, and Ba isotopic compositions of the AB1 grains with SN model predictions indicates that the s-process isotopic compositions of AB1 grains resulted from neutron-capture processes occurring during the progenitor massive stars’ pre-SN evolution rather than from an explosive neutron-capture process. In addition, the observations of (1) explosive H burning occurring in the C-rich regions of the progenitor SNe of X grains as suggested by the isotopic compositions of X grains, and (2) explosive H burning occurring both at the bottom of the He/C zone and at the top of the He/N zone as suggested by model simulations, imply that explosive H burning is a common phenomenon in outer SN zones.

The History and Impact of the CNO Cycles in Nuclear Astrophysics

The carbon cycle, or Bethe-Weizsacker cycle, plays an important role in astrophysics as one of the most important energy sources for quiescent and explosive hydrogen burning in stars. This paper presents the intellectual and historical background of the idea of the correlation between stellar energy production and the synthesis of the chemical elements in stars on the example of this cycle. In particular, it addresses the contributions of Carl Friedrich von Weizsacker and Hans Bethe, who provided the first predictions of the carbon cycle. Further, the experimental verification of the predicted process as it developed over the following decades is discussed, as well as the extension of the initial carbon cycle to the carbon-nitrogen-oxygen (CNO) multi-cycles and the hot CNO cycles. This development emerged from the detailed experimental studies of the associated nuclear reactions over more than seven decades. Finally, the impact of the experimental and theoretical results on our present understanding of hydrogen burning in different stellar environments is presented, as well as the impact on our understanding of the chemical evolution of our universe.

Experimental Approach to Explosive Hydrogen Burning in X-Ray Bursts and Core-Collapse Supernovae

Recent experimental challenges to study the explosive hydrogen burning at extremely high temperatures are discussed. Along the nucleosynthetic pathway, discussions were made especially for the medium mass region, where we have possible waiting points and bottle necks which influence not only the light curve but also the production of heavy elements including the anomalously abundant p-nuclei at A = 80-100. Two approaches were discussed for the problem; one is the precision mass measurements and the other one is the half-live measurement of very short lived isotopes. A scope of the field is also discussed.

Study of the Characteristics of Elementary Processes in a Chain Hydrogen Burning Reaction in Oxygen

The characteristics of possible chain explosive hydrogen burning reactions in an oxidizing medium are calculated on the potential energy surface. Specifically, reactions H2 + O2 → H2O + O, H2 + O2 → HO2 + H, and H2 + O2 → OH + OH are considered. Special attention is devoted to the production of a pair of fast highly reactive OH radicals. Because of the high activation threshold, this reaction is often excluded from the known kinetic scheme of hydrogen burning. However, a spread in estimates of kinetic characteristics and a disagreement between theoretical predictions with experimental results suggest that the kinetic scheme should be refined.

Design of SECAR a recoil mass separator for astrophysical capture reactions with radioactive beams

A recoil mass separator SECAR has been designed for the purpose of studying low-energy (p,γ) and (α,γ) reactions in inverse kinematics with radioactive beams for masses up to about A = 65. Their reaction rates are of importance for our understanding of the energy production and nucleosynthesis during explosive hydrogen and helium burning. The radiative capture reactions take place in a windowless hydrogen or He gas target at the entrance of the separator, which consists of four Sections. The first Section selects the charge state of the recoils. The second and third Sections contain Wien Filters providing high mass resolving power to separate efficiently the intense beam from the few reaction products. In the following fourth Section, the reaction products are guided into a detector system capable of position, angle and time-of-flight measurements. In order to accept the complete kinematic cone of recoil particles including multiple scattering in the target in the center of mass energy range of 0.2 MeV to 3.0 MeV, the system must have a large polar angle acceptance of ± 25 mrad. This requires a careful minimization of higher order aberrations. The present system will be installed at the NSCL ReA3 accelerator and will be used with the much higher beam intensities of the FRIB facility when it becomes available.

Shell and explosive hydrogen burning

The nucleosynthesis of light elements, from helium up to silicon, mainly occurs in Red Giant and Asymptotic Giant Branch stars and Novae. The relative abundances of the synthesized nuclides critically depend on the rates of the nuclear processes involved, often through non-trivial reaction chains, combined with complex mixing mechanisms. In this review, we summarize the contributions made by LUNA experiments in furthering our understanding of nuclear reaction rates necessary for modeling nucleosynthesis in AGB stars and Novae explosions.

Underground study of theO17(p,γ)F18reaction relevant for explosive hydrogen burning

Background: The O17(p,γ)F18 reaction affects the production of key isotopes (e.g., F18 and O18) in the explosive hydrogen burning that powers classical novae. Under these explosive conditions, the reaction rate is dominated by contributions from a narrow resonance at Ec.m.=183keV and by the combined contributions of direct capture and low-energy tails of broad resonances. At present, the astrophysical reaction rate is not well constrained because of the lack of data in the energy region appropriate to classical novae. Purpose: This study aims at the measurement of the O17(p,γ)F18 reaction cross section in order to determine its reaction rate in the temperature region appropriate to explosive hydrogen burning in novae. Method: The O17(p,γ)F18 reaction cross section was measured using both the prompt detection of the emitted γ rays and an activation technique. Measurements were carried out at the Laboratory for Underground Nuclear Astrophysics (Gran Sasso, Italy) where the strongly reduced cosmic-ray-induced background allows for improved sensitivity compared to previous studies. Results: The O17(p,γ)F18 reaction cross section was measured in the range Ec.m.=160 to 370keV. The strength of the Ec.m.=183keV resonance, ωγ=1.67±0.12μeV, was determined with unprecedented precision. The total S factor was obtained through a combined fit of prompt γ-ray and activation results. An overall global fit including other existing data sets was also carried out and a recommended astrophysical reaction rate is presented. Conclusions: The reaction rate uncertainty attained in this work is now below the required precision for nova models. We verified, following a full set of hydrodynamic nova models, that the abundances of oxygen and fluorine isotopes obtained with the present reaction rate are determined with 10% precision and put firmer constraints on observational signatures of novae events.5 MoreReceived 2 August 2013DOI:https://doi.org/10.1103/PhysRevC.89.015803©2014 American Physical Society

Evidence for the existence of the astrophysically important 6.40-MeV state of31S

Proton-unbound excited states of ${}^{31}$S have been populated with the ${}^{32}$S($d,t$)${}^{31}$S reaction at a beam energy of 24 MeV. Tritons corresponding to ${}^{31}$S states with ${E}_{x}{(}^{31}S)\ensuremath{\approx}6.3$--$7.1$ MeV were momentum analyzed with a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph at six angles ranging from ${\ensuremath{\theta}}_{\mathrm{lab}}={15}^{\ensuremath{\circ}}$ to $58.{5}^{\ensuremath{\circ}}$. We report a statistically significant detection of an astrophysically important state at ${E}_{x}{(}^{31}S)=6402$ (2) keV, whose existence as a third state in this region has been under debate. Using updated $A=31$ nuclear structure information, we present a new set of proposed ${}^{31}$S-${}^{31}$P mirror assignments for ${}^{31}$S, in which this state is tentatively assigned a spin of 7/2. This level, corresponding to a ${}^{30}$P + $p$ resonance at 271 keV, is likely to have a significant influence on the ${}^{30}$P($p,\ensuremath{\gamma}$)${}^{31}$S reaction rate in explosive hydrogen burning in classical novae.

Resonant scattering of22Na + pstudied by the thick-target inverse-kinematic method

Background: In presolar low-density graphite grains, an extraordinarily large ${}^{22}$Ne/${}^{20}$Ne ratio or even nearly pure ${}^{22}$Ne is found, pointing to the condensation of radioactive ${}^{22}$Na in grains. Supernovae and neon-rich novae are the main events that produce ${}^{22}$Na via the explosive hydrogen burning process. The ${}^{22}$Na($p$, $\ensuremath{\gamma}$)${}^{23}$Mg reaction is one of the key reactions that influences the ${}^{22}$Na abundance in ejecta.Purpose:The present work aims to explore the proton resonant states in ${}^{23}$Mg relevant to the astrophysical ${}^{22}$Na($p$, $\ensuremath{\gamma}$)${}^{23}$Mg reaction. The determined ${}^{23}$Mg resonant parameters can be used to evaluate the ${}^{22}$Na($p$, $\ensuremath{\gamma}$)${}^{23}$Mg reaction rate.Method:A low-energy ${}^{22}$Na radioactive ion beam is produced via the ${}^{1}$H(${}^{22}$Ne, ${}^{22}$Na)$n$ reaction, and used to measure the experimental excitation function of the ${}^{22}$Na + $p$ resonant scattering with a conventional thick-target inverse kinematic method. $R$-matrix analysis is applied to deduce the ${}^{23}$Mg resonance parameters from the experimental excitation function.Results: Three proton resonance states in ${}^{23}$Mg are observed. Spins/parities and the proton partial widths are determined. The deduced excitation energies agree with the compiled values.Conclusions: The new spin and parity assignments allow us to perform a shell-model calculation of the $\ensuremath{\gamma}$ widths of the ${}^{23}$Mg resonant states for the evaluation of the ${}^{22}$Na($p$, $\ensuremath{\gamma}$)${}^{23}$Mg astrophysical reaction rate. The two $s$-wave resonant states established in this work at 8.793 and 8.916 MeV in ${}^{23}$Mg, respectively, increase the total reaction rate by about 5% at a temperature greater than 2 GK.

Level structure of30S and its importance in the26Si(α,p)29P and29P(p,γ)30S reaction rates

The level structure of ${}^{30}$S was studied via the ${}^{28}$Si(${}^{3}$He,n) and ${}^{32}$S(p,t) reactions at the Nuclear Science Laboratory of the University of Notre Dame and the Research Center for Nuclear Physics of the University of Osaka, Japan. Important experimental information on the energy levels, decay branching ratios, and tentative spin assignments is extracted to calculate the reaction rates for ${}^{29}$P(p,$\ensuremath{\gamma}$)${}^{30}$S and ${}^{26}$Si($\ensuremath{\alpha}$,p)${}^{29}$P, which play a critical role in the reaction flow in explosive hydrogen burning.

One-proton breakup of24Si and the23Al(p,γ)24Si reaction in type I x-ray bursts

Background: To understand explosive hydrogen burning in stars and to explore various explosive scenarios such as type I x-ray bursts (XRBs), reliable reaction rates are needed. The cross sections for radiative proton capture on near-dripline nuclei are necessary for the determination of the reaction rates, but cannot be measured directly.Purpose: To determine the reaction rate for the radiative proton capture reaction ${}^{23}\mathrm{Al}{(p,\ensuremath{\gamma})}^{24}\mathrm{Si}$ using indirect methods and, as a consequence, evaluate if sequential $2p$ capture on ${}^{22}$Mg seed nuclei is significant at high temperatures.Method: Nonresonant radiative proton capture on ${}^{23}$Al is investigated using the one-proton breakup of ${}^{24}$Si at 61 MeV/nucleon and the asymptotic normalization coefficient (ANC) for ${}^{24}{\mathrm{Si}}_{\mathrm{gs}}\ensuremath{\rightarrow}{}^{23}\mathrm{Al}+p$ is deduced.Results: From the ANC, the nonresonant component of the astrophysical $S$-factor for the ${}^{23}\mathrm{Al}{(p,\ensuremath{\gamma})}^{24}\mathrm{Si}$ reaction is determined and, using other new experimental data the resonant component is re-evaluated.Conclusions: The ${}^{23}\mathrm{Al}{(p,\ensuremath{\gamma})}^{24}\mathrm{Si}$ reaction is of interest for type I XRB nucleosynthesis and its reaction rate can affect both the ${}^{22}$Na abundance and the total energy output. New determinations of the rates for the ${}^{22}\mathrm{Mg}{(p,\ensuremath{\gamma})}^{23}\mathrm{Al}{(p,\ensuremath{\gamma})}^{24}\mathrm{Si}$ reaction chain are provided here and we point to the need that they be included in XRB scenarios.

Novae - The study of the reactive flow

There is a wide consensus in the astrophysics community that the mechanism underlying the observed Classical Nova eruptions is a surface thermonuclear runaway. We start this short review with the main observational facts that lead to the theoretical model of a thermonuclear runaway that takes place in an accreted hydrogen rich envelope placed on top of a cool degenerate core of a white dwarf. According to the theory, the accreted envelope becomes unstable to convection days to weeks prior to the runaway. During the extreme stages of the runaway itself, when the burning is most efficient, the envelope is fully convective. Therefore, the elements processed under such extreme conditions are lifted to the outermost regions of the star. A significant fraction of the envelope is ejected during the outburst. The complicated combination of hydrodynamic instabilities and explosive hydrogen burning, close to the surface of the star, gives us a unique opportunity to study this complex reactive flow. The range of core masses, core temperatures and accretion rates introduce a whole range of burning temperatures and densities. Following the description of the "standard" cases, we then focus on rare, but still possible, portions of the relevant parameter space, in which "breakout" of the traditional CNO cycle can occur and lead to heavy element enrichment patterns caused only by breakout burning. We conclude our review with the main challenges that nova theorists face today, with special emphasis on problems related to the nucleosynthesis issues.

Alpha-Cluster Dominance in the $\alpha$p Process in Explosive Hydrogen Burning

1Center for Nuclear Study, University of Tokyo, Wako 351-0198, Japan 2Institute of Physics, VAST, Hanoi, Vietnam 3Japan Atomic Energy Agency, Tokai, Ibaraki 351-1195, Japan 4Department of Physics, Kyushu University, Fukuoka 812-8581, Japan 5Department of Physics, Tohoku University, Sendai 980-8578, Japan 6Department of Physics, Tsukuba University, Tsukuba 305-8571, Japan 7Faculty of Science, Yamagata University, Yamagata 990-8560, Japan 8Department of Physics, University of McMaster, Canada 9Universita di Catania and INFN-LNS, Catania, Italy 10Department of Physics, Seoul National University, Seoul, Korea 11Ewha Womans’ University, Seoul, Korea 12Institute of Modern Physics, Lanzhou, China 13Department of Physics, Chung Ang University, Seoul, Korea 14Max-Planck-Institut fur Astrophysik, D-85748 Garching, Germany