Oxygen Burning Research Articles

WASTE PROCESSING BY PLASMA ARC ELECTROLYTIC CENTRIFUGAL CONVERSION

The plasma-arc electrolytic centrifugal conversion process was developed for processing of crude ore materials and hydrocarbons to end products with simultaneous generation of energy carriers and energy [1]. The developed process may be used with a similar purpose for processing of various wastes. Industrial and household wastes have identical chemistry with common ore and hydrocarbons. Waste ore called “tailings” is comparable to payable ore and, just as in base ore, oxygen comprises about half of its composition. Household wastes for the most part contain organic compounds, including plastic, wood, paper, and form hydrocarbon mixtures containing various metals and nonmetals. Melted waste mixture contains almost every elements of the periodic table with various chemical elements acting as reaction catalysts. The wastes are processed under electric energy generated by hydrogen burning in oxygen. Hydrogen is recovered from hydrocarbon materials contained in household wastes and water contained in the feed. Oxygen is recovered from waste ore. Hydrogen and oxygen are stored in a methanol compound obtained from synthesis gas generated in the course of waste plasma melting.

Electron-capture supernovae in NS + He star systems and the double neutron star systems

ABSTRACT Electron-capture-supernovae (EC-SNe) provide an alternative channel for producing neutron stars (NSs). They play an important role in the formation of double NS (DNS) systems and the chemical evolution of galaxies, and contribute to the NS mass distribution in observations. It is generally believed that EC-SNe originate from e-captures on $\rm ^{24}Mg$ and $\rm ^{20}Ne$ in the massive degenerate oxygen–neon (ONe) cores with masses close to the Chandrasekhar limit (MCh). However, the origin of EC-SNe is still uncertain. In this paper, we systematically studied the EC-SNe in NS + He star systems by considering the explosive oxygen burning that may occur in the near-MCh ONe core. We provided the initial parameter spaces for producing EC-SNe in the initial orbital period − initial He star mass (log$P_{\rm orb}^{\rm i}-M_{\rm He}^{\rm i}$) diagram, and found that both $M_{\rm He}^{\rm i}$ and minimum $P_{\rm orb}^{\rm i}$ for EC-SNe increase with metallicity. Then, by considering NS kicks added to the newborn NS, we investigated the properties of the formed DNS systems after the He star companions collapse into NSs, such as the orbital periods, eccentricities, and spin periods of recycle pulsars (Pspin), etc. The results show that most of the observed DNS systems can be produced by NS kicks of $\lesssim$50 km s−1. In addition, we found that NSs could accrete more material if the residual H envelope on the He star companions is considered, which can form the mildly recycled pulsars (Pspin ∼ 20 ms) in DNS systems.

Generation of Autoantibodies in Metal-catalyzed Oxidatively Damaged DNA in Various Cancer Subjects.

Free radicals exist as unstable and highly reactive substances, occurring both in and outside the body. Free radicals are labeled as electron-hungry molecules formed from metabolism and endogenous burning of oxygen. They are transported in cells, upsetting the arrangement of molecules and instigating cellular injury. Hydroxyl radical (•OH) is one of the highly reactive free radicals, which damages the biomolecules in its close vicinity. In the present study, DNA was modified by the hydroxyl radical generated via the Fenton reaction. The •OH-oxidized/-modified DNA (Ox-DNA) was characterized by UV-visible and fluorescence spectroscopy. Thermal denaturation was performed to reveal the susceptibility of modified DNA toward heat. The role of Ox-DNA was also established in probing the presence of autoantibodies against Ox-DNA in the sera of cancer patients by direct binding ELISA. The specificity of autoantibodies was also checked by inhibition ELISA. In biophysical characterization, an increase in hyperchromicity and relative reduction of fluorescence intensity for Ox-DNA was reported compared to the native DNA analog. A thermal denaturation study revealed that Ox-DNA was highly susceptible to heat in comparison to the native conformers. The direct binding ELISA showed the prevalence of autoantibodies from cancer patient sera separated for immunoassay detection against the Ox-DNA. The generated autoantibodies against the Ox-DNA were detected as highly specific against bladder, head, neck, and lung cancer, which was further confirmed by the inhibition ELISA for the serum and IgG antibodies. The generated neoepitopes on DNA molecules are recognized as nonself by the immune system, which leads to the formation of autoantibodies in cancer patients. Therefore, our study confirmed that oxidative stress plays a role in the structural perturbation of DNA and makes it immunogenic.

Preparation and characterization of ethylene‐propylene‐diene monomer rubber/metal hydroxide composites by electron beam irradiation

AbstractEthylene‐propylene‐diene monomer (EPDM) rubber has widespread uses in various applications due to its ozone and oxidation stability, weatherability, electrical insulation, and elasticity. However, its further use is limited by low flame retardancy and low thermal stability. This study incorporated metal hydroxide flame retardant to enhance flame retardancy and electron beam cross‐linking to simultaneously enhance mechanical properties, thermal stability, and flame retardancy. The utilization of electron beam irradiation resulted in an increase in gel content and a decrease in swelling ratio with increasing absorbed doses, indicating the successful formation of cross‐linked network structures. The hot‐set elongation was significantly reduced to 0.6%–0.8%, while the decomposition temperature increased with increasing absorbed doses, indicating improved thermal stability. The results from limiting oxygen index measurements and burning tests demonstrate that the flame retardancy of EPDM was improved by the combined effect of metal hydroxide flame retardant and electron beam cross‐linking.Highlights EPDM/metal hydroxide composites were prepared via electron beam irradiation. Metal hydroxide flame retardants were incorporated to enhance flame retardancy. EBI simultaneously enhanced mechanical property and thermal stability.

IMPACT OF HAZARDOUS TOXIC FACTORS OF FIRES DURING MILITARY ACTIONS ON ECOSYSTEMS AND POPULATION LIFE ACTIVITIES

It has been analysed that wildfires are the most influential factor in environmental pollution. The ecological consequences of wildfires consist primarily of atmospheric air pollution with carbon monoxide, products of combustion of combustible materials, and the burning of oxygen. Plant combustion also contributes significantly to planetary environmental changes: climate warming, ozone depletion, acid rain, chemical and radioactive pollution of the atmosphere, water, and soil. With wildfires, soot particles, that is, carbon and products of incomplete combustion of wood, enter the air. Smoke, which is formed during fires, explosions, ignitions, etc., during the combustion of plant biomass (grasses, leaves from trees and shrubs, mosses, lichens, peatlands, forest litter and needles), is an aerosol-gas mixture containing hazardous substances dangerous to the environment and humans. Biomass burning is a global source of gas and dust pollution of the atmosphere and is considered one of the determining factors of ecotoxicological risks for public health. The negative impact of fires from military actions on the human body is acute and delayed. Acute exposure leads to hypoxia due to carbon oxides, increased consumption of oxygen from the air during combustion, irritation of the respiratory tract, and difficulty breathing. Delayed exposure is due to toxic, mutagenic, and carcinogenic effects. The toxic impact of fires on the life and health of the population is not only a threat to life but also an adverse effect on human organisms of heavy compounds of chemical substances represented by the combustion products of plant biomass of forests, peatlands, vast agricultural lands, including crop rotations and pastures. Often, this impact occurs in combination with typical atmospheric air pollution: emissions from industrial enterprises and motor vehicles. The peculiarity of such a combination is the complex synergistic effects of chemical compounds and high air temperature with significant relative duration (decades, months) of emergency environmental situations. Keywords: toxic factors, fires, ecosystem, human activity.

3D simulations of magnetoconvection in a rapidly rotating supernova progenitor

ABSTRACT We present a first 3D magnetohydrodynamic (MHD) simulation of oxygen, neon, and carbon shell burning in a rapidly rotating $16\hbox{-}\mathrm{M}_\odot$ core-collapse supernova progenitor. We also run a purely hydrodynamic simulation for comparison. After $\mathord \approx 180\mathrm{s}$ ($\mathord \approx$ 15 and 7 convective turnovers, respectively), the magnetic fields in the oxygen and neon shells achieve saturation at 1011 and 5 × 1010 G. The strong Maxwell stresses become comparable to the radial Reynolds stresses and eventually suppress convection. The suppression of mixing by convection and shear instabilities results in the depletion of fuel at the base of the burning regions, so that the burning shell eventually move outward to cooler regions, thus reducing the energy generation rate. The strong magnetic fields efficiently transport angular momentum outwards, quickly spinning down the rapidly rotating convective oxygen and neon shells and forcing them into rigid rotation. The hydrodynamic model shows complicated redistribution of angular momentum and develops regions of retrograde rotation at the base of the convective shells. We discuss implications of our results for stellar evolution and for the subsequent core-collapse supernova. The rapid redistribution of angular momentum in the MHD model casts some doubt on the possibility of retaining significant core angular momentum for explosions driven by millisecond magnetars. However, findings from multidimensional models remain tentative until stellar evolution calculations can provide more consistent rotation profiles and estimates of magnetic field strengths to initialize multidimensional simulations without substantial numerical transients. We also stress the need for longer simulations, resolution studies, and an investigation of non-ideal effects.

Type Ia supernovae in NS+He star systems and the isolated mildly recycled pulsars

ABSTRACT Type Ia supernovae (SNe Ia) are successful cosmological distance indicators and important element factories in the chemical evolution of galaxies. They are generally thought to originate from thermonuclear explosions of carbon–oxygen white dwarfs in close binaries. However, the observed diversity among SNe Ia implies that they have different progenitor models. In this article, we performed the long-term evolution of NS+He star binaries with different initial He star masses ($M_{\rm He}^{\rm i}$) and orbital periods ($P_{\rm orb}^{\rm i}$) for the first time, in which the He star companions can explode as SNe Ia eventually. Our simulations indicate that after the He stars develop highly degenerate oxygen–neon (ONe) cores with masses near the Chandrasekhar limit, explosive oxygen burning can be triggered due to the convective Urca process. According to these calculations, we obtained an initial parameter space for the production of SNe Ia in the $\rm log\,$$P^{\rm i}_{\rm orb}{\text {--}}M^{\rm i}_{\rm He}$ plane. Meanwhile, we found that isolated mildly recycled pulsars can be formed after He stars explode as SNe Ia in NS+He star binaries, in which the isolated pulsars have minimum spin periods ($P_{\rm spin}^{\rm min}$) of ∼30–110 ms and final orbital velocities of ${\sim} \rm 60{\!-\!}360\, km\, s^{-1}$, corresponding to initial orbital periods of 0.07–10 d. Our work suggests that the NS+He star channel may contribute to the formation of isolated mildly recycled pulsars with velocity $\rm {\lesssim} 360\, km\, s^{-1}$ in observations, and such isolated pulsars should locate in the region of pulsars with massive white dwarf companions in the $P_{\rm spin} {\!-\!}\dot{P}_{\rm spin}$ diagram.

Mass Distribution of Black Holes with Effects of Convective Carbon Shell Burning on Pair-instability Pulsation and Fe Core Collapse

Motivated by the determination of black hole masses with gravitational-wave observations, we calculate the evolution of massive stars through presupernova stages and obtain the mass distribution of black holes. In the first part, we calculate the evolution of He stars with masses of 30–120 M ⊙. We study in detail how convective carbon shell burning controls pair-instability pulsations before and during oxygen burning and determine their final fates. In the second part, we calculate the evolution of H-rich stars with initial masses of 13–80 M ⊙ until Fe core collapse and obtain the possible black hole mass range by applying the criterion of the compactness parameters. From these models, we predict the mass distribution of black holes for stars that undergo Fe core collapse and pair-instability pulsation. The predicted masses for black holes range from 4.2 to 46 M ⊙, which are consistent with the gravitational-wave observations.

Thermonuclear and electron-capture supernovae from stripped-envelope stars

Context. When stripped from their hydrogen-rich envelopes, stars with initial masses between ∼7 and 11 M⊙ may develop massive degenerate cores and collapse. Depending on the final structure and composition, the outcome can range from a thermonuclear explosion, to the formation of a neutron star in an electron-capture supernova (ECSN). It has recently been demonstrated that stars in this mass range may be more prone to disruption than previously thought: they may initiate explosive oxygen burning when their central densities are still below ρc ≲ 109.6 g cm−3. At the same time, their envelopes expand significantly, leading to the complete depletion of helium. This combination makes them interesting candidates for type Ia supernovae–which we call (C)ONe SNe Ia–and this might have broader implications for the formation of neutron stars via ECSNe. Aims. To constrain the observational counterparts of (C)ONe SNe Ia and the key properties that enable them, it is crucial to constrain the evolution, composition, and precollapse structure of their progenitors, as well as the evolution of these quantities with cosmic time. In turn, this requires a detailed investigation of the final evolutionary stages preceding the collapse, and their sensitivity to input physics. Methods. Here, we modeled the evolution of 252 single, nonrotating helium stars covering the initial mass range 0.8 − 3.5 M⊙, with metallicities between Z = 10−4 and 0.02, and overshoot efficiency factors from fOV = 0.0 to 0.016 across all convective boundaries. We used these models to constrain several properties of these stars, including their central densities, compositions, envelope masses, and radii at the onset explosive oxygen ignition, as well as the final outcome as a function of initial helium star mass. We further investigate the sensitivity of these properties to mass loss rate assumptions using an additional grid of 110 models with varying wind efficiencies. Results. We find that helium star models with masses between ∼1.8 and 2.7 M⊙ are able to evolve onto 1.35−1.37 M⊙ (C)ONe cores that initiate explosive burning at central densities between log10(ρc/g cm−3) ∼ 9.3 and 9.6. We constrained the amount of residual carbon retained after core carbon burning as a function of initial conditions, and conclude that it plays a critical role in determining the final outcome: Chandrasekhar-mass degenerate cores that retain more than approximately 0.005 M⊙ of carbon result in (C)ONe SNe Ia, while those with lower carbon mass become ECSNe. We find that (C)ONe SNe Ia are more likely to occur at high metallicities, whereas at low metallicities ECSNe dominate. However, both SN Ia and ECSN progenitors expand significantly during the final evolutionary stages, so that for the most extended models, a further binary interaction may occur. We constrain the relative ratio between (C)ONe SNe Ia and SNe Ib/c to be 0.17−0.30 at Z = 0.02, and 0.03−0.13 at Z ≤ 10−3. Conclusions. We conclude with a discussion on potential observational properties of (C)ONe SNe Ia and their progenitors. In the few thousand years leading to the explosion, at least some progenitors should be identifiable as luminous metal-rich super-giants, embedded in hydrogen-free circumstellar nebulae.

Extreme Mass Loss in Low-mass Type Ib/c Supernova Progenitors

Many core-collapse supernovae (SNe) with hydrogen-poor and low-mass ejecta, such as ultra-stripped SNe and type Ibn SNe, are observed to interact with dense circumstellar material (CSM). These events likely arise from the core collapse of helium stars that have been heavily stripped by a binary companion and have ejected significant mass during the last weeks to years of their lives. In helium star models run to days before core collapse we identify a range of helium core masses ≈2.5–3 M ⊙ whose envelopes expand substantially due to the helium shell burning while the core undergoes neon and oxygen burning. When modeled in binary systems, the rapid expansion of these helium stars induces extremely high rates of late-stage mass transfer () beginning weeks to decades before core collapse. We consider two scenarios for producing CSM in these systems: either mass transfer remains stable and mass loss is driven from the system in the vicinity of the accreting companion, or mass transfer becomes unstable and causes a common envelope event (CEE) through which the helium envelope is unbound. The ensuing CSM properties are consistent with the CSM masses (∼10−2–1 M ⊙) and radii (∼1013–1016 cm) inferred for ultra-stripped SNe and several type Ibn SNe. Furthermore, systems that undergo a CEE could produce short-period neutron star binaries that merge in less than 100 Myr.

The magneto-rotational instability in massive stars

Context. The magneto-rotational instability (MRI) has been proposed as a mechanism to transport angular momentum (AM) and chemical elements in theoretical stellar models. Aims. Using as a prototype a massive star of 15 M⊙ with solar metallicity, we explore the effects of the MRI on the evolution of massive stars. Methods. We used the Geneva Stellar Evolution Code to simulate the evolution of various models, up to the end of oxygen burning, including the MRI through effective, one-dimensional, diffusion coefficients. We consider different trigger conditions (depending on the weighting of chemical gradients through an arbitrary but commonly used factor) and different treatments of meridional circulation as either advective or diffusive. We also compare the MRI with the Tayler-Spruit (TS) dynamo in models that included both instabilities. Results. The MRI triggers throughout stellar evolution. Its activation is highly sensitive to the treatment of meridional circulation and the existence of chemical gradients. The MRI is very efficient at transporting both matter and AM, leading to noticeable differences in rotation rates and chemical structure, which may be observable in young main sequence stars. While the TS dynamo is the dominant mechanism for transferring AM, the MRI remains relevant in models where both instabilities are included. Extrapolation of our results suggests that models that include the MRI tend to develop more compact cores, which likely produce failed explosions and black holes, than models where only the TS dynamo is included (where explosions and neutron stars may be more frequent). Conclusions. The MRI can be an important factor in massive star evolution but is very sensitive to the implementation of other processes in the model. The transport of AM and chemical elements due to the MRI alters the rotation rates and the chemical makeup of the star from the core to the surface and may change the explodability properties of massive stars.

Forbidden electron capture on Na24 and Al27 in degenerate oxygen-neon stellar cores

Background: Stars with an initial mass of $\ensuremath{\approx}7$--11 solar masses form degenerate oxygen-neon cores following carbon burning. Electron captures in such cores can trigger runaway oxygen burning, resulting in either a collapse or a thermonuclear explosion. Previous work constrained the contribution of the forbidden ${0}_{\text{g.s.}}^{+}\ensuremath{\rightarrow}{2}_{\text{g.s.}}^{+}$ transition to the $^{20}\mathrm{Ne}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{20}\mathrm{F}$ rate and discussed its significance for the evolution of the core.Purpose: We provide a detailed description of the formalism used in previous work and apply it to two further forbidden transitions that are relevant to degenerate oxygen-neon cores: the ${4}_{\text{g.s.}}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}$ transition in $^{24}\mathrm{Na}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Ne}$ and the $5/{2}_{\text{g.s.}}^{+}\ensuremath{\rightarrow}1/{2}_{\text{g.s.}}^{+}$ transition in $^{27}\mathrm{Al}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{27}\mathrm{Mg}$.Method: The relevant nuclear matrix elements are determined through shell model calculations and constraints from the conserved vector current (CVC) theory. We then investigate the astrophysical impact using the stellar evolution code mesa (Modules for Experiment in Stellar Astrophysics) and through timescale arguments.Results: In the relevant temperature range, the forbidden transitions substantially reduce the threshold densities for $^{24}\mathrm{Na}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Ne}$ and $^{27}\mathrm{Al}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{27}\mathrm{Mg}$. In the mesa models, $^{24}\mathrm{Na}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Ne}$ now occurs immediately following the onset of $^{24}\mathrm{Mg}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Na}$. The impact on the overall evolution is uncertain; this is due to known difficulties in accounting for convective instabilities triggered by the $A=24$ electron captures. The transition between $^{27}\mathrm{Al}$ and $^{27}\mathrm{Mg}$ may have a minor effect on the early evolution but is unlikely to affect the outcome.Conclusions: The studied transitions should be included when calculating weak interaction rates between $^{24}\mathrm{Na}$ and $^{24}\mathrm{Ne}$ for temperatures ${log}_{10}(T[\text{K}])\ensuremath{\lesssim}8.5$ and between $^{27}\mathrm{Al}$ and $^{27}\mathrm{Mg}$ for ${log}_{10}(T[\text{K}])\ensuremath{\lesssim}8.8$.

Presupernova ultralight axionlike particles

We calculate the production of ultra-light axion-like particles (ALPs) in a nearby supernova progenitor. Once produced, ALPs escape from the star and a part of them is converted into photons during propagation in the Galactic magnetic field. It is found that the MeV photon flux that reaches Earth may be detectable by gamma ray telescopes for ALPs lighter than ~1 neV when Betelgeuse undergoes oxygen and silicon burning. The dependence of the gamma ray flux on the stellar mass is much smaller than the uncertainty that originates from the Galactic magnetic field. If ALPs are lighter than ~0.1 neV and the supernova progenitor is close enough to the Solar System, the gamma ray flux is insensitive to the distance d because the ALP-photon conversion probability is proportional to d^2. (Non-)detection of gamma rays from a supernova progenitor with next-generation gamma ray telescopes just after pre-supernova neutrino alerts would lead to an independent constraint on ALP parameters as stringent as a SN 1987A limit.

Progenitor and close-in circumstellar medium of type II supernova 2020fqv from high-cadence photometry and ultra-rapid UV spectroscopy

ABSTRACT We present observations of SN 2020fqv, a Virgo-cluster type II core-collapse supernova (CCSN) with a high temporal resolution light curve from the Transiting Exoplanet Survey Satellite (TESS) covering the time of explosion; ultraviolet (UV) spectroscopy from the Hubble Space Telescope (HST) starting 3.3 d post-explosion; ground-based spectroscopic observations starting 1.1 d post-explosion; along with extensive photometric observations. Massive stars have complicated mass-loss histories leading up to their death as CCSNe, creating circumstellar medium (CSM) with which the SNe interact. Observations during the first few days post-explosion can provide important information about the mass-loss rate during the late stages of stellar evolution. Model fits to the quasi-bolometric light curve of SN 2020fqv reveal 0.23 M⊙ of CSM confined within 1450 R⊙ (1014 cm) from its progenitor star. Early spectra (<4 d post-explosion), both from HST and ground-based observatories, show emission features from high-ionization metal species from the outer, optically thin part of this CSM. We find that the CSM is consistent with an eruption caused by the injection of ∼5 × 1046 erg into the stellar envelope ∼300 d pre-explosion, potentially from a nuclear burning instability at the onset of oxygen burning. Light-curve fitting, nebular spectroscopy, and pre-explosion HST imaging consistently point to a red supergiant (RSG) progenitor with $M_{\rm ZAMS}\approx 13.5\!-\!15 \, \mathrm{M}_{\odot }$, typical for SN II progenitor stars. This finding demonstrates that a typical RSG, like the progenitor of SN 2020fqv, has a complicated mass-loss history immediately before core collapse.

Halogen‐Free Polyphosphazene‐Based Flame Retardant Cathode Materials for Li–S Batteries

Novel, eco‐friendly, and halogen‐free polyphosphazene‐based flame retardant (FR) cathodes for Li–S batteries are prepared by the inverse vulcanization of elemental sulfur with polybis(2‐acrylamidoethoxy)phosphazene (poly(AAE)) resulting in the covalently bonded different wt% sulfur content polymers. Due to the polar amide groups, poly(AAE)‐xS (x refer to the wt% of sulfur) composites assist in anchoring intermediate lithium polysulfide species. The structural characterizations of the poly(AAE)‐xS polymers are conducted by elemental analysis, Fourier‐transform infrared spectroscopy, differential scanning calorimeter, thermogravimetric analysis, and scanning electron microscopy. Their FR properties are elevated by calculation of the limiting oxygen index values and burning tests. Later, lithium‐ion storage mechanism of poly(AAE)‐55S polymer is evaluated, including cyclic voltammetry, rate performance tests and electrochemical impedance spectroscopy measurements. To compare with the electrochemical performance of poly(AAE)‐55S, a noncovalent blend polymer mixture and bare sulfur are also tested. It is found that covalently bonded poly(AAE)‐55S obtained by the inverse vulcanization of elemental sulfur and poly(AAE) results much pronounced capacities. Herein, new insights into the design and development of alternative cathode materials for safer Li–S batteries, both providing FR properties and good lithium polysulfides adsorption capabilities, are offered.

Chemical modeling for methane oxy-combustion in Liquid Rocket Engines

Methane–oxygen burning is considered for many future rocket engines for practicality and cost reasons. As this combustion is slower than hydrogen–oxygen, flame ignition and stability may be more difficult to obtain. To address these questions, numerical simulation with realistic chemistry is appropriate. However the high pressure and turbulence intensity encountered in rocket engines enhance drastically the stiffness of methane oxy-combustion. In this work, Analytically Reduced Chemistry (ARC) is proposed for accurate chemistry description at a reasonable computational cost. An ARC scheme is specifically derived for typical rocket engine conditions. It is validated by comparison with its parent skeletal mechanism on a series of laminar flames. Then the numerical stiffness of chemistry is overcome with an original approach for time integration, allowing to run simulations close to the acoustic time step whatever the chemical stiffness. It is demonstrated on laminar cases that the flame structure is well preserved, and that numerical stability is ensured while decreasing significantly the computational cost. The performance of ARC with the fast time integration method is finally demonstrated in a 3D Large-Eddy Simulation of a lab-scale Liquid Rocket Engine combustion chamber, where a detailed flame analysis is conducted.

3D simulations of oxygen shell burning with and without magnetic fields.

We present a first 3D magnetohydrodynamic (MHD) simulation of convective oxygen and neon shell burning in a non-rotating [Formula: see text] star shortly before core collapse to study the generation of magnetic fields in supernova progenitors. We also run a purely hydrodynamic control simulation to gauge the impact of the magnetic fields on the convective flow and on convective boundary mixing. After about 17 convective turnover times, the magnetic field is approaching saturation levels in the oxygen shell with an average field strength of [Formula: see text], and does not reach kinetic equipartition. The field remains dominated by small-to-medium scales, and the dipole field strength at the base of the oxygen shell is only [Formula: see text]. The angle-averaged diagonal components of the Maxwell stress tensor mirror those of the Reynolds stress tensor, but are about one order of magnitude smaller. The shear flow at the oxygen-neon shell interface creates relatively strong fields parallel to the convective boundary, which noticeably inhibit the turbulent entrainment of neon into the oxygen shell. The reduced ingestion of neon lowers the nuclear energy generation rate in the oxygen shell and thereby slightly slows down the convective flow. Aside from this indirect effect, we find that magnetic fields do not appreciably alter the flow inside the oxygen shell. We discuss the implications of our results for the subsequent core-collapse supernova and stress the need for longer simulations, resolution studies, and an investigation of non-ideal effects for a better understanding of magnetic fields in supernova progenitors.

Modelling of coal combustion in a drop tube furnace in air and oxy fuel environment

The present energy crisis and the growing concern over emission lead to the development of various low emission technologies for efficient utilization of fossil fuels. These technologies offer the benefits to continue using fossil fuels without harming the environment. Oxyfuel combustion technology is one of the promising low emission combustion techniques that have the advantage of being a better retrofit option as well. A non comprehensive modelling study on coal combustion in air and oxy fuel mode was carried out using Fluent, a CFD tool. A numerical study to investigate the effects of balance gas N2 and CO2 and the effects of oxygen concentration on coal burning have been carried out. A drop tube furnace with higher heating rate simulates the real time furnace to some extent was chosen for modelling. The computational results show that, for the same oxygen concentration, the particle and burning temperatures were much lower and the burning of volatiles and char are delayed in oxy case compared to air case. It implied that coal particles take longer to burn completely in the presence of CO2. This will affect the entire processes such as heat transfer, flame stability, emission and ash characteristics inside the furnace. The less intense or slower burning in oxy case is attributed to the higher molar specific heat of CO2 and lower thermal and mass diffusivity in CO2. This ill effects in oxy case can be overcome by increasing the oxygen concentration above 21% so that the burning characteristics as in air (with 21% O2) case can be achieved. The trends obtained from this study are in agreement with the findings from the literature.

Study of fusion hindrance in the system 12C+24Mg

The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behaviour of heavy-ion fusion at extreme sub-barrier energies, including reactions involving lighter systems, e.g. reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the S-factor vs. energy. Whether there is an S-factor maximum at very low energies for systems with a positive fusion Q-value is an experimentally challenging question. Our aim has been to search evidence for fusion hindrance in 12C + 24Mg which is a medium-light systems with positive Q-value for fusion, besides the heavier cases where hindrance is recognised to be a general phenomenon. The experiment has been performed at the XTU Tandem accelerator of LNL by directly detecting the fusion evaporation residues at very forward angles. The excitation function has been extended down to ≃10μb, i.e. 4 orders of magnitude lower than previous measurements and we observe that the S-factor develops a clear maximum vs. energy. Coupled-Channels calculations using a Woods-Saxon potential give a good account of the data near and above the barrier but over predict the cross sections at very low energies. Therefore the hindrance phenomenon is clearly recognised in 12 C + 24 Mg with an energy threshold that nicely fits the systematics in several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value (as=1.6mb) is indeed found for this system. It may be possible to extend the measurements further down in energy.

Impact of forbidden electron capture in intermediate-mass stars

Intermediate-mass stars (7-11 M⊙) form degenerate ONe cores following carbon burning. In some cases the cores grow dense enough to trigger electron capture on various nuclei. The double electron capture 20Ne(e^-, v e)20F(e^-, νe )20O releases enough heat to trigger runaway oxygen burning. Depending on the conditions at ignition the outcome is either a collapse to a neutron star or a thermonuclear explosion with an ONeFe remnant. We have investigated the impact of two forbidden transitions on the ignition conditions using the MESA stellar evolution code. The 0+ → 2+ transition between the ground states of 20Ne and 20F has been measured experimentally and is found to decrease the ignition density while pushing the point of ignition off-centre. We have also studied the 2+ → 4+ transition between 24Na and 24Ne through a shell model calculation. Its impact is found to be marginal in our simulations, but it might play a role in the development of convectional instabilities.