Nova Nucleosynthesis Research Articles

Trojan Horse measurement of the 18F(p, $\alpha$ )15O astrophysical S(E)-factor

Crucial information on novae nucleosynthesis is linked to the abundance of 18F , which, due to great improvements in gamma-ray astronomy, can be detected in explosive environments. Therefore, the reaction network producing and destroying this radioactive isotope has been extensively studied in the last years. Among those reactions, the 18F(p,\(\alpha\))15O cross section has been measured by means of several dedicated experiments, both using direct and indirect methods. The presence of resonances in the energy region of astrophysical interest has been reported by many authors. In the present work a report on a recent experiment performed via the Trojan Horse Method (THM) is presented and the results are given and compared with the ones known in the literature, both direct and indirect. Data arising from THM measurements are then averaged and the reaction rate calculated in the novae energy range.

Nova nucleosynthesis and production of the radioisotope 18F

Novae are cataclysmic stellar explosions driven by nuclear physics under extreme conditions. An important probe of nova conditions could come from the observation of long- lived radioisotopes such as 18F that are produced during the explosion process. To interpret such observations, however, the rates of reactions creating and destroying 18F need to be determined. A variety of direct and indirect studies have greatly reduced uncertainties in nova 18F production. Recent results are reviewed along with prospects for future progress.

Statistical methods for thermonuclear reaction rates and nucleosynthesis simulations

Rigorous statistical methods for estimating thermonuclear reaction rates and nucleosynthesis are becoming increasingly established in nuclear astrophysics. The main challenge being faced is that experimental reaction rates are highly complex quantities derived from a multitude of different measured nuclear parameters (e.g., astrophysical S-factors, resonance energies and strengths, particle and γ-ray partial widths). We discuss the application of the Monte Carlo method to two distinct, but related, questions. First, given a set of measured nuclear parameters, how can one best estimate the resulting thermonuclear reaction rates and associated uncertainties? Second, given a set of appropriate reaction rates, how can one best estimate the abundances from nucleosynthesis (i.e., reaction network) calculations? The techniques described here provide probability density functions that can be used to derive statistically meaningful reaction rates and final abundances for any desired coverage probability. Examples are given for applications to s-process neutron sources, core-collapse supernovae, classical novae, and Big Bang nucleosynthesis.

β Decay as a Probe of Explosive Nucleosynthesis in Classical Novae

Classical novae are common thermonuclear explosions in the Milky Way galaxy, occurring on the surfaces of white-dwarf stars that are accreting hydrogen-rich material from companion stars. Nucleosynthesis in classical novae depends on radiative proton-capture reaction rates on radioactive nuclides. Many of these reactions cannot be measured directly at current accelerator facilities due to the lack of intense, high-quality, radioactive-ion beams at the relevant energies. Since most of these reactions proceed via resonant capture, their rates can be determined indirectly by measuring the properties of the resonances. At the National Superconducting Cyclotron Laboratory, we have used the β-delayed γ decays of 26P and 31Cl to populate resonances in 26Si and 31S and study the radiative proton captures on 25Al and 30P, respectively. These were two out of the three most important nuclear-physics uncertainties associated with the observable products of nova nucleosynthesis. The 26P experiment has enabled a more accurate estimate of the nova contribution to the long-lived Galactic 26Al detected with γ-ray telescopes. The 31Cl experiment, currently under analysis, will calibrate potential nova thermometers and mixing meters based on elemental abundance ratios, and facilitate the identification of pre-solar nova grain candidates found in primitive meteorites based on isotopic ratios.

Isotopic 32S/33S ratio as a diagnostic of presolar grains from novae

Measurements of sulphur isotopes in presolar grains can help to identify the astrophysical sites in which these grains were formed. A more precise thermonuclear rate of the 33S(p,γ)34Cl reaction is required, however, to assess the diagnostic ability of sulphur isotopic ratios. We have studied the 33S(3He,d)34Cl proton-transfer reaction at 25 MeV using a high-resolution quadrupole–dipole–dipole–dipole magnetic spectrograph. Deuteron spectra were measured at ten scattering angles between 10° and 55°. Twenty-four levels in 34Cl over Ex=4.6–5.9 MeV were observed, including three levels for the first time. Proton spectroscopic factors were extracted for the first time for levels above the 33S + p threshold, spanning the energy range required for calculations of the thermonuclear 33S(p,γ)34Cl rate in classical nova explosions. We have determined a new 33S(p,γ)34Cl rate using a Monte Carlo method and have performed new hydrodynamic nova simulations to determine the impact on nova nucleosynthesis of remaining nuclear physics uncertainties in the reaction rate. We find that these uncertainties lead to a factor of ≤5 variation in the 33S(p,γ)34Cl rate over typical nova peak temperatures, and variation in the ejected nova yields of SCa isotopes by ≤20%. In particular, the predicted 32S/33S ratio is 110–130 for the nova model considered, compared to 110–440 with previous rate uncertainties. As recent type II supernova models predict ratios of 130–200, the 32S/33S ratio may be used to distinguish between grains of nova and supernova origin.

MESA and NuGrid simulations of classical novae: CO and ONe nova nucleosynthesis

Classical novae are the result of thermonuclear flashes of hydrogen accreted by CO or ONe white dwarfs, leading eventually to the dynamic ejection of the surface layers. These are observationally known to be enriched in heavy elements, such as C, O and Ne that must originate in layers below the H-flash convection zone. Building on our previous work, we now present stellar evolution simulations of ONe novae and provide a comprehensive comparison of our models with published ones. Some of our models include exponential convective boundary mixing to account for the observed enrichment of the nova ejecta even when accreted material has a solar abundance distribution. Our models produce maximum temperature evolution profiles and nucleosynthesis yields in good agreement with models that generate enriched ejecta by assuming that the accreted material was pre-mixed. We confirm for ONe novae the result we reported previously, i.e.\ we found that $^3$He could be produced {\it in situ} in solar-composition envelopes accreted with slow rates ($\dot{M} < 10^{-10}\,M_\odot/\mbox{yr}$) by cold ($T_{\rm WD} < 10^7$ K) CO WDs, and that convection was triggered by $^3$He burning before the nova outburst in that case. In addition, we now find that the interplay between the $^3$He production and destruction in the solar-composition envelope accreted with an intermediate rate, e.g.\ $\dot{M} = 10^{-10}\,M_\odot/\mbox{yr}$, by the $1.15\,M_\odot$ ONe WD with a relatively high initial central temperature, e.g.\ $T_{\rm WD} = 15\times 10^6$ K, leads to the formation of a thick radiative buffer zone that separates the bottom of the convective envelope from the WD surface. (Abridged)

The 30P(p, γ)31S reaction in classical novae: progress and prospects

The unknown thermonuclear rate of the 30P(p, γ)31S reaction at classical-nova temperatures currently prohibits the accurate modeling of nova nucleosynthesis in the A ⩾ 30 region. This is hindering the calibration of nova thermometers based on observed O/S, S/Al, O/P, and P/Al abundance ratios in nova ejecta, the calibration of a meter to probe mixing at the core-envelope interface in novae based on the observed Si/H abundance ratio, and the identification of candidate pre-solar nova grains found in primitive meteorites based on laboratory measurements of their 30Si/28Si isotopic ratios. Each of these diagnostics could address key questions in our understanding of classical novae if the 30P(p, γ)31S rate were known. We review progress on the determination of the 30P(p, γ)31S rate leading to a critical assessment of current interpretations of published data and prospects for future work.

The unreasonable effectiveness of experiments in constraining nova nucleosynthesis

Classical nova explosions arise from thermonuclear ignition in the envelopes of accreting white dwarfs in close binary star systems. Detailed observations of novae have stimulated numerous studies in theoretical astrophysics and experimental nuclear physics. These phenomena are unusual in nuclear astrophysics because most of the thermonuclear reaction rates thought to be involved are constrained by experimental measurements. This situation allows for rather precise statements to be made about which measurements are still necessary to improve the nuclear physics input to astrophysical models. We briefly discuss desired measurements in these environments with an emphasis on recent experimental progress made to better determine key rates.

Constraining nova observables: Direct measurements of resonance strengths in33S(p,γ)34Cl

The 33S(p,\gamma)34Cl reaction is important for constraining predictions of certain isotopic abundances in oxygen-neon novae. Models currently predict as much as 150 times the solar abundance of 33S in oxygen-neon nova ejecta. This overproduction factor may, however, vary by orders of magnitude due to uncertainties in the 33S(p,\gamma)34Cl reaction rate at nova peak temperatures. Depending on this rate, 33S could potentially be used as a diagnostic tool for classifying certain types of presolar grains. Better knowledge of the 33S(p,\gamma)34Cl rate would also aid in interpreting nova observations over the S-Ca mass region and contribute to the firm establishment of the maximum endpoint of nova nucleosynthesis. Additionally, the total S elemental abundance which is affected by this reaction has been proposed as a thermometer to study the peak temperatures of novae. Previously, the 33S(p,\gamma)34Cl reaction rate had only been studied directly down to resonance energies of 432 keV. However, for nova peak temperatures of 0.2-0.4 GK there are 7 known states in 34Cl both below the 432 keV resonance and within the Gamow window that could play a dominant role. Direct measurements of the resonance strengths of these states were performed using the DRAGON recoil separator at TRIUMF. Additionally two new states within this energy region are reported. Several hydrodynamic simulations have been performed, using all available experimental information for the 33S(p,\gamma)34Cl rate, to explore the impact of the remaining uncertainty in this rate on nucleosynthesis in nova explosions. These calculations give a range of ~ 20-150 for the expected 33S overproduction factor, and a range of ~ 100-450 for the 32S/33S ratio expected in ONe novae.

Low-energy radioactive ion beam production of 22Mg

The 22Mg nucleus plays an important role in nuclear astrophysics, specially in the 22Mg(α,p)25Al and proton capture 22Mg(p,γ)23Al reactions. It is believed that 22Mg is a waiting point in the αp-process of nucleosynthesis in novae. We proposed a direct measurement of the 22Mg+α resonance reaction in inverse kinematics using a radioactive ion (RI) beam. A 22Mg beam of 3.73MeV/u was produced at CRIB (Center for Nuclear Study (CNS) low-energy RI Beam) facility of the University of Tokyo located at RIKEN (Japan) in 2011. In this paper we present the results about the production of the 22Mg beam used for the direct measurement of the scattering reaction 22Mg(α,α)22Mg, and the stellar reaction 22Mg(α,p)25Al in the energy region concerning an astrophysical temperature of T9=1–3GK.

Recommendations for Monte Carlo nucleosynthesis sampling

Context: Recent reaction rate evaluations include reaction rate uncertainties that have been determined in a statistically meaningful manner. Furthermore, reaction rate probability density distributions have been determined and published in the form of lognormal parameters with the specific goal of pursuing Monte Carlo nucleosynthesis studies. Aims: To test and assess different methods of randomly sampling over reaction rate probability densities and to determine the most accurate method for estimating elemental abundance uncertainties. Methods: Experimental Monte Carlo reaction rates are first computed for the 22Ne+alpha, 20Ne(p,g)21Na, 25Mg(p,g)26Al, and 18F(p,alpha)15O reactions, which are used to calculate reference nucleosynthesis yields for 16 nuclei affected by nucleosynthesis in massive stars and classical novae. Five different methods of randomly sampling over these reaction rate probability distributions are then developed, tested, and compared with the reference nucleosynthesis yields. Results: Given that the reaction rate probability density distributions can be described accurately with a lognormal distribution, Monte Carlo nucleosynthesis variations arising from the parametrised estimates for the reaction rate variations agree remarkably well with those obtained from the true rate samples. Most significantly, the most simple parametrisation agrees within just a few percent, meaning that Monte Carlo nucleosynthesis studies can be performed reliably using lognormal parametrisations of reaction rate probability density functions.

First Direct Measurement of theO17(p,γ)F18Reaction Cross Section at Gamow Energies for Classical Novae

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive (18)F, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The (17)O(p,γ)(18)F reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes, but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range E(c.m.)~/= 200-370 keV appropriate to hydrogen burning in classical novae. In addition, the E(c.m.)=183 keV resonance strength, ωγ=1.67±0.12 μeV, has been measured with the highest precision to date. The uncertainty on the (17)O(p,γ)(18)F reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis.

Nuclear Physics Constraints on the Characteristics of Astrophysical Thermonuclear Flashes

We review the nuclear physics that is associated with the outbursts of Type Ia (thermonuclear) supernova explosions and with the thermonuclear runaway events that define the outbursts of both classical novae and recurrent novae. We describe how distinguishing characteristics of these two classes of astrophysical explosion are strongly dependent both upon fuel ignition in degenerate matter and upon the rates of critical charged-particle reaction rates and weak interaction rates. In this centennial celebration of the important contributions of Rutherford and his collaborators to our understanding of the structure of the nucleus of an atom, it is quite interesting to note the evolution of the α-particle scattering experiments described in Rutherford's seminal paper (Rutherford 1911) to current studies of α-particle induced reactions and their defining roles in studies of stellar, nova, and supernova nucleosynthesis. We identify and discuss for example: (1) the manner in which (α, p) reactions in proximity to the Z = N line carry the major flows from 12C and 16O to 56Ni in Type Ia supernovae; and (2) the critical role of the 15O(α, γ)19Ne reaction in possibly effecting "breakout" of the Hot CNO cycles at the highest temperatures achievable in Classical Novae. In this contribution, we first review the current status our understanding of Type Ia supernova events and then that of Classical Novae.

Proton capture reaction of 33S in nova V1065 Centauri

Previous studies have shown that 33S(p, γ)34Cl is the most important reaction that affects the abundance of 33S in the product of nova nucleosynthesis. In this paper, a more accurate thermonuclear reaction rate of 33S(p, γ)34Cl in the nova is calculated based on the newly measured 34Cl nuclear resonance levels. The electron screening correction and the non-resonance and narrow-resonance contributions are considered. The calculations are also combined with the recent observational data of nova V1065 Centauri and show that the thermonuclear reaction rates of 33S(p, γ) 34Cl are significantly different in the improved method. Because these results can affect the isotopic ratio of sulfur in the nova ejecta significantly, we make an estimate of the values of 32S/33S and 33S/33S⊙, which can be used as a diagnostic tool for the novae.

Structure of23Al from the one-proton breakup reaction and astrophysical implications

The ground state of the proton-rich nucleus 23Al has been studied by one-proton removal on a carbon target at about 50 MeV/nucleon using the EXOGAM + SPEG experimental setup at GANIL. Longitudinal momentum distributions of the 22Mg breakup fragments, inclusive and in coincidence with gamma rays de-exciting the residues, were measured. The ground-state structure of 23Al is found to be a configuration mixing of a d-orbital valence proton coupled to four core states - 0$^{+}_{gs}$, 2$^{+}_{1}$, 4$^{+}_{1}$, 4$^{+}_{2}$. We confirm the ground state spin and parity of 23Al as $J^{\pi} = 5/2^{+}$. The measured exclusive momentum distributions are compared with extended Glauber model calculations to extract spectroscopic factors and asymptotic normalization coefficients (ANCs). The spectroscopic factors are presented in comparison with those obtained from large-scale shell model calculations. We determined the asymptotic normalization coefficient of the nuclear system $^{23}$Al$_{gs}$ $\rightarrow$ $^{22}$Mg(0$^{+}$) + p to be $C^{2}_{d_{5/2}}$($^{23}Al_{gs}$) = (3.90 $\pm$ 0.44) $\times$ 10$^{3}$ fm$^{-1}$, and used it to infer the stellar reaction rate of the direct radiative proton capture $^{22}$Mg(p,$\gamma$)$^{23}$Al. Astrophysical implications related to $^{22}$Na nucleosynthesis in ONe novae and the use of one-nucleon breakup at intermediate energies as an indirect method in nuclear astrophysics are discussed.

Improving theP30(p,γ)S31rate in oxygen-neon novae: Constraints onJπvalues for proton-threshold states inS31

Calculation of the thermonuclear $^{30}\mathrm{P}$($p$,\ensuremath{\gamma})$^{31}\mathrm{S}$ rate in oxygen-neon nova explosions depends critically upon nuclear structure information for states within \ensuremath{\sim}600 keV of the ${}^{30}\mathrm{P}+p$ threshold in $^{31}\mathrm{S}$. We have studied the $^{31}\mathrm{P}$(${}^{3}\mathrm{He},t$)$^{31}\mathrm{S}$ reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Tritons corresponding to the states ${E}_{x}$($^{31}\mathrm{S}$) \ensuremath{\sim} 6.1--7.1 MeV were observed at ten angles between ${\ensuremath{\theta}}_{\mathrm{lab}}$ $=$ 10\ifmmode^\circ\else\textdegree\fi{} and 55\ifmmode^\circ\else\textdegree\fi{}. States that were only tentatively identified in past studies have been observed unambiguously. For the first time, we have measured and analyzed angular distributions of the $^{31}\mathrm{P}$(${}^{3}\mathrm{He},t$)$^{31}\mathrm{S}$ reaction. We present, also for the first time, a consistent set of experimental spin constraints for all except one of the critical proton-threshold states in $^{31}\mathrm{S}$. Hydrodynamic nova simulations have been calculated in order to assess the impact on nova nucleosynthesis of remaining uncertainties in ${J}^{\ensuremath{\pi}}$ values of $^{31}\mathrm{S}$ states and the unknown relevant proton spectroscopic factors. We find that these uncertainties may lead to a factor of up to 20 variation in the $^{30}\mathrm{P}$($p$,\ensuremath{\gamma})$^{31}\mathrm{S}$ rate over typical nova peak temperatures, which may then lead to a factor of up to 4 variation in the nova yields of Si-Ar isotopes.

Indirect nuclear physics techniques for studying nova nucleosynthesis

Indirect nuclear physics techniques for studying nova nucleosynthesis

Remeasurement of the 193 keV resonance inO17(p,α)N14

A recently discovered resonance at 193 keV determines the thermonuclear rates of the {sup 17}O +p reactions at temperatures important for the nucleosynthesis in classical novae (T=0.1-0.4 GK). We report on a remeasurement of this resonance in the {sup 17}O(p,{alpha}){sup 14}N reaction by using a different kind of target compared to the previous study. Special emphasis is placed on Monte Carlo simulations of the experiment in order to better understand certain effects that have been disregarded previously. Our measured value of the resonance strength amounts to ({omega}{gamma}){sub p{alpha}}=(1.66{+-}0.17)x10{sup -3} eV, in agreement with the previously reported result. As a byproduct of our study, we find that the inhomogeneity of the foil placed in front of the {alpha}-particle detector determines the resolution in the pulse-height spectrum, and thus constrains the signal-to-noise ratio in searches of very weak (p,{alpha}) resonances.

Experimental determination of theO17(p,α)N14andO17(p,γ)F18reaction rates

The $^{17}\mathrm{O}$($p,\ensuremath{\alpha}$)$^{14}\mathrm{N}$ and $^{17}\mathrm{O}$($p,\ensuremath{\gamma}$)$^{18}\mathrm{F}$ reactions are of major importance to hydrogen-burning nucleosynthesis in a number of different stellar sites. In particular, $^{17}\mathrm{O}$ and $^{18}\mathrm{F}$ nucleosynthesis in classical novae is strongly dependent on the thermonuclear rates of these two reactions. The previously estimated rate for $^{17}\mathrm{O}$($p,\ensuremath{\alpha}$)$^{14}\mathrm{N}$ carries very large uncertainties in the temperature range of classical novae ($T=0.01$--0.4 GK), whereas a recent measurement has reduced the uncertainty of the $^{17}\mathrm{O}$($p,\ensuremath{\gamma}$)$^{18}\mathrm{F}$ rate. We report on the observation of a previously undiscovered resonance at ${E}_{c.m.}=183.3$ keV in the $^{17}\mathrm{O}$($p,\ensuremath{\alpha}$)$^{14}\mathrm{N}$ reaction, with a measured resonance strength $\ensuremath{\omega}{\ensuremath{\gamma}}_{p\ensuremath{\alpha}}=(1.6\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ eV. We studied in the same experiment the $^{17}\mathrm{O}$($p,\ensuremath{\gamma}$)$^{18}\mathrm{F}$ reaction by an activation method, and the resonance strength was found to amount to $\ensuremath{\omega}{\ensuremath{\gamma}}_{p\ensuremath{\gamma}}=(2.2\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ eV. The excitation energy of the corresponding level in $^{18}\mathrm{F}$ was determined to be $5789.8\ifmmode\pm\else\textpm\fi{}0.3$ keV in a Doppler shift attenuation method measurement, which yielded a value of $\ensuremath{\tau}&lt;2.6$ fs for the level lifetime. The $^{17}\mathrm{O}$($p,\ensuremath{\alpha}$)$^{14}\mathrm{N}$ and $^{17}\mathrm{O}$($p,\ensuremath{\gamma}$)$^{18}\mathrm{F}$ reaction rates were calculated using the measured resonance properties and reconsidering some previous analyses of the contributions of other levels or processes. The $^{17}\mathrm{O}$($p,\ensuremath{\alpha}$)$^{14}\mathrm{N}$ rate is now well established below $T=1.5$ GK, with uncertainties reduced by orders of magnitude in the temperature range $T=0.1$--0.4 GK. The uncertainty in the $^{17}\mathrm{O}$($p,\ensuremath{\gamma}$)$^{18}\mathrm{F}$ rate is somewhat larger because of remaining obscurities in the knowledge of the direct capture process. These new resonance properties have important consequences for $^{17}\mathrm{O}$ nucleosynthesis and \ensuremath{\gamma}-ray emission of classical novae.

Segmented linear radiofrequency quadrupole/laser ion source project at TRIUMF

Reactions such as 25 Al(p,γ ) 26 Si are the key to understand the production of 26g Al and 26m Al in our galaxy. Experimental results could provide important constraints on nova nucleosynthesis and modelling where 26 Al is believed to be produced. To achieve such measurements, high-intensity and high-purity radioactive beams are required. However, production targets at ISOL-type facilities such as ISAC at TRIUMF produce high-intensity alkali beams by surface ionization on hot transfer tubes hampering the measurement of isotopes of interest. To overcome this issue, an ion source combining a segmented linear radiofrequency quadrupole (RFQ) to a laser ion source is being built. Its main function is to suppress alkali impurities whilst allowing for fast-release of short-live di sotopes. The beam production method, the RFQ/laser ion source and the removal of alkali contaminants are discusse di n this paper.