Nucleosynthesis Models Research Articles

Low Mass Stars or Intermediate Mass Stars? The Stellar Origin of Presolar Oxide Grains Revealed by Their Isotopic Composition

Among presolar grains, oxide ones are made of oxygen, aluminum, and a small fraction of magnesium, produced by the 26Al decay. The largest part of presolar oxide grains belong to the so-called group 1 and 2, which have been suggested to form in Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars, respectively. However, standard stellar nucleosynthesis models cannot account for the 17O/16O, 18O/16O, and 26Al/27Al values recorded in those grains. Hence, for more than 20 years, the occurrence of mixing phenomena coupled with stellar nucleosynthesis have been suggested to account for this peculiar isotopic mix. Nowadays, models of massive AGB stars experiencing Hot Bottom Burning or low mass AGB stars where Cool Bottom Process, or another kind of extra-mixing, is at play, nicely fit the oxygen isotopic mix of group 2 oxide grains. The largest values of the 26Al/27Al ratio seem somewhat more difficult to account for.

Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars

Context. Among carbon-enhanced metal-poor (CEMP) stars, some are found to be enriched in slow-neutron capture (s-process) elements (and are then tagged CEMP-s), some have overabundances in rapid-neutron capture (r-process) elements (tagged CEMP-r), and some are characterized by both s- and r-process enrichments (tagged CEMP-rs). The current distinction between CEMP-s and CEMP-rs is based on their [Ba/Fe] and [Eu/Fe] ratios, since barium and europium are predominantly produced by the s- and the r-process, respectively. The origin of the abundance differences between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that the i-process, whose site still remains to be identified, could better reproduce CEMP-rs abundances than the s-process. Aims. We propose a more robust classification method for CEMP-s and CEMP-rs stars using additional heavy elements other than Ba and Eu. Once a secure classification is available, it should then be possible to assess whether the i-process or a variant of the s-process better fits the peculiar abundance patterns of CEMP-rs stars. Methods. We analyse high-resolution spectra of 24 CEMP stars and one r-process enriched star without carbon-enrichment, observed mainly with the high-resolution HERMES spectrograph mounted on the Mercator telescope (La Palma) and also with the UVES spectrograph on VLT (ESO Chile) and HIRES spectrograph on KECK (Hawaii). Stellar parameters and abundances are derived using MARCS model atmospheres. Elemental abundances are computed through spectral synthesis using the TURBOSPECTRUM radiative transfer code. Stars are re-classified as CEMP-s or -rs according to a new classification scheme using eight heavy element abundances. Results. Within our sample of 25 objects, the literature classification is globally confirmed, except for HE 1429−0551 and HE 2144−1832, previously classified as CEMP-rs and now as CEMP-s stars. The abundance profiles of CEMP-s and CEMP-rs stars are compared in detail, and no clear separation is found between the two groups; it seems instead that there is an abundance continuum between the two stellar classes. There is an even larger binarity rate among CEMP-rs stars than among CEMP-s stars, indicating that CEMP-rs stars are extrinsic stars as well. The second peak s-process elements (Ba, La, Ce) are slightly enhanced in CEMP-rs stars with respect to first-peak s-process elements (Sr, Y, Zr), when compared to CEMP-s stars. Models of radiative s-process nucleosynthesis during the interpulse phases reproduce well the abundance profiles of CEMP-s stars, whereas those of CEMP-rs stars are explained well by low-metallicity 1 M⊙ models experiencing proton ingestion. The global fitting of our i-process models to CEMP-rs stars is as good as the one of our s-process models to CEMP-s stars. Stellar evolutionary tracks of an enhanced carbon composition (consistent with our abundance determinations) are necessary to explain the position of CEMP-s and CEMP-rs stars in the Hertzsprung–Russell diagram using Gaia DR2 parallaxes; they are found to lie mostly on the red giant branch (RGB). Conclusions. CEMP-rs stars present most of the characteristics of extrinsic stars such as CEMP-s, CH, barium, and extrinsic S stars; they can be explained as being polluted by a low-mass, low-metallicity thermally-pulsing asymptotic giant branch (TP-AGB) companion experiencing i-process nucleosynthesis after proton ingestion during its first convective thermal pulses. As such, they could be renamed CEMP-sr stars, since they represent a particular manifestation of the s-process at low-metallicities. For these objects a call for an exotic i-process site may not necessarily be required anymore.

The Origin of Cosmic Structures Part 2—HI Rings

Researchers have long been aware of the existence of large HI structures and over the past decades, a large observational record of their properties has been assembled. Despite all the work that has been done, however, there is no consensus about their origin. In this paper, we will show that these structures can readily be understood within the framework of a new model of cosmology based on time-varying curvature and an unconventional model of nucleosynthesis. According to this model, all galaxies came into existence at the same time during nucleosynthesis as large proton gas clouds whose detailed structure was fixed by an imprint established in the vacuum during an initial Plank inflation. One of the predictions of the model is that all these proto-galaxy gas clouds reached the zero-velocity point of their expansion with sizes many times larger than their present-day size. The large outer HI rings are simply regions of this gas that remained at the maximum size as the core compacted to its present-day size. Similarly, the inner rings are regions that did compact with the galaxy core.

Enhanced triple-α reaction reduces proton-rich nucleosynthesis in supernovae.

The rate of the triple-α reaction that forms 12C affects1,2 the synthesis of heavy elements in the Ga-Cd range in proton-rich neutrino-driven outflows of core-collapse supernovae3-5. Initially, these outflows contain only protons and neutrons; these later combine to form α particles, then 12C nuclei via the triple-α reaction, and eventually heavier nuclei as the material expands and cools. Previous experimental work6,7 demonstrated that despite the high temperatures encountered in these environments, the reaction is dominated by the well characterized Hoyle state resonance in 12C nuclei. At sufficiently high nucleon densities, however, proton- and neutron-scattering processes may alter the effective width of the Hoyle state8,9. This raises the questions of what the reaction rate in supernova outflows is, and how changes affect nucleosynthesis predictions. Here we report that in proton-rich core-collapse supernova outflows, these hitherto neglected processes enhance the triple-α reaction rate by up to an order of magnitude. The larger reaction rate suppresses the production of heavy proton-rich isotopes that are formed by the νp process3-5 (where ν is the neutrino and p is the proton) in the innermost ejected material of supernovae10-13. Previous work on the rate enhancement mechanism9 did not anticipate the importance of this enhancement for proton-rich nucleosynthesis. Because the in-medium contribution to the triple-α reaction rate must be presentat high densities, this effect needs to be included in supernova nucleosynthesis models. This enhancement also differs from earlier sensitivity studies that explored variations of the unenhanced rate by a constant factor1,2, because the enhancement depends on the evolving thermodynamic conditions. The resulting suppression of heavy-element nucleosynthesis for realistic conditions casts doubt on the νp process being the explanation for the anomalously high abundances of 92,94Mo and 96,98Ru isotopes in the Solar System1,3,14 and for the signatures of early Universe element synthesis in the Ga-Cd range found in the spectra of ancient metal-poor stars15-20.

Monte Carlo variations as a tool to assess nuclear physics uncertainties in nucleosynthesis studies

The propagation of uncertainties in reaction cross sections and rates of neutron-, proton-, and α-induced reactions into the final isotopic abundances obtained in nucleosynthesis models is an important issue in studies of nucleosynthesis and Galactic Chemical Evolution. We developed a Monte Carlo method to allow large-scale postprocessing studies of the impact of nuclear uncertainties on nucleosynthesis. Temperature-dependent rate uncertainties combining realistic experimental and theoretical uncertainties are used. From detailed statistical analyses uncertainties in the final abundances are derived as probability density distributions. Furthermore, based on rate and abundance correlations an automated procedure identifies the most important reactions in complex flow patterns from superposition of many zones or tracers. The method already has been applied to a number of nucleosynthesis processes.

Study of the contribution of the 7Be(d, p) reaction to the 7Li problem in the Big-Bang Nucleosynthesis

Our research goal is to measure the cross section of the 7Be(d, p) reaction to find a solution for the cosmological the 7Li problem, which is an overestimation of the primordial 7Li abundance in the standard Big-Bang nucleosynthesis (BBN) model. We developed a method to produce an unstable 7Be target to measure the reaction in normal kinematics in a high-resolution measurement. We performed an experiment to produce the 7Be target and also to measure the cross section of the 7Be(d, p)8Be reaction with the produced 7Be target at the tandem facility, Kobe University. A 2.36 MeV proton beam irradiated a Li target to transmute 7Li particles to 7Be particles via the 7Li(p, n)7Be reaction. We produced 2.6×1013 7Be particles in the target after two days’ proton irradiation. After the target production, the beam ion was changed to deuterons with energies of 0.6, 1.0, and 1.6 MeV to measure the 7Be(d, p) reaction. The outgoing protons were measured by two ΔE-E silicon telescopes placed at 30° and 45°. We report the preliminary results of this experiment, including the 7Be(d, p) cross section.

Direct and Indirect Measurements for a Better Understanding of the Primordial Nucleosynthesis

The Big Bang Nucleosynthesis (BBN) model is a great success of nuclear astrophysics, because of the outstanding agreement between observational and predicted light elements abundances, except for the so-called problem. In this context, experimental efforts to measure the relevant reactions have brought to an increased level of accuracy in measuring primordial abundances and the introduction of indirect methods has allowed to overcome the natural limitations of direct measurements in the energy range of interest for BBN. Here we review the results obtained from the application of the Trojan Horse Method to some of the most influential reactions of the standard network, such as $^2$H(d,p)$^3$H, $^2$H(d,n)$^3$He, $^3$He(d,p)$^4$He, $^7$Li(p,$\alpha$)$^4$He and $^7$Be(n,$\alpha$)$^4$He. The relevant cross sections have been thus used as new inputs of a classical BBN code resulting in important constraints that advice on a possible solution of the lithium problem outside of nuclear physics.

Radiative Width of the Hoyle State from γ-Ray Spectroscopy.

The cascading 3.21 and 4.44MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65MeV excitation energy in ^{12}C, excited by the ^{12}C(p,p^{'}) reaction at 10.7MeV proton energy. From the proton-γ-γ triple coincidence data, a value of Γ_{rad}/Γ=6.2(6)×10^{-4} was obtained for the radiative branching ratio. Using our results, together with Γ_{π}^{E0}/Γ from Eriksen etal. [Phys. Rev. C 102, 024320 (2020)PRVCAN2469-998510.1103/PhysRevC.102.024320] and the currently adopted Γ_{π}(E0) values, the radiative width of the Hoyle state is determined as Γ_{rad}=5.1(6)×10^{-3} eV. This value is about 34% higher than the currently adopted value and will impact models of stellar evolution and nucleosynthesis.

Isotopic Abundances in Presolar SiC Grains accounted by s-Processing from MHD-induced Mixing in low mass AGB stars

In the past, the observational evidence that s-process elements from Sr to Pb are produced by stars ascending the so-called Asymptotic Giant Branch (or AGB) could not be explained by self-consistent models, forcing researchers to extensive parameterisations. The crucial point is to understand how protons can be injected from the envelope into the He-rich layers, yielding to the formation of 13C and then the activation of the 13C(α,n)16O reaction. In the last decade, some physically-based mixing mechanisms have been considered to solve this problem. Nowadays, a big step forward in s-process studies would be to understand what is among the suggested ones the physical model better accounts for the observational constrains. In this paper we analyse a model where the 13C forms as a feedback of MHD processes in the stellar plasma. We compare results of nucleosynthesis models for low mass AGB stars (M<3M©), developed from the MHD scenario, with the record of isotopic abundance ratios of s-elements in presolar SiC grains, which were shown to offer precise constraints on the 13C reservoir. We find that n-captures driven by magnetically-induced mixing can well account for the SiC data and that this is due to the fact that our 13C distribution fullfills the above constraints rather accurately. We show comparisons between model predictions and measurements for isotopes of Sr, Zr, Ba, Mo and Ru as representative examples of light and heavy s-elements.

The nature of s-process nucleosynthesis in low mass AGB stars based on individual Barium star observations

Barium stars are now primaries in a binary system with a former asymptotic giant branch (AGB) star. Here we compare some available AGB nucleosynthesis models and the observed s-process abundances of individual Ba star measurements to constrain the nature of the s-process in low mass AGB stars. After correcting the models with a dilution factor calculated for [Ce/Fe], we found that some of the sample stars show higher abundances for light s-process elements than the model predictions. This might be attributed to diffusive mixing in the stars.

First inverse kinematics study of the Ne22(p,γ)Na23 reaction and its role in AGB star and classical nova nucleosynthesis

The abundances of sodium and oxygen are observed to be anti-correlated in all well-studied globular clusters. Asymptotic giant branch (AGB) stars undergoing hot bottom burning (HBB) are thought to be prime candidates for producing sodium-rich oxygen-poor material and expelling it into the cluster ISM. The 22Ne(p,gamma)23Na reaction has been shown to strongly influence the amount of 23Na produced during HBB. This reaction is also important for classical novae nucleosynthesis, with sensitivity studies showing that the abundances of several isotopes in the Ne-Al region are significantly altered when varying the reaction rate between available compilations. Here we present the first inverse kinematics measurements of key resonances strengths as well as the direct capture S-factor. Together, this study represents the largest centre of mass energy range (149-1222 keV) over which this reaction has been measured in a single experiment. Our results for low-energy resonances at Ecm=149, 181 and 248 keV are in good agreement with recent forward kinematics results; we also find a direct capture S-factor consistent with the literature value of 62 keV.b. However, in the case of the important reference resonance at Ecm = 458 keV we find a strength value of wg=0.44 +/- 0.02 eV, which is significantly lower than recent results. Using our new recommended rate we explore the impact of these results on both AGB star and classical novae nucleosynthesis. In the case of AGB stars we see very little abundance changes with respect to the rate included in the STARLIB-2013. However, we observe changes of up to a factor of 2 in isotopes produced in both the carbon-oxygen (CO) and oxygen-neon (ONe) classical novae models considered here. The 22Ne(p,gamma)23Na reaction rate is now sufficiently well constrained to not significantly contribute toward abundance uncertainties from classical novae nucleosynthesis models.

Calculations of astrophysical reaction rates using ENDF/B-VIII.0 library

Recent observations of neutron stars merger (GW170817) renewed interest in stellar nucleosynthesis. Stellar nucleosynthesis modeling requires fully traceable, unbiased, high fidelity nuclear data. The Evaluated Nuclear Data File (ENDF) libraries contain complete collections of reaction data sets over the nuclear industry standard 10−5 eV - 20 MeV energy span. For the first time, the (n,γ), (n,fission), (n,p), and (n,α) astrophysical reaction rates were computed within the 0.01-10 GK range of temperatures using the ENDF/B-VIII.0 library, and nuclear Reaction rate Library for astrophysics (REACLIB) fit parameters were deduced. The present results were used to estimate the slow neutron capture timescale. These findings demonstrate the potential astrophysical applications of the ENDF libraries and the complementary relations between the nuclear industry and astrophysics data developments.

Efficient method for estimation of fission fragment yields of r -process nuclei

$\textbf{Background}$ More than half of all the elements heavier than iron are made by the rapid neutron capture process (or r process). For very neutron-rich astrophysical conditions, such at those found in the tidal ejecta of neutron stars, nuclear fission determines the r-process endpoint, and the fission fragment yields shape the final abundances of $110\le A \le 170$ nuclei. The knowledge of fission fragment yields of hundreds of nuclei inhabiting very neutron-rich regions of the nuclear landscape is thus crucial for the modeling of heavy-element nucleosynthesis. $\textbf{Purpose}$ In this study, we propose a model for the fast calculation of fission fragment yields based on the concept of shell-stabilized prefragments defined with help of the nucleonic localization functions. $\textbf{Methods}$ To generate realistic potential energy surfaces and nucleonic localizations, we apply Skyrme Density Functional Theory. The distribution of the neck nucleons among the two prefragments is obtained by means of a statistical model. $\textbf{Results}$ We benchmark the method by studying the fission yields of $^{178}$Pt, $^{240}$Pu, $^{254}$Cf, and $^{254,256,258}$Fm and show that it satisfactorily explains the experimental data. We then make predictions for $^{254}$Pu and $^{290}$Fm as two representative cases of fissioning nuclei that are expected to significantly contribute during the r-process nucleosynthesis occurring in neutron star mergers. $\textbf{Conclusions}$ The proposed framework provides an efficient alternative to microscopic approaches based on the evolution of the system in a space of collective coordinates all the way to scission. It can be used to carry out global calculations of fission fragment distributions across the r-process region.

Unveiling pure-metal ejecta X-ray emission in supernova remnants through their radiative recombination continuum

Context. Spectral analysis of X-ray emission from ejecta in supernova remnants (SNRs) is hampered by the low spectral resolution of CCD detectors, which typically creates a degeneracy between the best-fit values of chemical abundances and the plasma emission measure. The combined contribution of shocked ambient medium and ejecta to the emerging X-ray emission further complicates the determination of the ejecta mass and chemical composition. This degeneracy leads to big uncertainties in mass estimates and can introduce a bias in the comparison between the ejecta chemical composition derived from the observations and the yields predicted by explosive nucleosynthesis models. Aims. We explore the capabilities of present and future spectral instruments with the aim of identifying a spectral feature that may allow us to discriminate between metal-rich and pure-metal plasmas in X-ray spectra of SNRs. Methods. We studied the behavior of the most common X-ray emission processes of an optically thin plasma in the high-abundance regime. We investigated spectral features of bremsstrahlung, radiative recombination continua (RRC), and line emission, by exploring a wide range of chemical abundances, plasma temperatures, and ionization parameters. We then synthesized X-ray spectra from a state-of-the-art 3D hydrodynamic simulation of Cas A, by using the response matrix from the Chandra ACIS-S charged-coupled device detector and that of the XRISM/Resolve X-ray calorimeter spectrometer. Results. We found that a bright RRC shows up when the plasma is made of pure-metal ejecta, and a high spectral resolution is needed to actually identify this ejecta signature. We tested and verified the applicability of our novel diagnostic tool and we propose a promising target for the future detection of such spectral feature: the southeastern Fe-rich clump of Cas A. Conclusions. While there is no way to unambiguously reveal pure-metal ejecta emission with CCD detectors, X-ray calorimeters will be able to pinpoint the presence of pure-metal RRC and to recover correctly absolute mass and the chemical composition of the ejecta, opening a new window on the link between progenitor star, supernova and SNRs.

Quantified limits of the nuclear landscape

The chart of the nuclides is limited by particle drip lines beyond which nuclear stability to proton or neutron emission is lost. Predicting the range of particle-bound isotopes poses an appreciable challenge for nuclear theory as it involves extreme extrapolations of nuclear masses beyond the regions where experimental information is available. Still, quantified extrapolations are crucial for a variety of applications, including the modeling of stellar nucleosynthesis. We use microscopic nuclear mass models and Bayesian methodology to provide quantified predictions of proton and neutron separation energies as well as Bayesian probabilities of existence throughout the nuclear landscape all the way to the particle drip lines. We apply nuclear density functional theory with several energy density functionals. To account for uncertainties, Bayesian Gaussian processes are trained on the separation-energy residuals for each individual model, and the resulting predictions are combined via Bayesian model averaging. This framework allows to account for systematic and statistical uncertainties and propagate them to extrapolative predictions. We characterize the drip-line regions where the probability that the nucleus is particle-bound decreases from $1$ to $0$. In these regions, we provide quantified predictions for one- and two-nucleon separation energies. According to our Bayesian model averaging analysis, 7759 nuclei with $Z\leq 119$ have a probability of existence $\geq 0.5$. The extrapolations obtained in this study will be put through stringent tests when new experimental information on exotic nuclei becomes available. In this respect, the quantified landscape of nuclear existence obtained in this study should be viewed as a dynamical prediction that will be fine-tuned when new experimental information and improved global mass models become available.

Measurement of the 154Gd(n,γ) cross section and its astrophysical implications

The neutron capture cross section of 154Gd was measured from 1 eV to 300 keV in the experimental area located 185 m from the CERN n_TOF neutron spallation source, using a metallic sample of gadolinium, enriched to 67% in 154Gd. The capture measurement, performed with four C6D6 scintillation detectors, has been complemented by a transmission measurement performed at the GELINA time-of-flight facility (JRC-Geel), thus minimising the uncertainty related to sample composition. An accurate Maxwellian averaged capture cross section (MACS) was deduced over the temperature range of interest for s process nucleosynthesis modelling. We report a value of 880(50) mb for the MACS at kT=30 keV, significantly lower compared to values available in literature. The new adopted 154Gd(n,γ) cross section reduces the discrepancy between observed and calculated solar s-only isotopic abundances predicted by s-process nucleosynthesis models.

Spin assignments for $$^{23}\hbox {Mg}$$ levels and the astrophysical $$^{22}\hbox {Na}(p,\gamma )^{23}\hbox {Mg}$$ reaction

The $$^{22}\hbox {Na}(p,\gamma )^{23}\hbox {Mg}$$ reaction is responsible for destruction of the long-lived radionuclide $$^{22}\hbox {Na}$$ produced during nova explosions. Since the reaction proceeds through resonances from levels in $$^{23}\hbox {Mg}$$ above the proton threshold at 7.581 MeV, the properties of these levels such as excitation energies, spins, and parities are crucial ingredients to determine the $$^{22}\hbox {Na}(p,\gamma )^{23}\hbox {Mg}$$ reaction rate. Despite recent studies of these levels, their spins are not well constrained in many cases. We have measured the $$^{24}\mathrm{Mg}(p,d)^{23}\hbox {Mg}$$ transfer reaction to determine spectroscopic properties of these levels at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The spin of the $$E_{x}$$ = 7.788 MeV level in $$^{23}\hbox {Mg}$$ is constrained to be $$J^{\pi }$$ = (3/2$$^+$$, 5/2$$^+$$) through the present work. The astrophysical $$^{22}\hbox {Na}(p,\gamma )^{23}\hbox {Mg}$$ reaction rate at nova temperatures is updated accordingly. Nova nucleosynthesis model calculations using the newly updated $$^{22}\hbox {Na}(p,\gamma )^{23}\hbox {Mg}$$ reaction rate shows that the final weighted abundance of the radionuclide $$^{22}\hbox {Na}$$ is increased by 42% compared to that obtained by using the previous $$^{22}\hbox {Na}(p,\gamma )^{23}\hbox {Mg}$$ reaction rate of Sallaska et al. for a 1.35 $$M_{\odot }$$ ONeMg white dwarf.

Dark energy from Higgs potential

We derive the ratio of dark energy to baryon matter content in the universe from a Higgs potential matching a description of baryon matter on an intrinsic configuration space. The match determines the Higgs mass and self-coupling parameters and introduces a constant term in the Higgs potential. The constant term is taken to give dark energy contributions from detained neutrons, both primordial and piled-up neutrons from nuclear processes in stars. This corresponds to the dark energy content increasing with time. The two contributions possibly give rise to the primordial inflation and the later accelerated recession, respectively. The ensuing inflation during nucleosynthesis may explain the primordial lithium-seven deficit relative to the standard Big Bang nucleosynthesis model predictions. From the observed helium and stellar metallicity contents, we get a dark energy to baryon matter ratio of 14.5(0.7) to compare with the observed value of 13.9(0.2).

Calibration of CR-39 solid-state track detectors for study of laser-driven nuclear reactions

It is of particular interest to investigate nuclear fusion reactions generated by high-intensity lasers in plasma environments that are similar to real astrophysical conditions. We have experimentally investigated $$^2 \hbox {H}({d},{p})^3 \hbox {H}$$, one of the most crucial reactions in big bang nucleosynthesis models, at the Shenguang-II laser facility. In this work, we present a new calibration of CR-39 solid-state track detectors, which are widely employed as the main diagnostics in this type of fusion reaction experiment. We measure the dependence of the track diameter on the proton energy. It is found that the track diameters of protons with different energies are likely to be identical. We propose that in this case, the energy of the reaction products can be obtained by considering both the diameters and gray levels of these tracks. The present results would be very helpful for analyzing the $$^2 \hbox {H}(d,p)^3 \hbox {H}$$ reaction products recorded with the same batch of CR-39 solid-state track detectors.

A Nucleosynthetic Origin for the Southwestern Fe-rich Structure in Kepler’s Supernova Remnant

Abstract Chandra X-ray observations of Kepler’s supernova remnant indicate the existence of a high-speed Fe-rich ejecta structure in the southwestern region. We report strong K-shell emission from Fe-peak elements (Cr, Mn, Fe, Ni), as well as Ca, in this Fe-rich structure, implying that those elements could be produced in the inner area of the exploding white dwarf. We found Ca/Fe, Cr/Fe, Mn/Fe, and Ni/Fe mass ratios of 1.0%–4.1%, 1.0%–4.6%, 1%–11%, and 2%–30%, respectively. In order to constrain the burning regime that could produce this structure, we compared these observed mass ratios with those in 18 one-dimensional Type Ia nucleosynthesis models (including both near-M Ch and sub-M Ch explosion models). The observed mass ratios agree well with those around the middle layer of incomplete Si burning in Type Ia nucleosynthesis models with a peak temperature of ∼(5.0–5.3) × 109 K and a high metallicity, Z &gt; 0.0225. Based on our results, we infer the necessity for some mechanism to produce protruding Fe-rich clumps dominated by incomplete Si-burning products during the explosion. We also discuss the future perspectives of X-ray observations of Fe-rich structures in other Type Ia supernova remnants.