MgAl Cycles Research Articles

Measurement of the 27Al(p, α)24Mg fusion reaction at astrophysical energies via the Trojan Horse Method

In astrophysics, the abundance of 26Al is essential for understanding nucleosynthesis in the Milky Way and Galactic core-collapse supernovae rates. Detection methods involve γ-ray lines and comparing 26Mg overabundance with the common Mg isotope in meteorites. Therefore, stable isotopes 27Al and 24Mg play a crucial role and the MgAl cycle affecting aluminum and magnesium production has to be carefully studied. Recent surveys reveal complexities in stellar populations whose understanding may also benefit from better constraining the closure of the MgAl cycle. The 27Al(p, α)24Mg fusion reaction, a key 27Al destruction channel, is central to these scenarios. Due to uncertainties, the Trojan Horse Method is applied, allowing high-precision spectroscopy on the compound nucleus 28Si. It reveals crucial fusion cross section information in the astrophysically relevant energy range. The indirect measurement by means of the 2H(27Al,α24Mg)n process made it possible to assess the contribution of the 84.3 keV resonance and to set upper limits on nearby resonances. This study evaluates the THM recommended rate’s impact on intermediate-mass asymptotic giant branch stars, showing a notable increase in surface aluminum abundance at lower masses due to fusion cross section modification, while 24Mg remains largely unaffected.

MgAl burning chain in ω Centauri

In this study, we report the results of the analysis of Fe, Mg, Al, and Si abundances analysis for a sample of 439 stars in ω Centauri, using high-resolution spectra obtained with the VLT/FLAMES multi-object spectrograph. Our analysis reveals the presence of four distinct Fe populations, with the main peak occurring at a low metallicity, consistent with previous literature findings. We observe a discrete and pronounced Mg–Al anticorrelation, which exhibits variations in shape and extension as a function of metallicity. Specifically, this anticorrelation is present in stars with metallicities lower than approximately −1.3 dex, while it becomes less evident or absent for higher [Fe/H] values. Additionally, we detect (anti)correlations between Mg and Si, and between Al and Si, whose extensions also vary with metallicity, similar to the Mg–Al anticorrelation. These results suggest that the MgAl cycle plays a crucial role in the formation of multiple populations in ω Centauri, with the presence of all (anti)correlations at metallicities lower than –1.3 dex, providing evidence for the burning of Mg at very high temperatures (> 108 K), at least in the metal-poor regime. Furthermore, we observe a clear trend of stars with [Al/Fe] > +0.5 dex as a function of metallicity, confirming for the first time the existence of the two channels of Al production and destruction. This evidence can help to provide further constraints on the potential nature of the polluters responsible for the observed chemical anomalies in this stellar system. Finally, we find that the two most metal-poor populations identified in our sample are compatible with null or very small metallicity dispersion and we discuss how this result fits into a scenario where ω Centauri is the remnant of a disrupted nucleated dwarf galaxy.

Stellar wind yields of very massive stars

ABSTRACT The most massive stars provide an essential source of recycled material for young clusters and galaxies. While very massive stars (VMSs, M>100 $\rm {\rm M}_{\odot }$) are relatively rare compared to O stars, they lose disproportionately large amounts of mass already from the onset of core H-burning. VMS have optically thick winds with elevated mass-loss rates in comparison to optically thin standard O-star winds. We compute wind yields and ejected masses on the main sequence, and we compare enhanced mass-loss rates to standard ones. We calculate solar metallicity wind yields from MESA stellar evolution models in the range 50–500 $\rm {\rm M}_{\odot }$, including a large nuclear network of 92 isotopes, investigating not only the CNO-cycle, but also the Ne–Na and Mg–Al cycles. VMS with enhanced winds eject 5–10 times more H-processed elements (N, Ne, Na, Al) on the main sequence in comparison to standard winds, with possible consequences for observed anticorrelations, such as C–N and Na–O, in globular clusters. We find that for VMS 95 per cent of the total wind yields is produced on the main sequence, while only ∼ 5 per cent is supplied by the post-main sequence. This implies that VMS with enhanced winds are the primary source of 26Al, contrasting previous works where classical Wolf–Rayet winds had been suggested to be responsible for galactic 26Al enrichment. Finally, 200 $\rm {\rm M}_{\odot }$ stars eject 100 times more of each heavy element in their winds than 50 $\rm {\rm M}_{\odot }$ stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50 $\rm {\rm M}_{\odot }$ stars.

Updated reaction rate of Mg25(p,γ)Al26 and its astrophysical implication

Al26 with a half-life of 7.17×105 years is one of the most significant nuclides in γ-ray astronomy and presolar grains of meteorites. Its main production mechanism in the H-burning MgAl cycle is the Mg25(p,γ)Al26 reaction. In the temperature region of 0.05–0.3 GK of astrophysical interest, the astrophysical Mg25(p,γ)Al26 reaction rate is dominated by the resonant capture of several low-energy resonances. In this work, we report the results of a complete experimental investigation of the Ec.m.=92, 130, and 189 keV resonances in the Mg25(p,γ)Al26 reaction with the Jinping Underground Nuclear Astrophysics Experimental Facility. The updated thermonuclear Mg25(p,γ)Al26 reaction rate is (32–39)% higher than that obtained at the Laboratory for Underground Nuclear Astrophysics around 0.07–0.09 GK, mainly due to the 32% enhancement of the 92-keV resonance strength. The astrophysical impact of our new rate on the Al26 yield in a 5 M⊙ low-metallicity asymptotic giant branch star is investigated, in which an increase of (45–79)% in the Al26 yield is found by adopting our new Mg25(p,γ)Al26 rates.5 MoreReceived 19 December 2022Revised 22 March 2023Accepted 24 April 2023DOI:https://doi.org/10.1103/PhysRevC.107.065801©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasH & He burningH & He induced nuclear reactionsHydrostatic stellar nucleosynthesisNuclear astrophysicsRadiative captureProperties20 ≤ A ≤ 38Nuclear Physics

The study of the 20Ne(p,γ)21Na reaction at LUNA

The NeNa and MgAl cycles have been the subject of much experimental activity during the last decade because of their relevance to the synthesis of Ne, Na, and Mg isotopes during the H burning in several astrophysical scenarios. Key reactions in these cycles are also believed to be the main agents of the observed anti-correlations in O-Na and Al-Mg abundances exhibited by the stars of Galactic globular clusters. The 20Ne(p,γ)21Na is the first reaction and the bottleneck of the NeNa cycle: having the slowest reaction rate, it controls the speed of the entire cycle. In order to better constrain the overall astrophysical reaction rate of this important reaction, the LUNA collaboration has started a new experimental effort to study the 366 keV resonance and to improve the knowledge of the cross section at proton energies below 400 keV. This contribution describes the experimental setup and preliminary results.

A New Reaction Rate of the 27Al(p/α)24Mg Reaction Based on Indirect Measurements at Astrophysical Energies and Implications for 27Al Yields of Intermediate-mass Stars

The abundance of 26Al carries a special role in astrophysics, since it probes active nucleosynthesis in the Milky Way and constrains the Galactic core-collapse supernovae rate. It is estimated through the detection of the 1809 keV γ-line and from the superabundance of 26Mg in comparison with the most abundant Mg isotope (A = 24) in meteorites. For this reason, high precision is necessary also in the investigation of the stable 27Al and 24Mg isotopes. Moreover, these nuclei enter the so-called MgAl cycle, playing an important role in the production of Al and Mg. Recently, high-resolution stellar surveys have shown that the Mg–Al anticorrelation in red-giant stars in globular clusters may hide the existence of multiple stellar populations, and that the relative abundances of Mg isotopes may not be correlated with Al. The common thread running through these astrophysical scenarios is the 27Al(p,α)24Mg reaction, which is the main 27Al destruction channel and directly correlates its abundance with the 24Mg one. Since available reaction rates show large uncertainties owing to the vanishingly small cross section at astrophysical energies, we have applied the Trojan Horse Method to deduce the reaction rate with no need of extrapolation. The indirect measurement made it possible to assess the contribution of the 84 keV resonance and to lower upper limits on the strength of nearby resonances. In intermediate-mass AGB stars experiencing hot bottom burning, a sizeable increase in surface aluminum abundance is observed at the lowest masses, while 24Mg is essentially unaffected by the change in the reaction rate.

Investigation of direct capture in the Na23(p,γ)Mg24 reaction

The $^{23}\mathrm{Na}(p,\ensuremath{\gamma})^{24}\mathrm{Mg}$ reaction plays an important role in the nucleosynthesis of elements in the hot bottom burning environment of asymptotic giant branch stars by providing a breakout path from the NeNa cycle to the MgAl cycle. At temperatures above $\ensuremath{\approx}0.06$ GK, the underlying nuclear reaction contributions to the rate are primarily narrow resonances, but at lower temperatures direct and broad resonance tail contributions come to dominate. While there have been recent studies to improve the uncertainties associated with these narrow resonances, little attention has been paid to the nonresonant component. In this work, experimental measurements are reported over the energy range from 0.5 and 1.05 MeV proton beam energy, with a focus on studying the off-resonance region of the cross section. Several transitions were observed where two broad resonances dominate the energy range and whose low energy tails contribute strongly to the low-energy, nonresonant, cross section. In addition, a clear signature of direct capture has been observed for the first time in the $^{23}\mathrm{Na}(p,\ensuremath{\gamma})^{24}\mathrm{Mg}$ reaction.

Chemical Composition of a Palomar 12 Blue Straggler

Abstract With the equivalent area of a 16m telescope, ESPRESSO in 4UT mode allows to inaugurate high resolution spectroscopy for solar-type stars belonging to extragalactic globular clusters. We determine the chemical composition of an extragalactic blue straggler. The star has a G magnitude of 19.01 and belongs to the globular cluster Pal12, that is associated to the Sagittarius dwarf galaxy. Abundances are computed by using high resolution spectroscopy and LTE analysis. Two 50 minutes ESPRESSO spectra, co-added, provide a Signal to Noise Ratio of ∼25 with a resolving power R∼70000. This allows us to measure with good precision abundance of several (13) elements. Li could help to distinguish between formation models of Blue Stragglers; we are able to set a 3σ upper limit of Li=3.1, which is still too high to discriminate between competing models. The abundances we retrieve for the BS are compatible with those of giant stars of Pal 12 published in literature, re-analyzed by us using the same procedure and line list. Small differences are present, that can be ascribed to NLTE effects, but for Mg the BS shows a large under-abundance. The most likely explanation is that the BS atmosphere is dominated by gas processed through the Mg-Al cycle, but we have no suitable Al or Na lines to confirm this hypothesis. We show that ESPRESSO with 4UT can be used to derive precise abundances for solar-type stars fainter than magnitude 19. At these magnitudes a proper sky subtraction is needed and in crowded field the targets must be chosen with outmost care, to avoid contamination of the sky fibre from nearby stars.

Exploring the astrophysical energy range of the 27Al[formula omitted]Mg reaction: A new recommended reaction rate

The 26Al abundance holds a special role in present-day astrophysics, since it is a probe of active nucleosynthesis in the Galaxy and a valuable constraint of Galactic core-collapse supernovae rate. It is estimated through the detection of the 1809-keV γ-line of the daughter 26Mg and from the superabundance of 26Mg in comparison with the most abundant 24Mg isotope in meteorites. Accurate knowledge of the reaction rates involving 26Al, its stable counterpart 27Al and 24Mg is then mandatory. Moreover, these nuclei enter the MgAl cycle playing an important role in the production of Al and Mg isotopes. Recently, high-resolution stellar surveys have shown that the Mg-Al anti-correlation in red giants of globular clusters may hide the existence of multiple stellar populations, and that the relative abundances of Mg isotopes may not show correlation with Al.The common thread running through these astrophysical scenarios is the 27Al(p,α)24Mg reaction, which is the main 27Al destruction channel and directly correlates its abundance with the 24Mg one. Since available reaction rates show an order of magnitude uncertainty owing to the vanishingly small cross section at astrophysical energies, we have applied the Trojan Horse Method to deduce the reaction rate with no need of extrapolation. The indirect measurement made it possible to assess the contribution of the 84-keV resonance and to lower the upper limits on the strength of nearby resonances, with potential important impact for astrophysics. In particular, modifications in the 27Al and 24Mg abundances up to ∼30% are predicted for intermediate mass stars.

Revised decay properties of the key 93-keV resonance in the Mg 25 (p,γ) reaction and its influence on the MgAl cycle in astrophysical environments

Revised decay properties of the key 93-keV resonance in the Mg 25 (p,γ) reaction and its influence on the MgAl cycle in astrophysical environments

Direct measurement of the resonance strengths and branching ratios of low-energy (p, γ) reactions on Mg isotopes * *Supported by the National Natural Science Foundation of China (11490563, 11961141003) and the Continuous Basic Scientific Research Project (WDJC-2019-13)

Proton capture reactions on Mg isotopes are significant in the Mg-Al cycle in stellar H-burning. In particular, the resonance strengths and branching ratios of low-energy resonances in 25Mg( )26Al reactions determine the production of 26Al, which is one of the most important long-lived radioactive nuclei in nuclear astrophysics. In this article, we report our first experiment using the intense proton beam of approximately 2 mA provided by the JUNA accelerator ground laboratory and a new technique that can minimize the composition change of targets under intense beam irradiation. The resonance strengths and branching ratios of E = 214, 304, and 326 keV resonances in the reactions of 24Mg( )25Al, 25Mg( )26Al, and 26Mg( )27Al, respectively, were measured with high accuracy. The success of this experiment provides a good calibration for the nuclear astrophysical experiment at the Jinping underground laboratory.

Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

The 92-keV resonance in the 25Mg(p,γ)26Al reaction plays a key role in the production of 26Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26gAl, or isomeric state, 26mAl. It is the ground state that is critical as the source of cosmic γ rays. It is therefore important to precisely determine the ground-state branching fraction f0 of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f0=0.52(2)stat(6)syst. The f0 value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26gAl and its cosmic 1809-keV γ rays.

Indirect measurement of the 57.7 keV resonance strength for the astrophysical γ -ray source of the Mg25(p,γ)Al26 reaction

$^{25}\mathrm{Mg}(p,\ensuremath{\gamma})^{26}\mathrm{Al}$ is the most important reaction in the Mg-Al cycle in the hydrogen burning regions of stars. Its cross sections at stellar energies are essential to understand the issues of radioactive $^{26}\mathrm{Al}$ in the galaxy and meteorites. The 57.7 keV resonance dominate the $^{25}\mathrm{Mg}(p,\ensuremath{\gamma})^{26}\mathrm{Al}$ astrophysical reaction rates at relative low temperature, but it is very difficult to measure its resonance strength directly, and the indirect measurement results deviate by a factor of about 2 by far. In this work, the angular distributions of $^{25}\mathrm{Mg}(^{7}\mathrm{Li},^{6}\mathrm{He})^{26}\mathrm{Al}$ leading to 6.364 MeV and eleven low-lying states in $^{26}\mathrm{Al}$ have been measured by the Q3D magnetic spectrometer of the HI-13 tandem accelerator. The spectroscopic factors were derived and used to deduce the proton width and 57.7 keV resonance strength. The astrophysical $^{25}\mathrm{Mg}(p,\ensuremath{\gamma})^{26}\mathrm{Al}$ reaction rates at stellar energies have been updated by using the present result.

A Comparison of the Chemical Composition of Main-sequence and Giant Stars in the Open Cluster NCC 752

The chemical composition of stars in open clusters provides the best information on the chemical evolution of stars via comparison of the main sequence stars with the evolved giants. This is a case study of the abundances in the dwarfs and giants in the old, open cluster, NGC 752. It is also a pilot program for automated abundance determinations, including equivalent width measurements, stellar parameter determinations, and abundance analysis. We have found abundances of 31 element-ion combinations in 23 dwarfs and 6 giants. The mean cluster abundance of Fe is solar with [Fe/H]$=-0.01\pm0.06$ with no significant difference between the dwarfs and giants. We find that the cluster abundances of other elements, including alpha-elements, to be at, or slightly above solar levels. We find some evidence for CNO processing in the spectra of the giants. The enhancement of Na in giants indicates that the NeNa cycle has occurred. The abundances of Mg and Al are similar in the dwarfs and giants indicating that the hotter MgAl cycle has not occurred. We find no evidence of s-process enhancements in the abundances of heavy elements in the giants.

Direct measurements of low-energy resonance strengths of the 23Na(p,γ)24Mg reaction for astrophysics

The NeNa and the MgAl cycles play a fundamental role in the nucleosynthesis of asymptotic giant branch stars undergoing hot bottom burning. The Na23(p,γ)24Mg reaction links these two cycles and a precise determination of its rate is required to correctly estimate the contribution of these stars to the chemical evolution of various isotopes of Na, Mg and Al. At temperatures of 50≲T≲110MK, narrow resonances at Ep=140 and 251keV are the main contributors to the reaction rate, in addition to the direct capture that dominates in the lower part of the temperature range. We present new measurements of the strengths of these resonances at the Laboratory for Underground Nuclear Astrophysics (LUNA). We have used two complementary detection approaches: high efficiency with a 4π BGO detector for the 140keV resonance, and high resolution with a HPGe detector for the 251keV resonance. Thanks to the reduced cosmic ray background of LUNA, we were able to determine the resonance strength of the 251keV resonance as ωγ=482(82)μeV and observed new gamma ray transitions for the decay of the corresponding state in Mg24 at Ex=11931keV. With the highly efficient BGO detector, we observed a signal for the 140keV resonance for the first time in a direct measurement, resulting in a strength of ωγ140=1.46−0.53+0.58neV (68% CL). Our measurement reduces the uncertainty of the Na23(p,γ)24Mg reaction rate in the temperature range from 0.05 to 0.1GK to at most −35%+50% at 0.07GK. Accordingly, our results imply a significant reduction of the uncertainties in the nucleosynthesis calculations.

Resonant nuclear reaction 23Mg (p,γ) 24Al in strongly screening magnetized neutron star crust**Supported by National Natural Science Foundation of China (11565020), the Counterpart Foundation of Sanya (2016PT43), the Special Foundation of Science and Technology Cooperation for Advanced Academy and Regional of Sanya (2016YD28), the Scientific Research Starting Foundation for 515 Talented Project of Hainan Tropical Ocean University (RHDRC201701) and the

Based on the relativistic theory of superstrong magnetic fields (SMF), by using three models those of Lai (LD), Fushiki (FGP), and our own (LJ), we investigate the influence of SMFs due to strong electron screening (SES) on the nuclear reaction 23Mg (p,γ) 24Al in magnetars. In a relatively low density environment (e.g., ρ7<0.01) and 1<B12<102, our screening rates are in good agreement with those of LD and FGP. However, in relatively high magnetic fields (e.g., B12>102), our reaction rates can be 1.58 times and about three orders of magnitude larger than those of FGP and LD, respectively (B12, ρ7 are in units of 1012G, 107g cm−3). The significant increase of strong screening rate can imply that more 23Mg will escape from the Ne-Na cycle due to SES in a SMF. As a consequence, the next reaction, 24Al (β+, ν) 24Mg, will produce more 24Mg to participate in the Mg-Al cycle. Thus, it may lead to synthesis of a large amount of A>20 nuclides in magnetars.

23Na(p,γ)24Mg Cross Section Measurements

As a link between the NeNa and MgAl cycles in stellar burning, the reaction 23 Na(p,γ )24 Mg is of interest for various astrophysical scenarios, such as AGB stars. A combined effort at the Laboratory for Underground Nuclear Astrophysics (LUNA) and the Nuclear Science Laboratory (NSL) at the University of Notre Dame aims at a cross section determination for the this reaction, to constrain the astrophysical reaction rate by improving the knowledge of the resonance strengths and the non-resonant component. Experiments at LUNA benefit from the underground location at the Gran Sasso National Laboratory which allows for the measurement of resonances at low energies with high sensitivity in a low background environment. Measurements at the University of Notre Dame pursue a determination of the non-resonant cross section at higher energies. We present the two experiments and the status of the data analysis.

Two groups of red giants with distinct chemical abundances in the bulge globular cluster NGC 6553 through the eyes of APOGEE

Multiple populations revealed in globular clusters (GCs) are important windows to the formation and evolution of these stellar systems. The metal-rich GCs in the Galactic bulge are an indispensable part of this picture, but the high optical extinction in this region has prevented extensive research. In this work, we use the high resolution near-infrared (NIR) spectroscopic data from APOGEE to study the chemical abundances of NGC 6553, which is one of the most metal-rich bulge GCs. We identify ten red giants as cluster members using their positions, radial velocities, iron abundances, and NIR photometry. Our sample stars show a mean radial velocity of $-0.14\pm5.47$ km s$^{-1}$, and a mean [Fe/H] of $-0.15\pm 0.05$. We clearly separate two populations of stars in C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where the multiple stellar population phenomenon is found until now. Substantial chemical variations are also found in Na, O, and Al. However, the two populations show similar Si, Ca, and iron-peak element abundances. Therefore, we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do not seem to have significantly polluted the second generation stars. Comparing with other GC studies, NGC 6553 shows similar chemical variations as other relatively metal-rich GCs. We also confront current GC formation theories with our results, and suggest possible avenues for improvement in the models.

Cross Section Measurements for the 23Na(p,γ)24Mg Reaction at LUNA

LUNA, the Laboratory for Underground Nuclear Astrophysics, is an accelerator facility for measurements of nuclear cross sections of astrophysical interest. The greatly reduced cosmic ray background at LUNA's underground location in the Gran Sasso National Laboratory (LNGS) allows direct measurements of weak reactions at low energies.One of the reactions currently under study at LUNA is 23Na(p,γ)24Mg, which links the NeNa and MgAl cycles in stellar burning. The LUNA facility is presented, with a focus on the current experimental efforts to study the reaction 23Na(p,γ)24Mg.

Determination of the 25Mg(p, γ)26Al resonance strength at E c.m.=58 keV via shell model calculation

The observation of 26Al is an useful tool for γ-ray astronomy and in studies of galactic chemical evolution. The most likely mechanism for 26A1 nucleosynthesis is in the hydrogen burning MgAl cycle, and the 26A1 production reaction 25Mg(p, γ)26Al at the important temperature range below T = 0.2 GK is still not well known. The spectroscopic factor of 58 keV resonance level in 26A1 is determined with shell model calculation and then used to deduce the resonance strength of the 25Mg(p, γ)26Al reaction. The result provides a reference for the future 25Mg(p, γ)26Al direct measurement at Jinping underground laboratory.