Stellar Burning Research Articles

Sub-Coulomb nuclear studies using Indirect Methods: Recent results with the Trojan Horse Method

Our knowledge on how stars evolve depends on understanding two key factors: energy production and chemical evolution. Both factors are determined by thermonuclear reactions. Unfortunately, directly measuring them in a laboratory can be challenging: often, the reaction cross section are too small or require radioactive targets that are impractical to work with. To overcome these challenges, indirect techniques have been developed. One such technique, the Trojan Horse Method, makes use of transfer reactions to determine cross sections of reactions relevant in stellar burning processes. Here we will explore the Trojan Horse Method in the relevant details, explaining how it is used in nuclear astrophysics and we will delve into examples of THM measurements.

Nuclear astrophysics studies with the Trojan Horse Method

Our understanding of how stars evolve relies on two critical factors: energy production and chemical evolution. These factors are influenced by thermonuclear reactions. However, directly measuring these reactions in a laboratory setting can be challenging due to small reaction cross sections or the need for radioactive targets that are impractical to handle. To overcome these challenges, scientists have developed indirect techniques. One such method is the Trojan Horse Method (THM), which utilizes transfer reactions to determine cross sections of reactions relevant to stellar burning processes. In this paper, we will delve into the features of the Trojan Horse Method, its application in nuclear astrophysics, and provide examples of THM measurements.

Nuclear astrophysicists at war

AbstractThe question of energy production in stars stimulated an entire generation of young physicists in the 1930s who came to work in this field exploring the fundamentals of quantum and nuclear physics. Their experience and methodologies were essential to the Manhattan Project, facilitating the rapid development of the atomic bomb. The experience and knowledge gained from the Manhattan Project then flowed back to nuclear astrophysics after the war and led to its further development. This paper is motivated by the question that was raised in the film Oppenheimer, which asks whether “a bomb can set the atmosphere on fire?”. Seeking an answer requires a close intellectual exchange between the physics of the atomic bomb and the physics of stellar burning; this exchange is the topic of this paper.

Carbon burning at stellar energies

Fusion reactions with light nuclei play an essential role in understanding the energy production, the nucleosynthesis of chemical elements and the evolution of massive stars. The measurement of key fusion reactions at stellar energies is thus of interest, but highly challenging since the associated cross sections are extremely small, of the sub-nanobarn range. Among these reactions, the fusion of carbon nuclei, which drives the stellar carbon burning phase, is deeply connected with essential microscopic features such as the impact of symmetries, the access to quantum states, emerging of resonances or Pauli repulsion effects. These may manifest themselves in exceptional behaviour of the S-factor of this particular system and the precision of extrapolations to deep sub-barrier energies is limited. The present contribution discusses recent experimental results of the nuclear astrophysics community on the measurement of the carbon + carbon fusion reaction down to the astrophysics region. The interplay between nuclear structure, nucleosynthesis and stellar evolution is addressed.

A novel fractal-fractional analysis of the stellar helium burning network using extended operational matrix method

The second stage, in which the star uses nuclear fuel in its interior, represents the helium burning phase. At that stage, three elements are synthesised: carbon, oxygen, and neon. This paper aims to establish a numerical solution for the helium burning system (HBN) fractal-fractional differential equations (FFDEs). The extended operative matrix method (OM) is employed in the solution of a system of differential equations. The product abundances of the four elements (helium, carbon, oxygen and neon) were obtained in a form of divergent series. These divergent series are then accelerated using Euler-Abell transformation (EUAT) and Pade approximation (EUAT-PA) to obtain more reliable results. Nine fractal-fractional (FF) gas models are calculated, and fractal-fractional parameters’ influence on product abundances is discussed. The findings show that modeling nuclear burning networks with the OM fractal-fractional derivative produces excellent results, establishing it as an accurate, resilient, and trustworthy approach, and the fractional HB models can have a considerable impact on stellar model calculations.

The resonances in the ^{22}Ne+alpha fusion reactions

The interplay and correlation between the ^{22}Ne(alpha ,gamma )^{26}Mg and the competing ^{22}Ne(alpha ,n)^{25}Mg reaction plays an important role for the interpretation of the ^{22}Ne(alpha ,n)^{25}Mg reaction as a neutron source in the s- and n-processes. This paper provides a summary and new data on the alpha -cluster and single-particle structure of the compound nucleus ^{26}Mg and the impact on the reaction rate of these two competing processes in stellar helium burning environments.

Nuclear data - an essential tool in nuclear astrophysics

Nuclear processes play an essential role for the evolution of many astrophysical objects and they are key to the origin of the elements in the Universe. Our understanding of the Universe has benefitted from the tremendous progress in nuclear physics which became possible due to novel experimental facilities and improved instrumentation as well as due to advances in theoretical modelling. The talk exemplifies this progress for three selected topics: solar and stellar hydrostatic burning, neutron-star mergers as a site of heavy-element production by the r-process and the influence of electron capture on nuclei for the core-collapse in massive stars leading to supernova explosions.

Measurement of 19F(p, γ)20Ne reaction suggests CNO breakout in first stars.

Proposed mechanisms for the production of calcium in the first stars (population III stars)-primordial stars that formed out of the matter of the Big Bang-are at odds with observations1. Advanced nuclear burning and supernovae were thought to be the dominant source of the calcium production seen in all stars2. Here we suggest a qualitatively different path to calcium production through breakout from the 'warm' carbon-nitrogen-oxygen (CNO) cycle through a direct experimental measurement of the 19F(p, γ)20Ne breakout reaction down to a very low energy point of 186 kiloelectronvolts, reporting a key resonance at 225 kiloelectronvolts. In the domain of astrophysical interest2, at around 0.1 gigakelvin, this thermonuclear 19F(p, γ)20Ne rate is up to a factor of 7.4 larger than the previous recommended rate3. Our stellar models show a stronger breakout during stellar hydrogen burning than previously thought1,4,5, and may reveal the nature of calcium production in population III stars imprinted on the oldest known ultra-iron-poor star, SMSS0313-67086. Our experimental result was obtained in the China JinPing Underground Laboratory7, which offers an environment with an extremely low cosmic-ray-induced background8. Our rate showcases the effect that faint population III star supernovae can have on the nucleosynthesis observed in the oldest known stars and first galaxies, which are key mission targets of the James Webb Space Telescope9.

Direct measurement of the low-energy resonances in Ne22(α,γ)Mg26 reaction

The $^{22}\rm{Ne}(\alpha,\gamma)^{26}\rm{Mg}$ is an important reaction in stellar helium burning environments as it competes directly with one of the main neutron sources for the s-process, the $^{22}\rm{Ne}(\alpha,n)^{25}\rm{Mg}$ reaction. The reaction rate of the $^{22}\rm{Ne}(\alpha,\gamma)^{26}\rm{Mg}$ is dominated by the low energy resonances at $E_{\alpha}^{lab}$ = 650 and 830 keV respectively. The $E_{\alpha}^{lab}$ = 830 keV resonance has been measured previously, but there are some uncertainties in the previous measurements. We confirmed the measurement of the $E_{\alpha}^{lab}$ = 830 keV resonance using implanted $^{22}$Ne targets. We obtained a resonance strength of $\omega\gamma$ = 35 $\pm$ 4 $\mu eV$, and provide a weighted average of the present and previous measurements of $\omega\gamma$ = 35 $\pm$ 2 $\mu eV$ with reduced uncertainties compared to previous studies. We also attempted to measure the strength of the predicted resonance at $E_{\alpha}^{lab}$ = 650 keV directly for the first time and found an upper limit of $\omega\gamma$ $\mathrm{<0.15}$ $\mu eV$ for the strength of this resonance. In addition, we also studied the $E_{P}^{lab}$= 851 keV resonance in $^{22}\rm{Ne}(p,\gamma)^{23}\rm{Na}$, and obtained a resonance strength of $\omega\gamma$ = 9.2 $\pm$ 0.7 eV with significantly lower uncertainties compared to previous measurements.

Formation of black holes in the pair-instability mass gap: Evolution of a post-collision star

ABSTRACT The detection of GW190521 by the LIGO–Virgo collaboration has revealed the existence of black holes (BHs) in the pair-instability (PI) mass gap. Here, we investigate the formation of BHs in the PI mass gap via star–star collisions in young stellar clusters. To avoid PI, the stellar-collision product must have a relatively small core and a massive envelope. We generate our initial conditions from the outputs of a hydrodynamical simulation of the collision between a core helium burning star (∼58 M⊙) and a main-sequence star (∼42 M⊙). The hydrodynamical simulation allows us to take into account the mass lost during the collision (∼12 M⊙) and to build the chemical composition profile of the post-collision star. We then evolve the collision product with the stellar evolution codes parsec and mesa. We find that the post-collision star evolves through all the stellar burning phases until core collapse, avoiding PI. At the onset of core collapse, the post-collision product is a blue supergiant star. We estimate a total mass-loss of about 1 M⊙ during the post-collision evolution, due to stellar winds and shocks induced by neutrino emission in a failed supernova. The final BH mass is ≈87 M⊙. Therefore, we confirm that the collision scenario is a suitable formation channel to populate the PI mass gap.

The Effect of Supernova Convection On Neutron Star and Black Hole Masses

Our understanding of the convective-engine paradigm driving core-collapse supernovae has been used for two decades to predict the remnant mass distribution from stellar collapse. These predictions improve as our understanding of this engine increases. In this paper, we review our current understanding of convection (in particular, the growth rate of convection) in stellar collapse and study its effect on the remnant mass distribution. We show how the depth of the mass gap between neutron stars and black holes can help probe this convective growth. We include a study of the effects of stochasticity in both the stellar structure and the convective seeds caused by stellar burning. We study the role of rotation and its effect on the pair-instability mass gap. Under the paradigm limiting stellar rotation to those stars in tight binaries, we determine the effect of rotation on the remnant mass distribution.

Measurement of Low-Energy Resonance Strengths in the O18(α,γ)Ne22 Reaction

The ^{18}O(α,γ)^{22}Ne reaction is an essential part of a reaction chain that produces the ^{22}Ne(α,n)^{25}Mg neutron source for both the weak and main components of the slow neutron-capture process. At temperatures of stellar helium burning, the astrophysically relevant resonances in the ^{18}O(α,γ)^{22}Ne reaction that dominate the reaction rate occur at α particle energies E_{lab} of 472 and 569 keV. However, previous experiments have shown the strengths of these two resonances to be very weak, and only upper limits or partial resonance strengths could be obtained. This Letter reports the first direct measurement of the total resonance strength for the 472- and 569-keV resonances, 0.26±0.05 and 0.63±0.30 μeV, respectively. New resonance strengths for the resonances at α particle energies of 662.1, 749.9, and 767.6 keV are also provided. These results were achieved in an experiment optimized for background suppression and detection efficiency. The experiment was performed at the Sanford Underground Research Facility, in the 4850-foot underground cavity dedicated to the Compact Accelerator System for Performing Astrophysical Research. The experimental end station used the γ-summing High EffiCiency TOtal absorption spectrometeR. Compared to previous works, the results decrease the stellar reaction rate by as much as ≈46_{-11}^{+6}% in the relevant temperature range of stellar helium burning.

Oxygen Formation in the Light of Gamma-Beams: [Precision Measurements of the 12C(α, γ) Reaction with Gamma-Beams and a Time Projection Chamber

The carbon/oxygen (C/O) ratio at the end of stellar helium burning is not known with sufficient accuracy due to the large uncertainty in the cross-section of the 12C(α, γ)16O reaction. We developed a new method to measure this reaction that is significantly different from the experimental efforts of the past four decades. Data were measured with vanishingly small background, inside one detector, that also serves as an active target (AT-TPC). Angular distributions of the 12C(α, γ)16O reaction were obtained by measuring the inverse 16O(γ, α)12C reaction with gamma-beams (from the HIγS facility) and an optical readout time projection chamber (O-TPC) detector. We agree with current world data on the measured total reaction cross-section. We further evidence the strength of our method with angular distributions measured over the 1–resonance at Ecm ~ 2.4 MeV. We extract the E1-E2 mixing phase angle (ϕ12) over this resonance and obtained values that follow the trend of prediction based on unitarity of the scattering amplitude. Our technique promises to yield results that will surpass the quality of the currently available data. We continue these measurements to lower energies at the HIγS facility of TUNL at Duke University, and anticipate measurements down to Ecm = 1.1 MeV at the ELI-NP, the EU facility in Romania..

Precision measurements on oxygen formation in stellar helium burning with gamma-ray beams and a Time Projection Chamber

The carbon/oxygen (C/O) ratio at the end of stellar helium burning is the single most important nuclear input to stellar evolution theory. However, it is not known with sufficient accuracy, due to large uncertainties in the cross-section for the fusion of helium with 12C to form 16O, denoted as 12C(α, γ)16O. Here we present results based on a method that is significantly different from the experimental efforts of the past four decades. With data measured inside one detector and with vanishingly small background, angular distributions of the 12C(α, γ)16O reaction were obtained by measuring the inverse 16O(γ, α)12C reaction with gamma-beams and a Time Projection Chamber (TPC) detector. We agree with current world data for the total reaction cross-section and further evidence the strength of our method with accurate angular distributions measured over the 1− resonance at Ecm ~ 2.4 MeV. Our technique promises to yield results that will surpass the quality of the currently available data.

The ^{27}hbox {Al}(hbox {p},alpha )^{24}hbox {Mg} reaction at astrophysical energies studied by means of the Trojan Horse Method applied to the ^2hbox {H}(^{27}hbox {Al},alpha ^{24}hbox {Mg})hbox {n} reaction

The ^{27}hbox {Al}(hbox {p},alpha )^{24}hbox {Mg} reaction, which drives the destruction of ^{27}Al and the production of ^{24}hbox {Mg} in stellar hydrogen burning, has been investigated via the Trojan Horse Method (THM), by measuring the ^2hbox {H}(^{27}hbox {Al},alpha ^{24}hbox {Mg})hbox {n} three-body reaction. The experiment covered a broad energy range (E_mathrm{c.m.}le ,1.5,hbox {MeV}), aiming to investigate those of interest for astrophysics. The results confirm the THM as a valuable technique for the experimental study of fusion reactions at very low energies and suggest the presence of a rich pattern of resonances in the energy region close to the Gamow window of stellar hydrogen burning (70–120 keV), with potential impact on astrophysics. To estimate such an impact a second run of the experiment is needed, since the background due the three-body reaction hampered to collect enough data to resolve the resonant structures and extract the reaction rate.

Conformable Fractional Models of the Stellar Helium Burning via Artificial Neural Networks

The helium burning phase represents the second stage that the star used to consume nuclear fuel in its interior. In this stage, the three elements, carbon, oxygen, and neon, are synthesized. The present paper is twofold: firstly, it develops an analytical solution to the system of the conformable fractional differential equations of the helium burning network, where we used, for this purpose, the series expansion method and obtained recurrence relations for the product abundances, that is, helium, carbon, oxygen, and neon. Using four different initial abundances, we calculated 44 gas models covering the range of the fractional parameterα=0.5−1with stepΔα=0.05. We found that the effects of the fractional parameter on the product abundances are small which coincides with the results obtained by a previous study. Secondly, we introduced the mathematical model of the neural network (NN) and developed a neural network algorithm to simulate the helium burning network using a feed-forward process. A comparison between the NN and the analytical models revealed very good agreement for all gas models. We found that NN could be considered as a powerful tool to solve and model nuclear burning networks and could be applied to the other nuclear stellar burning networks.

Low energy cross section of 18O(p,γ)19F

The observation of oxygen isotopes in giant stars sheds light on mixing processes operating in their interiors. Due to the very strong correlation between nuclear burning and mixing processes it is very important to reduce the uncertainty on the cross sections of the nuclear reactions that are involved. In this paper we focus our attention on the reaction 18O(p,γ)19F. While the 18O(p, α)15N channel is thought to be dominant, the (p,γ) channel can still be an important component in stellar burning in giants, depending on the low energy cross section. So far only extrapolations from higher-energy measurements exist and recent estimates vary by orders of magnitude. These large uncertainties call for an experimental reinvestigation of this reaction.We present a direct measurement of the 18O(p,γ)19F cross section using a high-efficiency 4π BGO summing detector at the Laboratory for Underground Nuclear Astrophysics (LUNA). The reaction cross section has been directly determined for the first time from 140 keV down to 85 keV and the different cross section components have been obtained individually. The previously highly uncertain strength of the 90 keV resonance was found to be three orders of magnitude lower than an indirect estimate based on nuclear properties of the resonant state and a factor of 20 lower than a recently established upper limit. This result excludes the possibility that the 90 keV resonance can contribute significantly to the stellar reaction rate.In addition the strengths and branching ratios of resonances between 150 and 400 keV have been determined with much improved precision and sensitivity using a HPGe detector, including a first measurement of branching ratios of the 216 keV resonance. Preliminary results are presented.

Impact of O16(e,e′α)C12 measurements on the C12(α,γ)O16 astrophysical reaction rate

The $^{12}$C($\alpha,\gamma$)$^{16}$O reaction, an important component of stellar helium burning, has a key role in nuclear astrophysics. It has direct impact on the evolution and final state of massive stars and also influences the elemental abundances resulting from nucleosynthesis in such stars. Providing a reliable estimate for the energy dependence of this reaction at stellar helium burning temperatures has been a longstanding and important goal. In this work, we study the role of potential new measurements of the reaction, $^{16}$O($e,e'\alpha$)$^{12}$C reaction, in reducing the overall uncertainty. A multilevel $R$-matrix analysis is used to make extrapolations of the astrophysical S factor for the $^{12}$C($\alpha,\gamma$)$^{16}$O reaction to the stellar energy of 300 keV. The statistical precision of the $S$-factor extrapolation is determined by performing multiple fits to existing $E1$ and $E2$ ground state capture data, including the impact of possible future measurements of the $^{16}$O($e,e'\alpha$)$^{12}$C reaction. In particular, we consider a proposed MIT experiment that would make use of a high-intensity low-energy electron beam that impinges on a windowless oxygen gas target as a means to determine the total $E1$ and $E2$ cross sections for this reaction.

Direct anthropic bound on the weak scale from supernovæ explosions

Core-collapse supernovae presumably explode because trapped neutrinos push the material out of the stellar envelope. This process is directly controlled by the weak scale $v$: we argue that supernova explosions happen only if fundamental constants are tuned within a factor of few as $v \sim \Lambda_{\rm QCD}^{3/4} M_{\rm Pl}^{1/4}$, such that neutrinos are trapped in supernovae for a time comparable to the gravitational time-scale. We provide analytic arguments and simulations in spherical approximation, that need to be validated by more comprehensive simulations. The above result can be important for fundamental physics, because core-collapse supernova explosions seem anthropically needed, as they spread intermediate-mass nuclei presumably necessary for `life'. We also study stellar burning, finding that it does not provide anthropic boundaries on $v$.

Cross section of the reaction 18O(p,γ)19F at astrophysical energies: The 90 keV resonance and the direct capture component

The observation of oxygen isotopes in giant stars sheds light on mixing processes operating in their interiors. Due to the very strong correlation between nuclear burning and mixing processes it is very important to reduce the uncertainty on the cross sections of the nuclear reactions that are involved. In this paper we focus our attention on the reaction O18(p,γ)19F. While the O18(p,α)15N channel is thought to be dominant, the (p,γ) channel can still be an important component in stellar burning in giants, depending on the low energy cross section. So far only extrapolations from higher-energy measurements exist and recent estimates vary by orders of magnitude. These large uncertainties call for an experimental reinvestigation of this reaction. We present a direct measurement of the O18(p,γ)19F cross section using a high-efficiency 4π BGO summing detector at the Laboratory for Underground Nuclear Astrophysics (LUNA). The reaction cross section has been directly determined for the first time from 140 keV down to 85 keV and the different cross section components have been obtained individually. The previously highly uncertain strength of the 90 keV resonance was found to be 0.53 ± 0.07 neV, three orders of magnitude lower than an indirect estimate based on nuclear properties of the resonant state and a factor of 20 lower than a recently established upper limit, excluding the possibility that the 90 keV resonance can contribute significantly to the stellar reaction rate.