Shell Burning Research Articles

The Evolution of Massive Helium Stars, Including Mass Loss

Abstract The evolution of helium stars with initial masses in the range 1.6–120 is studied, including the effects of mass loss by winds. These stars are assumed to form in binary systems when their expanding hydrogenic envelopes are promptly lost just after helium ignition. Significant differences are found with single-star evolution, chiefly because the helium core loses mass during helium burning rather than gaining it from hydrogen shell burning. Consequently, presupernova stars for a given initial mass function have considerably smaller mass when they die and will be easier to explode. Even accounting for this difference, the helium stars with mass loss develop more centrally condensed cores that should explode more easily than their single-star counterparts. The production of low-mass black holes may be diminished. Helium stars with initial masses below 3.2 experience significant radius expansion after helium depletion, reaching blue supergiant proportions. This could trigger additional mass exchange or affect the light curve of the supernova. The most common black hole mass produced in binaries is estimated to be about 9 . A new maximum mass for black holes derived from pulsational pair-instability supernovae is derived, 46 , and a new potential gap at 10–12 is noted. Models pertinent to SN 2014ft are presented, and a library of presupernova models is generated.

Between winter storm surges – Human occupation on a growing Mid-Holocene transgression maximum (Tapes) beach ridge at Longva, Western Norway

Substantial amounts of archaeological material have been found intermixed with beach pebbles and cobbles on the Tapes beach ridge at Longva on the island Flemsøya/Skuløya in Western Norway. The artefacts show that the beach ridge was settled in the Late Mesolithic. The most significant remains are fireplaces, birch bark from the floor of a tent/hut, fish sinkers and middens containing numerous waste flakes and lithic tools. Radiocarbon dating, mainly of burnt hazelnut shells, shows two periods of occupation. The older and longer period is dated to between 7600 and 6800 cal yr BP, and the younger phase to between 6200 and 5900 cal yr BP. Pollen analysis revealed open vegetation at the beach ridge during the occupation periods. Based on the beach ridge deposits and radiocarbon dates, we reconstructed the Tapes transgression maximum high tide sea level to 8.2–9.0 m between 7600 and 5600 cal yr BP. We conclude that the late Mesolithic inhabitants at Longva occupied the beach ridge while it was growing. During the largest storm surges – most likely to have been in the winter months – the sea would have washed over their settlements and deposited pebbles and cobbles on top of their remains. We suggest that the inhabitants abandoned the settlement before each stormy season, but returned and restored the site the following spring or summer.

The Helium Abundance of NGC 6791 from Modeling of Stellar Oscillations

Abstract The helium abundance of stars is a strong driver of evolutionary timescales; however, it is difficult to measure in cool stars. We conduct an asteroseismic analysis of NGC 6791, an old, metal-rich open cluster that previous studies have indicated also has a high helium abundance. The cluster was observed by Kepler and has unprecedented light curves for many of the red giant branch stars in the cluster. Previous asteroseismic studies with Kepler data have constrained the age through grid-based modeling of the global asteroseismic parameters ( and ). However, with the precision of Kepler data, it is possible to do detailed asteroseismology of individual mode frequencies to better constrain the stellar parameters, something that has not been done for these cluster stars as yet. In this work, we use the observed mode frequencies in 27 hydrogen shell burning red giants to better constrain initial helium abundance (Y 0) and age of the cluster. The distributions of helium abundance and age for each individual red giant are combined to create a final probability distribution for age and helium abundance of the entire cluster. We find a helium abundance of Y 0 = 0.297 ± 0.003 and a corresponding age of 8.2 ± 0.3 Gyr.

Slowing the spins of stellar cores

ABSTRACT The angular momentum (AM) evolution of stellar interiors, along with the resulting rotation rates of stellar remnants, remains poorly understood. Asteroseismic measurements of red giant stars reveal that their cores rotate much faster than their surfaces, but much slower than theoretically predicted, indicating an unidentified source of AM transport operates in their radiative cores. Motivated by this, we investigate the magnetic Tayler instability and argue that it saturates when turbulent dissipation of the perturbed magnetic field energy is equal to magnetic energy generation via winding. This leads to larger magnetic field amplitudes, more efficient AM transport, and smaller shears than predicted by the classic Tayler–Spruit dynamo. We provide prescriptions for the effective AM diffusivity and incorporate them into numerical stellar models, finding they largely reproduce (1) the nearly rigid rotation of the Sun and main sequence stars, (2) the core rotation rates of low-mass red giants during hydrogen shell and helium burning, and (3) the rotation rates of white dwarfs. We discuss implications for stellar rotational evolution, internal rotation profiles, rotational mixing, and the spins of compact objects.

Mycotoxin concentrations in rice from three climatic locations in Africa as affected by grain quality, production site, and storage duration.

Information on the mycotoxin contamination of rice in Africa is limited although the risk of contamination is high. In this study, domestic milled rice processed by actors using suboptimal methods was purchased and total fumonisin (FUM), zearalenone, and aflatoxin concentrations determined at 0, 90, and 180 days after storage. Three different climatic locations, Cotonou (Benin) in the Guinea savanna, Yaoundé (Cameroon) in the Tropical forest, and N'diaye (Senegal) in the Sahel, were selected as storage sites. Subsets of the samples collected from Glazoue (Benin), Ndop (Cameroon), and Dagana (Senegal) were stored in plastic woven bags under room conditions in the respective sites with or without calcium oxide (burnt scallop shell—BSS, 0.1% w/w) treatment. Multivariance analysis showed that FUM concentration was positively influenced by the duration of storage only while zearalenone concentration was negatively influenced by relative humidity and head rice but positively by impurities. Zearalenone concentration was also influenced by sample collection/storage location, processing type, and duration of storage. Aflatoxin concentration was influenced negatively by storage room temperature and head rice but positively by impurities and chalky grains. In addition, aflatoxin concentration was influenced by collection/storage location and processing type. BSS treatment followed by storage for 6 months had no effect on the concentration of the three assessed mycotoxins. Strategies to reduce the risk of mycotoxin contamination in study sites will include the improvement of physical rice quality through better pre‐ and postharvest practices and proper packaging of both treated rice and untreated rice in hermetic systems before marketing and storage.

INTERNAL BALLISTICS OF CLASSIC GUN WITH COMPOSITE CHARGE

The paper presents a lumped parameter mathematical model considering the changes of thermodynamic properties for combustion products of a composite propelling charge, and of a burning cartridge casing or shell as well, when shot with classic guns. In addition a method was proposed for considering not coincidental instants of ignition for particular components of the charge and also for powder grains of each component, and the heat flow into the walls containing the space with combustion products. Some results of numerical computations are shown for 125 mm 2A46 tank gun firing a hard core projectile. Moreover an evaluation of accuracy of the results is given on the basis of experimental data. Maximum pressure and muzzle velocity were basic criteria at the verification of the model. Analysis of accuracy for solutions of model equations allows a conclusion that the proposed mathematical model may be useful at the designing process of ammunition and guns.

Final evolution of super-AGB stars and supernovae triggered by electron capture

AbstractStars of 8–10 M⊙ form a strongly electron-degenerate oxygen–neon–magnesium core which is more massive than ∼1.1 M⊙, and become super-Asymptotic Giant Branch stars. The oxygen–neon–magnesium core increases its mass through H and He shell burning. The core contracts accordingly and the central density increases. In the high density core, electron capture takes place and further boosts the core contraction. When electron capture on 20Ne starts, it induces oxygen–neon deflagration. It remains a theoretical question whether neutron star can be formed after the deflagration has started. If the star collapses, the following explosion is known as an electron capture supernova. In this article, we give a brief overview on the development of idea in the presupernova evolution and the hydrodynamics behaviour of electron capture supernovae. Using standard stellar evolutionary models that show rather high ignition density, we show that the collapse can occur in a wide range of model parameter. However, future study remains important. We also review the possible observables of electron capture supernovae and discuss their applications to the light curve model for the Crab supernova 1054.

PKM - PENGUSAHA ARANG TEMPURUNG KELAPA DESA BANTAN AIR KECAMATAN BANTAN KABUPATEN BENGKALIS PROVINSI RIAU

Bantan Air Village, which is part of Bantan District, Bengkalis Regency, has been established by two SMEs and is engaged in processing coconut shells into charcoal. Both of these MSMEs already have burning stoves but have the same problem, namely the smoke produced from burning coconut shells is released into the air without further treatment. The smoke that is discharged can of course disrupt the health of workers or the community around the place of business. Thus, it is necessary to apply more applicable science and technology to reduce smoke pollution and even convert the smoke waste into something that has a selling value. The proposer and the two partners have agreed to solve this problem, namely the manufacture of condensers to turn smoke into liquid, so that the aim of reducing pollution and increasing revenue can be achieved. This PKM activity begins with a preliminary survey to the location of partners and determines the solution to the problem, followed by the making of a condenser, provides training in how to operate equipment, care and guidance. The output target of this PKM has been achieved by producing a condenser to reduce smoke from the burning of coconut shells and produce products in the form of liquid smoke that is ready to sell. In addition, the community and MSME workers have also been given counseling and training in the use of condensers so that the knowledge and skills of workers can increase

Investigating the effect of eggshell burning temperature on the mechanical properties of artificial teeth

In the field of health, ceramics are used for the repair, reconstruction and replacement of the teeth and bones and the body tissue, which is now very possible to be developed into bio-ceramics. Bio-ceramic materials can be used for the manufacture of artificial teeth, wherein this material consists of kaolin, quartz, feldspar, and eggshell. The use of eggshells as an added ingredient and reinforcement in the artificial teeth has been studied and reviewed in some literatures, but they report the effect of temperature on the burning of egg shells is still lacking. This paper investigates the effect of eggshell burning temperature on the mechanical properties of artificial teeth. The effect of different burning temperatures on hardness is also presented.

Helium giant stars as progenitors of rapidly fading Type Ibc supernovae

ABSTRACT Type I rapidly fading supernovae (RFSNe) appear to originate from hydrogen-free stars with large radii that produce predominantly shock-cooling light curves, in contrast with more typical $^{56}\rm {Ni}$-rich SNe Ibc. However, it remains to be determined what types of stars would produce bright shock-cooling light curves without significant contribution from radioactive nickel. Bare helium stars in the mass range ${\sim }2\hbox{--}4 \, \rm {M}_\odot$ are known to hydrostatically develop radii as large as 100 $\rm {R}_\odot$ or more due to strong He and C shell burning outside of a core with a sharp density gradient. We produce several such stellar models and demonstrate that, when exploded, these helium giants can naturally produce RFSN light curves. Since many prototypical SNe Ibc should come from large-radius stars in this mass range as well, we predict that these RFSNe may be distinct from SNe Ibc solely due to the absence of substantial $^{56}\rm {Ni}$.

Turbulent mixing and nuclear burning in stellar interiors

The turbulent burning of nuclei is a common phenomenon in the evolution of stars. Here we examine a challenging case: the merging of the neon and oxygen burning shells in a 23 M$_{\odot}$ star. A previously unknown quasi-steady state is established by the interplay between mixing, turbulent transport, and nuclear burning. The resulting stellar structure has two burning shells within a single convection zone. We find that the new neon burning layer covers an extended region of the convection zone, with the burning peak occurring substantially below where the Damk\"ohler number first becomes equal to unity. These characteristics differ from those predicted by 1D stellar evolution models of similar ingestion events. We develop the mean-field turbulence equations that govern compositional evolution, and use them to interpret our data set. An important byproduct is a means to quantify sub-grid-scale effects intrinsic to the numerical hydrodynamic scheme. For implicit large eddy simulations, the analysis method is particularly powerful because it can reveal where and how simulated flows are modified by resolution, and provide straightforward physical interpretations of the effects of dissipation or induced transport. Focusing on the mean-field composition variance equations for our analysis, we recover a Kolmogorov rate of turbulent dissipation without it being imposed, in agreement with previous results which used the turbulent kinetic energy equation.

Heating mollusc shells - A radiocarbon and microstructure perspective from archaeological shells recovered from Kalba, Sharjah Emirate, UAE

This publication aims to shed light on the influence that prior heating (burning) of mollusc shells during human activity may have on the results of radiocarbon dating. We compare the geochemical and mineralogical composition of heated and unheated shells of Anadara uropigimelana and Terebralia palustris recovered from Neolithic and Bronze Age archaeological contexts at Kalba, Sharjah Emirate, in the United Arab Emirates (UAE). Our research examined whether the heating of shells impacts on the determination of reservoir effects, or whether in spite of heating, this material remains a viable material for precise 14C measurements. Our results show that both heated and non-heated shells of A. uropigimelana and T. palustris provide consistent results, although the mineral composition of the shells changes from aragonite to calcite. Our results are important, since some of our selection of shells did not initially appear to have been heated. A heating process will then usually be detected as a greyish, marble like structure when cutting the shells. As a result of this work, we have also developed insights into prehistoric cooking practices of shells collected in Arabia. Our results provide archaeologists and associated researchers with confidence when assessing the results of radiocarbon dating during their studies of shells that might have been heated.

X-ray, UV, and optical observations of the accretion disk and boundary layer in the symbiotic star RT Crucis

Compared to mass transfer in cataclysmic variables, the nature of accretion in symbiotic binaries in which red giants transfer material to white dwarfs (WDs) has been difficult to uncover. The accretion flows in a symbiotic binary are most clearly observable, however, when there is no quasi-steady shell burning on the WD to hide them. RT Cru is the prototype of such non-burning symbiotics, with its hard (δ-type) X-ray emission providing a view of its innermost accretion structures. In the past 20 yr, RT Cru has experienced two similar optical brightening events, separated by ~4000 days and with amplitudes of ΔV ~ 1.5 mag. After Swift became operative, the Burst Alert Telescope (BAT) detector revealed a hard X-ray brightening event almost in coincidence with the second optical peak. Spectral and timing analyses of multi-wavelength observations that we describe here, from NuSTAR, Suzaku, Swift/X-Ray Telescope (XRT) + BAT + UltraViolet Optical Telescope (UVOT) (photometry) and optical photometry and spectroscopy, indicate that accretion proceeds through a disk that reaches down to the WD surface. The scenario in which a massive, magnetic WD accretes from a magnetically truncated accretion disk is not supported. For example, none of our data show the minute-time-scale periodic modulations (with tight upper limits from X-ray data) expected from a spinning, magnetic WD. Moreover, the similarity of the UV and X-ray fluxes, as well as the approximate constancy of the hardness ratio within the BAT band, indicate that the boundary layer of the accretion disk remained optically thin to its own radiation throughout the brightening event, during which the rate of accretion onto the WD increased to 6.7 × 10−9M⊙ yr−1 (d/2 kpc)2. For the first time from a WD symbiotic, the NuSTAR spectrum showed a Compton reflection hump at E > 10 keV, due to hard X-rays from the boundary layer reflecting off of the surface of the WD; the reflection amplitude was 0.77 ± 0.21. The best fit spectral model, including reflection, gave a maximum post-shock temperature of kT = 53 ± 4 keV, which implies a WD mass of 1.25 ± 0.02 M⊙. Although the long-term optical variability in RT Cru is reminiscent of dwarf-novae-type outbursts, the hard X-ray behavior does not correspond to that observed in well-known dwarf nova. An alternative explanation for the brightening events could be that they are due to an enhancement of the accretion rate as the WD travels through the red giant wind in a wide orbit, with a period of about ~4000 days. In either case, the constancy of the hard X-ray spectrum while the accretion rate rose suggests that the accretion-rate threshold between a mostly optically thin and thick boundary layer, in this object, may be higher than previously thought.

A Model of the Mira–Type Star T UMi

Stellar evolution calculations were carried out from the main sequence to the final AGB stage of stars with initial masses from 1 to $2M_\odot$ and metallicity $Z=0.01$. Selected models of evolutionary sequences were used as initial conditions for solution of the equations of radiation hydrodynamics and time-dependent convection describing radial stellar pulsations. The study was aimed to construct the hydrodynamic models of Mira-type stars that show the secular decrease in the pulsation period commenced in 1970--th at $\Pi=315$ day. We show that such a condition for the period change is satisfied with evolutionary sequences from 1 to $1.2M_\odot$ and the best agreement with observations is obtained for models with initial mass $1.2M_\odot$. The pulsation period reduction is due to both the stellar radius decrease during the thermal pulse of the helium burning shell and mode switch from the fundamental mode to the first overtone. Theoretical estimates of the fundamental parameters of the star at the onset of pulsation period reduction are as follows: the mass is $M=0.93M_\odot$, the luminosity is $L = 4080L_\odot$, and the radius is $R=220R_\odot$. The mode switch occurs 35 years after the onset of period reduction.

Effect of elevated temperatures on compressive strength and microstructure of cement paste containing palm oil clinker powder

Residual compressive strength of cement after exposure to elevated temperatures is important when introduces a new supplementary cementitious material for cement based applications. Palm oil clinker (POC) is a residue produced upon burning of palm oil shells and fibres in a boiler of palm oil mill. The aim of this work is to evaluate the high temperature effect on compressive strength and microstructure change of palm oil clinker powder (POCP) containing cement paste. The palm oil clinker powder containing cement (POCPC) was prepared for 30% replacement of ordinary Portland cement (OPC). Elevated temperatures at 300 °C, 600 °C, and 800 °C have been maintained for 3 h for both OPC and POCPC paste. The compressive strengths of OPC and POCPC were measured at the curing age of 7, 28, 56 and 90 days. The mineralogy, IR spectroscopy, thermal analysis and microstructure were investigated using XRD, FTIR, TGA and FESEM, respectively. The compressive strength of POCPC is significantly higher than that of OPC sample when thermally activated at 600 °C and 800 °C for 3 h. The XRD, FTIR, TGA and FESEM results reveal that more hydration products (C-S-H) sustain in POCPC compare with OPC after exposure to elevated temperature that is the causes behind higher compressive strength in POCPC.

Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary systemφPersei

Stripped-envelope stars form in binary systems after losing mass through Roche-lobe overflow. They bear astrophysical significance as sources of UV and ionizing radiation in older stellar populations and, if sufficiently massive, as stripped supernova progenitors. Binary evolutionary models predict that they are common, but only a handful of subdwarfs with B-type companions are known. The question is whether a large population of such systems has evaded detection as a result of biases, or whether the model predictions are wrong. We reanalyze the well-studied post-interaction binaryφPersei. Recently, new data have improved the orbital solution of the system, which contains an ~1.2M⊙stripped-envelope star and a rapidly rotating ~9.6M⊙Be star. We compare with an extensive grid of evolutionary models using a Bayesian approach and constrain the initial masses of the progenitor to 7.2 ± 0.4M⊙for the stripped star and 3.8 ± 0.4M⊙for the Be star. The system must have evolved through near-conservative mass transfer. These findings are consistent with earlier studies. The age we obtain, 57 ± 9 Myr, is in excellent agreement with the age of theαPersei cluster. We note that neither star was initially massive enough to produce a core-collapse supernova, but mass exchange pushed the Be star above the mass threshold. We find that the subdwarf is overluminous for its mass by almost an order of magnitude, compared to the expectations for a helium core burning star. We can only reconcile this if the subdwarf resides in a late phase of helium shell burning, which lasts only 2–3% of the total lifetime as a subdwarf. Assuming continuous star formation implies that up to ~50 less evolved, dimmer subdwarfs exist for each system similar toφPersei, but have evaded detection so far. Our findings can be interpreted as a strong indication that a substantial population of stripped-envelope stars indeed exists, but has so far evaded detection because of observational biases and lack of large-scale systematic searches.

Mollusk carbonate thermal behaviour and its implications in understanding prehistoric fire events in shell middens

Archaeological shell middens are particularly important for reconstructing prehistoric human subsistence strategies. However, very little is known about shellfish processing, especially when related to the use of fire for dietary and disposal purposes. To shed light on prehistoric food processing techniques, an experimental study was undertaken on modern gastropod shells (Phorcus lineatus). The shells were exposed to high temperatures (200–700 °C) to investigate subsequent mineralogy and macro- and microstructural changes. Afterwards, the three-pronged approach was applied to archaeological shells from Haua Fteah cave, Libya (Phorcus turbinatus) and from shell midden sites in the United Arab Emirates (Anadara uropigimelana and Terebralia palustris) to determine exposure temperatures. Results indicated that shells from the Haua Fteah were exposed to high temperatures (600–700 °C) during the Mesolithic period (ca. 12.7–9 ka), whereas specimens from the Neolithic period (ca. 8.5–5.4 ka) were mainly exposed to lower temperatures (300–500 °C). The thermally-induced changes in A. uropigimelana and T. palustris shells from the South East Arabian archaeological sites were similar to those seen in Phorcus spp. suggesting a broad applicability of the experimental results at an interspecific level. Although heat significantly altered the appearance and mineralogy of the shells, 14CAMS ages obtained on burnt shells fit within the expected age ranges for their associated archaeological contexts, indicating that robust radiocarbon ages may still be obtained from burnt shells. Our study indicates that the combination of microstructural and mineralogical observations can provide important information to infer shellfish processing strategies in prehistoric cultures and their change through time.

The Betelgeuse Project II: asteroseismology

We explore the question of whether the interior state of massive red supergiant supernova progenitors can be effectively probed with asteroseismology. We have computed a suite of ten models with ZAMS masses from 15 to 25 m_sun in intervals of 1 m_sun, including the effects of rotation, with the stellar evolutionary code MESA. We estimate characteristic frequencies and convective luminosities of convective zones at two illustrative stages, core helium burning and off-center convective carbon burning. We also estimate the power that might be delivered to the surface to modulate the luminous output considering various efficiencies and dissipation mechanisms. The inner convective regions should generate waves with characteristic periods of ~20 days in core helium burning, ~10 days in helium shell burning, and 0.1 to 1 day in shell carbon burning. Acoustic waves may avoid both shock and diffusive dissipation relatively early in core helium burning throughout most of the structure. In shell carbon burning, years before explosion, the signal generated in the helium shell might in some circumstances be weak enough to avoid shock dissipation, but is subject to strong thermal dissipation in the hydrogen envelope. Signals from a convective carbon-burning shell are very likely to be even more severely damped within the envelope. In the most optimistic case, early in core helium burning, waves arriving close to the surface could represent luminosity fluctuations of a few millimagnitudes, but the conditions in the very outer reaches of the envelope suggest severe thermal damping there.

Turbulence Generation by Shock-Acoustic-Wave Interaction in Core-Collapse Supernovae

Convective instabilities in the advanced stages of nuclear shell burning can play an important role in neutrino-driven supernova explosions. In our previous work, we studied the interaction of vorticity and entropy waves with the supernova shock using a linear perturbations theory. In this paper, we extend our work by studying the effect of acoustic waves. As the acoustic waves cross the shock, the perturbed shock induces a field of entropy and vorticity waves in the post-shock flow. We find that, even when the upstream flow is assumed to be dominated by sonic perturbations, the shock-generated vorticity waves contain most of the turbulent kinetic energy in the post-shock region, while the entropy waves produced behind the shock are responsible for most of the density perturbations. The entropy perturbations are expected to become buoyant as a response to the gravity force and then generate additional turbulence in the post-shock region. This leads to a modest reduction of the critical neutrino luminosity necessary for producing an explosion, which we estimate to be less than ~5%.

Hydrogen and helium shell burning during white dwarf accretion

Type Ia supernovae (SNe Ia) are believed to be thermonuclear explosions of carbon oxygen (CO) white dwarfs (WDs) with masses close to the Chandrasekhar mass limit. How a CO WD accretes matter and grows in mass to this limit is not well understood, hindering our understanding of SN Ia explosions and the reliability of using SNe Ia as a cosmological distance indicator. In this work, we employed the stellar evolution code MESA to simulate the accretion process of hydrogen-rich material onto a 1.0 M⊙ CO WD at a high rate (over the Eddington limit) of 4.3 × 10−7M⊙ yr−1. The simulation demonstrates the characteristics of the double shell burning on top of the WD, with a hydrogen shell burning on top of a helium burning shell. The results show that helium shell burning is not steady (i.e. it flashes). Flashes from the helium shell are weaker than those in the case of accretion of helium-rich material onto a CO WD. The carbon to oxygen mass ratio resulting from the helium shell burning is higher than what was previously thought. Interestingly, the CO WD growing due to accretion has an outer part containing a small fraction of helium in addition to carbon and oxygen. The flashes become weaker and weaker as the accretion continues.