Helium Shell Research Articles

Silicon Isotopic Composition in Large Meteoritic SiC Particles and 22 Na Origin of 22 Ne

Large silicon carbide (SiC) particles extracted from acid-insoluble residues of carbonaceous chondrites are isotopically anomalous in both silicon and carbon and contain isotopically extreme noble gases. These particles are thought to have originated in mass outflows from red giant stars and to have existed in the interstellar medium at the time the solar system formed from an interstellar cloud. Calculations show that the silicon isotope correlations in those large SiC particles can be generated only in the most massive carbon stars. Consequently, the almost pure neon-22 ((22)Ne) in those particles must be interpreted as the condensation of radioactive sodium-22 ((22)Na) in the particles as they flowed away from the stars. The (22)Na is produced through proton capture by (21)Ne at the base of the surface convection zone. Neon-22 does not exist abundantly in helium shells hot enough to burn magnesium, which is necessary to establish the measured silicon isotopic composition.

SiC particles from asymptotic giant branch stars - MG burning and the s-process

The question of whether isotopically anomalous SiC particles found in meteorites originate in AGB stars is addressed. It is shown that if the peak helium shell flash temperatures of massive (6-9 solar masses) stars are about 10 percent larger than they are normally assumed to be, alpha particle reactions with the magnesium will become significant. Then the (Mg-29)(alpha, n)Si-29 reaction produces a large excess of Si-29. With a light element nuclear reaction network, the evolution of the silicon isotopic composition during AGB evolution is calculated. It is found that the experimentally determined correlation between excess Si-29 and excess Si-30 in SiC particles from carbonaceous chondrites can indeed be naturally produced in this way. It is suggested that if the large isotopically anomalous SiC particles carrying nearly pure-process krypton and xenon do indeed originate in AGB stars, those stars were massive and had peak shell flash temperatures near 450 KM.

On the nova-like eruptions of symbiotic binaries

We discuss three popular explanations for the nova-like eruptions observed in symbiotic binary stars and compare model predictions with recent observations. Most outbursts occur when unstable hydrogen shell burning causes a hot white dwarf to expand to a radius of 1–100 R⊙. This model predicts a long-duration, constant-luminosity phase following visual maximum, and observations of the symbiotic novae and several steady-burning sources confirm this feature of thermonuclear runaway calculations. At least two symbiotics erupt when the accretion rate from a circumstellar disc onto a solar-type central star increases by 1–2 orders of magnitude. Simple accretion models emit ∼ 50 per cent of the gravitational energy in the disc and the remainder in a hot boundary layer between the disc and central star. Observations of CI Cyg and AX Per support this picture for accretion rates below |$\sim 10^{-4} M_\odot\enspace \text {yr}^{-1}$|⁠, but boundary layer emission vanishes for higher accretion rates. Finally, we associate observations of extra reddening and the appearance of high-ionization emission lines in R Aqr with an increase in the Mira mass-loss rate initiated, perhaps, by a helium shell flash above the Mira's degenerate core.

Neutron cross sections of 122Te, 123Te, and 124Te between 1 and 60 keV.

The currently favored s process scenario of helium shell burning in low mass stars involves a range of thermal energies from kT=12 to 25 keV with most of the neutron exposure taking place at low temperatures. Therefore, differential cross sections are required down to the region of resolved resonances for the reliable determination of the Maxwellian-averaged cross sections typical of the stellar plasma. This work deals with the neutron capture cross sections of the important s only isotopes $^{122}\mathrm{Te}$, $^{123}\mathrm{Te}$, and $^{124}\mathrm{Te}$, which were measured between 1 and 60 keV neutron energy with a setup of Moxon-Rae detectors. The systematic uncertainties achieved in this experiment are \ensuremath{\sim}5%, but statistical uncertainties are smaller than 2%. In addition to the Moxon-Rae detectors, the setup includes a $^{6}\mathrm{Li}$ glass detector which could be used to determine the total neutron cross sections simultaneously. These results represent the first set of experimental data in this energy range.

The Effect of Helium Shell Flashes on Asymptotic Giant Branch Evolution

AbstractMass-loss has been incorporated into a series of evolutionary calculations of low to intermediate mass stars during Asymptotic Giant Branch (AGB) evolution. When helium shell flashes occur a superwind phase is a natural consequence of the mass-loss process. The structure of the stellar wind envelopes that result has been computed. The models may explain some previously curious features seen in OH/IR and CO line emission spectra.

Shell helium burning in low-mass stars. I - Models in thermal equilibrium

view Abstract Citations (20) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Shell Helium Burning in Low-Mass Stars. I. Models in Thermal Equilibrium Fujimoto, Masayuki Y. ; Iben, Icko, Jr. Abstract The paper is concerned with the structure of shell helium-burning stars with a carbon-oxygen core of relatively small mass. The analysis assumes thermal equilibrium, and model properties are presented in relation to the core mass and total model mass. Upper and lower limits on the mass fraction of the core are obtained; the variation of the envelope structure with the core configuration is discussed. Based on the results obtained, it is shown that the red giant structure is attributable to the power-law distribution of density, which is forced when envelope matter is of sufficiently high entropy that self-gravity can be ignored relative to the gravity produced by the inner core over a distance many time larger than the core radius. Publication: The Astrophysical Journal Pub Date: June 1991 DOI: 10.1086/170149 Bibcode: 1991ApJ...374..631F Keywords: Late Stars; Stellar Envelopes; Stellar Evolution; Stellar Interiors; Stellar Mass; Thermodynamic Equilibrium; Abundance; Helium; Stellar Cores; Stellar Gravitation; Stellar Models; Astrophysics; STARS: EVOLUTION; STARS: INTERIORS; STARS: LATE-TYPE full text sources ADS |

A new site for the astrophysical gamma-process

The study suggests that the requisite thermodynamic conditions may occur when carbon-oxygen white dwarfs explode by deflagration or detonation. When these stars undergo such explosive disruption, there will be a region near the surface where the burning temperature lies in the 2.4-3.2 range. To examine this astrophysical site, calculations are performed for an s-process nucleosynthesis during helium shell flashes and the nuclear transmission taking place when such mass zones are heated by the deflagration or detonation wave, and the results are compared with the solar-system distribution of the p-isotopes. It is demonstrated that Type Ia supernovas provide a viable site for the gamma process, and that the same thermodynamic conditions would also exist in Type II-p powered supernovas, provided that they are powered by detonation.

Numerical simulations of off-center detonations in helium shells

Results of two-dimensional simulations of detonations in helium shells, accreted above various C-O cores, are presented. These calculations comfirm results of one-dimensional calculations, showing that a detonation in a heavy helium layer of 0.1-0.3 M ○. , must ignite a detonation in the core, leading to the total disruption of the star. It is found that the mechanism works at two different modes. For stars of total mass above 1.2 M ○. , helium detonation develops into a double-detonation. In smaller white dwarfs, the off-center detonation propagates around the core, leaving a strong shock wave which travels into the core, a shock that is not strong enough to develop into a double-detonation.

FG Sagittae - No hot companion?

The nucleus of the planetary nebula He 1-5 (= PK 60 -7 deg 1), the variable star FG Sge, was observed with the SWP camera of the International Ultraviolet Explorer satellite to detect a hot companion of the star, if such a companion exists. The observation found no evidence for the existance of a hot companion in the 1200-2000 A range of the SWP camera and supported the contention that FG Sge underwent a helium shell flash during the past century, and that the surrounding nebula, He 1-5, is a nebula of fossil ionization. Despite the currently accepted fossil ionization model, constraints posed by the satellite detection limit, the observed H-beta flux, and the adopted radii for white dwarfs still allow the possibility of a putative hot companion photoionizing this nebula.

Thermal structure of accreting neutron stars and strange stars

view Abstract Citations (78) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Thermal Structure of Accreting Neutron Stars and Strange Stars Miralda-Escude, J. ; Haensel, P. ; Paczynski, B. Abstract Steady-state models of accreting neutron stars and strange stars are presented, and their properties as a function of accretion rate are analyzed. The models have steady-state envelopes, with stationary hydrogen burning taken into account, the helium shell flashes artificially suppressed, and the crust with a large number of secondary heat sources. The deep interiors are almost isothermal and are close to thermal equilibrium. A large number of models were calculated for many values of the accretion rates, with ordinary, pion-condensed, and strange cores, with and without secondary heat sources in the crust, and with the heavy element content of the accreting matter in the range Z = 0.0002-0.02. All models show a similar pattern of changes as the accretion rate is varied. For low accretion rates, the hydrogen burning shell is unstable; for intermediate rates, the hydrogen burning shell is stable, but helium burning is not; for high rates, the two shell sources burn together and are unstable. Publication: The Astrophysical Journal Pub Date: October 1990 DOI: 10.1086/169295 Bibcode: 1990ApJ...362..572M Keywords: Neutron Stars; Steady State; Stellar Interiors; Stellar Mass Accretion; Stellar Models; Temperature Distribution; Cobalt Isotopes; Heat Sources; Helium; Hydrogen; Iron Isotopes; Nickel Isotopes; X Rays; Astrophysics; STARS: ACCRETION; STARS: INTERIORS; STARS: NEUTRON; X-RAYS: BURSTS full text sources ADS |

Geometrical effects in off-center detonation of helium shells

The penetration of detonation waves from helium shells into C-O cores of accreting white dwarfs is studied. One-dimensional calculations using a Riemann solver show that for the spherically symmetric case the most relevant parameter for the formation of a double detonation is the location of the ignition surface. As long as ignition occurs at the interface, double detonation does not exist regardless of the accretion rates and the C-O core mass. A two-dimensional analysis, which resolves the interaction between a sliding Chapman-Jouguet front and adjacent medium, shows that the carbon near the interface is heated by the shock only slightly above 10 to the 9th K. Hence, some portion of the C-O core will burn and double detonation cannot be excluded. However, since rarefaction effects are not included in the analysis, detonation may not occur. 26 refs.

Successive detonations in accreting white dwarfs as an alternative mechanism for type I supernovae

Simulations of exploding helium shells in accreting white dwarfs show that the ingoing shock wave is strong enough to ignite the carbon at the center into an outgoing detonation wave. This mechanism provides a new way to form supernovae of type Ia, not only in heavy white dwarfs approaching the Chandrasekhar mass, but in a much wider range of masses. Thus, many of the problems which arise in current models, such as deflagration versus detonation, may be solved as explosions of white dwarfs of lesser mass are considered. 30 refs.

S-process nucleosynthesis - Classical approach and asymptotic giant branch models for low-mass stars

A critical comparison is made between the results of s-process nucleosynthesis obtained with the phenomenological classical approach and a stellar model for helium shell burning in low-mass stars. For the first time, close agreement is found between the abundances determined by the classical analysis and the results of a stellar model. The calculated abundances are found in good agreement with the s-process yields observed in solar material, and the corresponding quantities characterizing the neutron exposure are outlined in detail. Emphasis is laid on the information deduced from the abundance patterns in s-process branchings, i.e., neutron density and temperature. Despite the conceptual differences between the steady flow approximation of the classical approach and the dynamical environment of thermal pulses in low-mass stars, the results of both models are quite similar, but still obscured by the present uncertainties of the nuclear input data; further improvements are required to quantify the trends of the true physical conditions during the s-process which start to emerge from the above studies. 74 refs.

Evolutionary versus dynamical time scales for the evolution of the central stars of planetary nebulae

view Abstract Citations (51) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolutionary versus Dynamical Time Scales for the Evolution of the Central Stars of Planetary Nebulae McCarthy, J. K. ; Mould, J. R. ; Mendez, R. H. ; Kudritzki, R. P. ; Husfeld, D. ; Herrero, A. ; Groth, H. G. Abstract The disagreement between evolutionary and dynamical time scales for the evolution of the central stars of planetary nebulae (CSPN) through the H-R diagram is investigated. Three possible reasons for the disagreement are examined: (1) the nebulae could have experienced a phase of rapid photoionization of material ejected previously while the stars were still on the AGB; (2) the central stars could have undergone a late helium shell flash and returned to the AGB; and (3) the AGB-CSPN evolutionary transition times could have been increased by small additional amounts of residual envelope material remaining after the superwind mass-loss phase. Mechanism (3) is favored here because it can account for evolutionary ages both less than and greater than the corresponding dynamical ages. Publication: The Astrophysical Journal Pub Date: March 1990 DOI: 10.1086/168458 Bibcode: 1990ApJ...351..230M Keywords: Asymptotic Giant Branch Stars; Planetary Nebulae; Stellar Evolution; Absorption Spectra; Hertzsprung-Russell Diagram; Photoionization; Stellar Envelopes; Stellar Mass; Stellar Winds; Astrophysics; NEBULAE: PLANETARY; STARS: EVOLUTION full text sources ADS | data products SIMBAD (23)

The Origin and Evolution of Novae

AbstractThe cataclysmic binaries are products of the nonconservative evolution of close binaries with large initial mass ratios of components. The accretors in cataclysmic binaries can be helium or carbon-oxygen or oxygen-neon-magnesium white dwarfs. Their annual birthrates are ~0.005, ~0.005, and ~0.00005 respectively. In one-zone approximation of a thin accreting shell we estimate the critical masses of hydrogen and helium shells and recurrence time scales of thermonuclear runaways.

Stellar wind during helium nova outburst

The mass ejection process during helium shell flashes on a 1.3 solar masses white dwarf which accretes pure helium matter is examined. Full cycles of helium shell flashes are followed by two different kinds of approaches: one is a time-dependent hydrostatic calculation, and the other is the construction of the sequence consisting of static models and steady state models with wind mass loss. It is found that a stellar wind occurs if the shell flash is strong enough, i.e., the mass accretion rate is lower than 1.7 x 10 to the -6th solar masses/year. The mass accumulation ratio, i.e., the ratio of the processed matter which remains after a shell flash to the initial envelope mass, has been obtained. This ratio depends strongly on the mass accretion rate. If the binary size is small enough, further mass loss occurs due to the outer critical Roche lobe overflow. This mass loss seriously affects the existing scenarios on the neutron star formation induced by the accretion. 31 refs.

Peculiar Red Giants — What Kind of White Dwarfs do They Become?

AbstractAfter a brief commentary on the place of “peculiar red giants” in the overall scheme of stellar evolution, an outline is given of the various possibilities for post asymptotic giant branch (AGB) evolution. The behavior of a post-AGB model star is crucially dependent on where in a thermal pulse cycle the mass of the hydrogen-rich envelope is reduced to such an extent that departure from the AGB must follow on a thermal time scale. If departure from the AGB occurs while the model is still burning hydrogen, post-AGB behavior depends on the mass of the helium buffer zone (- zone containing predominantly helium which has been processed through the hydrogen-burning shell following the last thermal pulse on the AGB). If departure occurs at an arbitrary time during the hydrogen-burning phase, then: (1) in - 25% of all cases, the post-AGB model will experience a final helium shell flash, and, in consequence of additional mass loss, may become a non-DA white dwarf; (2) in - 60% of all cases, the model will cease burning hydrogen when the mass in its hydrogen-rich envelope is reduced to ∼ 10-4Mʘ and will evolve into a DA white dwarf; and (3) in - 15% of all cases, the model will experience a final hydrogen shell flash, but the outcome with regard to spectroscopic type is unclear. If departure from the AGB occurs while the model is burning helium, the result is either the same as in option (3) just described, or mass loss during the post-AGB helium-burning phase may turn the star into a non-DA white dwarf.

On the Formation of Close Binary White Dwarfs

Single stars of initial main-sequence mass larger than about 0.8-1.0⊙ and less than approximately 6-8M⊙ evolve into carbon-oxygen white dwarfs of mass in the range 0.55-1.1M⊙ in less than the Hubble time. Single stars do not make helium white dwarfs in a Hubble time. These statements are based on both observational and theoretical considerations. It is not yet established, but there are suspicions that single stars of initial mass in the range 8-10M⊙ may evolve into oxygen-neon white dwarfs of mass in the range 1.1-1.4M⊙. As a single star of the appropriate initial mass develops a hot, electron-degenerate core composed of carbon and oxygen or of oxygen and neon, it becomes large (R > 200R⊙) and luminous (L > 6000L⊙), with the luminosity being generated most of the time by hydrogen burning, interrupted quasiperiodically by a helium shell flash. The star is said to be an asymptotic giant branch (AGB) star. Within 105 - 106yr after arriving on the AGB, an envelope instability, whose nature is still being explored observationally and theoretically, leads to the ejection of most of the hydrogen-rich envelope of the star. The remnant shortly becomes the central star of a planetary nebula composed of the ejected material, and then cools into a white dwarf configuration.

Low-Mass Stars. II. The Core Mass--Luminosity Relations for Low-Mass Stars

view Abstract Citations (122) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Low-Mass Stars. II. The Core Mass--Luminosity Relations for Low-Mass Stars Boothroyd, Arnold I. ; Sackmann, I. -Juliana Abstract It was investigated whether the core mass-luminosity (Mc-L) relation that had been established in the literature for intermediate-mass stars (3 Msun < M < 9 Msun) can be extended to low-mass stars (0.8 Msun< Msun < 3 Msun), where many of the observations take place. Stars were evolved from the main sequence up the red giant branch, through the helium core flash and the horizontal branch phase, up to the asymptotic giant branch where helium shell flashes were followed. Two types of Mc-L relations were obtained, one for the red giant branch (when a single hydrogen-burning shell surrounds a degenerate helium core), and another one for the asymptotic giant branch (when two burning shells, of helium and hydrogen respectively, surround a degenerate carbon-oxygen core). Detailed calculations were carried out for a metal-poor case (Z = 0.001) for stars of initial masses 1.0 Msun, 1.2 Msun, 2.0 Msun, and 3.0 Msun and for a metal-rich case (Z= 0.02) for stars of initial masses 1.2 Msun and 3.0 Msun. The latest nuclear reaction rates were used, as well as the latest opacities (including some molecular opacities) and mass loss via a Reimers-type wind. The dependence of the relation on chemical composition was investigated. For the red giant branch, the relation for the metal- rich case (Z = 0.02, µ ≍ 0.624) was L = (6.86 Mc)7 for 0.3 Msun < Msun< 0.45 Msun, where all units are in solar units; the composition dependence was L ∝ µ 7(ZCNO)1/12 (µ is the envelope mean molecular weight, including free electrons). For the asymptotic giant branch, the Mc-L relation for the metal-rich case (Z = 0.02, µ ≍ 0.618) was L = 52,000 (Mc - 0.456) for 0.52 Msun < Mc < 0.7 Msun; the composition dependence was L ∝ µ 3(ZCNO)1/25. The Mc-L relation obtained for the low-mass asymptotic giant branch stars drops less steeply than would be expected from the previous higher Mc work; the difference is large at low core masses. Because of luminosity variations over the flash cycle, observers will see stars that do not lie on the Mc-L relation; the probability and extent of these deviations was described. The fortunate circumstance was established that, for the Mc relation of low-mass stars, (i) changes in the hydrogen-burning reaction rate have only a very minor effect; (ii) uncertainties in the convective mixing length have negligible effect; and (iii) there is no evidence that the star's total mass has any appreciable effect. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/166322 Bibcode: 1988ApJ...328..641B Keywords: MASS-LUMINOSITY RELATION; STARS: EVOLUTION; STARS: INTERIORS; STARS: LATE-TYPE full text sources ADS | Related Materials (3) Part 1: 1988ApJ...328..632B Part 3: 1988ApJ...328..653B Part 4: 1988ApJ...328..671B

Nucleosynthesis and Mixing in Low- and Intermediate-Mass AGB Stars

AbstractThe existence of carbon stars brighter than Mbol=-4 can be understood in terms of dredge up in thermally pulsing asymptotic giant branch (AGB) stars. As a low- or intermediate-mass star evolves on the AGB, the large fluxes engendered in a helium shell flash cause the base of the convective envelope to extend into the radiative, carbon-rich region, and transport nucleosynthesis products to the stellar surface. Numerical models indicate that AGB stars with sufficiently massive stellar envelopes can become carbon stars via this standard dredge-up mechanism. AGB stars with less massive stellar envelopes can become carbon stars when carbon recombines in the cool, carbon-rich region below the convective envelope.Neutron capture occurs on iron-seed nuclei during a shell flash, and the products of this nucleosynthesis are also carried to the stellar surface. The conversion of 22Ne into 25Mg can initiate neutron capture nucleosynthesis in largecore mass AGB stars, but only if these stars can survive their large mass loss rates. The current estimates of nuclear reaction rates do not allow for appreciable neutron capture nucleosynthesis via the 22Ne source in lower mass AGB stars. The carbon recombination that induces dredge up in AGB stars of small envelope mass, however, also induces mixing of 1H and 12C in such a way that ultimately a 13C neutron source is activated in these stars. The 13C source can provide an abundant supply of neutrons for the nucleosynthesis of both light and heavy elements. While the existence of neutron-nucleosynthesis products in AGB stellar atmospheres can be understood qualitatively in terms of an active neutron source, the combination of nuclear reaction theory and evolutionary models has yet to provide quantitative agreement with stellar observations.