Helium-burning Shell Research Articles

Noble Gases in the Murchison Meteorite: Possible Relics of s -Process Nucleosynthesis

The Murchison carbonaceous chondrite contains a new type of xenon component, enriched by up to 50 percent in five of the nine stable xenon isotopes, mass numbers 128 to 132. This component, comprising 5 x 10(-5) of the total xenon in the meteorite, is released at 1200 degrees to 1600 degrees C from a severely etched mineral fraction, and probably resides in some refractory mineral. Krypton shows a similar but smaller enrichment in the isotopes 80 and 82. Neon and helium released in the same interval also are quite anomalous, being highly enriched in the isotopes 22 and 3. These patterns are strongly suggestive of three nuclear processes believed to take place in red giants: the s process (neutron capture on a slow time scale), helium burning, and hydrogen shell burning. If this interpretation is correct, then primitive meteorites contain yet another kind of alien, presolar material: dust grains ejected from red giants.

A Shortlived, Deep Convective Envelope for Highly Evolved, Blue Stars?

An interesting new phenomenon was encountered while evolving a star with a core mass, Mc of 0.8 M0 and with a small envelope mass (0.015 M0) away from the red giant branch towards the nuclei of planetary nebulae, while taking the helium shell flashes into account. It was found that the top of the intershell carbon pocket (the carbon-enriched region in between the hydrogen- and helium-burning shells left behind by the flash) was expanded outwards and cooled immensely; namely, cooled to near 20,000°K! This means that the intershell carbon pocket was lifted out to near the photosphere, right into the shallow outer convective envelope surrounding the hydrogen- and helium-ionization zones! The carbon opacity at these cool temperatures is great. It seems likely that all the layers from the outer regions of the intershell carbon pocket right up to the surface will become convective. This would be a totally new type of deep convective envelope with a vast number of fascinating implications. Careful checks of this new phenomenon are now underway. (Supported in part by the National Aeronautics and Space Administration [NSG 7195].)

S-process studies - The effects of a pulsed neutron flux

Recent investigations of the astrophysical site of the s-process strongly suggest the helium-burning shell of the helium-shell-flashing stars of intermediate mass as the most likely site for the s-process event which produced the solar-system abundances. Previous analyses of branching in the s-process have been in terms of a constant temperature and density environment. The occurrence of periodic thermal instabilities seems to make this assumption inappropriate. The current effort is to reformulate the mathematics of the branched s-process to allow for the influence of thermal pulses. The effect of pulses on the unique isobars between branches is generally small; the largest effect of pulses is at those branch points where the branching ratio for ..beta..-decay during a pulse is small, but the ..beta..-decay lifetime during the interpulse period is shorter than the time between pulses. It is shown that the decay of the unstable branch nuclei between neutron irradiations allows the mean s-process neutron flux responsible for the solar-system abundances in such a pulsed environment to be considerably larger than that for a single continuous-exposure event.

Planetary Nebulae and Nitrogen Enrichment in the Galaxy

All stars in the mass interval 1-4Mʘprobably evolve through a double shell source phase before losing their entire envelopes through stellar wind and the ejection of planetary nebula shells. The remnant carbon-oxygen core rapidly evolves to a blue nucleus, illuminates the surrounding nebula for a few tens of thousands of years and finally cools off as a white dwarf (Paczynski, 1970, Harm and Schwarzschild 1975). A large fraction of the mass contained in the main-sequence stars of mass 1-4Mʘthus returns to the interstellar medium. A common feature of the double shell source (DSS) evolution of stars is the occurrence of He shell flashes. Evolutionary studies by Schwarzschild and Harm (1967), Iben (1975) and others show that the flash-driven convection zone carries helium burning products towards the hydrogen-rich layers. The consequences of mixing between the outer convective envelope and the intershell region have received a great deal of attention in recent years particularly in connection with the interpretation of carbon stars. If a deep temporary convection zone exists extending from the surface of the star to a point near the helium burning shell and mixing is allowed to take place for sufficiently long time Sackmann, Smith, and Despain (1974) found that the flash-produced12C underwent further ON0 processing enriching the surface of the star in12N. Iben (1976) has also speculated on the possibility of surface enrichment of14N during the DSS evolution of intermediate-mass stars. The subsequent loss of this envelope as a planetary nebula shell can thus cause nitrogen enrichment of the interstellar gas. In the present work we have evaluated the extent of this enrichment and have also derived the gradient of nitrogen abundance in the disc of the Galaxy based on the simple model of galactic evolution due to Talbot and Arnett (1973, hereinafter TA).

What quenches the helium shell flashes

An analysis is conducted of a typical stellar helium shell flash cycle. A sequence of 12 flashes for a population I star is considered. The changes taking place at a constant mass layer in the helium-burning shell are studied as the flash develops. A highly simplified mathematical description is presented, taking into account the three necessary conditions for the occurrence of the flash and the pressure-density and temperature-density relationships. Quantities which are helpful in explaining what triggers the flash are found to be useful in explaining what quenches the flash.

The pulsational properties of high-luminosity degenerate stars with He burning near the surface.

A sequence of 1 M/sub sun/ high-luminosity degenerate stellar models has been constructed with helium shell burning near the surface. The models have carbon-oxygen cores and helium envelopes. The luminosities lie in the range 0< or =log (L/L/sub sun/) < or =4. Values for periods, eigenfunctions, and stability integrals are presented. The models in the range 0< or =log(L/L/sub sun/) < or =3 are unstable in the fundamental mode. All models are stable by a large margin toward first harmonic pulsations.The models may be relevant to pulsating X-ray sources, white dwarfs, and recent discussions of helium accretion. (AIP)

The evolutionary status of Population II Cepheids

view Abstract Citations (179) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The evolutionary status of population II cepheids. Gingold, R. A. Abstract Several grids of post-horizontal-branch evolutionary models have been constructed assuming a range in either total mass or initial core mass on the horizontal branch. The less luminous Population II Cepheids (BL Herculis stars) are identified with stars of small envelope mass undergoing 'blueward noses' from the asymptotic giant branch due to readjustments between the hydrogen- and helium-burning shell luminosities following the exhaustion of helium in the core. The more luminous Cepheids (W Virginis stars) are identified with stars undergoing blueward loops from the second giant branch in response to helium shell flashes or finally evolving to the blue as the hydrogen-burning shell nears the stellar surface. The models predict relatively too few variables spread over too large a range in luminosity in the brighter group. Mass loss on the second giant branch is suggested to help resolve this problem. The assumption of a range in total mass on the zero-age horizontal branch produces a slightly better fit to the observed luminosity distribution of Population II Cepheids than does the assumption of a range in initial core mass. Publication: The Astrophysical Journal Pub Date: February 1976 DOI: 10.1086/154156 Bibcode: 1976ApJ...204..116G Keywords: Astronomical Models; Cepheid Variables; Stellar Evolution; Stellar Luminosity; Asymptotic Giant Branch Stars; Helium; Hydrogen; Nuclear Fusion; Stellar Mass; Stellar Structure; Astrophysics full text sources ADS | data products SIMBAD (2)

12C/13C ratios in stars ascending the giant branch the first time.

Stellar evolution calculations were carried out for 1 M/sub sun/ and 2 M/sub sun/ red giants in order to understand the observed /sup 12///sup 13/C ratios. Although very low values of 5 to 7 in some red supergiants can be understood in terms of thermally unstable helium shell burning models, there remains a problem of moderately low values (10 to 20) in stars not luminous enough for this process. In addition to the standard mixing at the base of the giant branch, which can give /sup 12/C/sup 13/C ratios in the region of 25 to 30, we consider: variation in initial composition; meridional mixing; mass loss; zero-age composition gradient; mixing during the helium flash; and instability of the hydrogen-burning shell. Of these, only the mass loss mechanism and the hydrogen shell instability produce values below approx.20. (AIP)

Helium-Shell Flashes in Population i Stars

Population I stars of 3, 3.3, 4, 5, and 6 M0 were evolved from the main sequence up to the first strong heliumshell flashes. It is found that the instability of helium-shell burning is initially oscillatory in all the models. Strong thermal pulses develop later. The interpulse period decreases strongly with increasing core mass. Subject headings: interiors, stellar - stellar evolution

Thermal Pulses in Helium Shell-Burning Stars. III

The shell- burning evolution of pure-helium stars in the mass range 0.5- 1.0 M/sub sub solar / studied, both including and neglecting neutrino energy losses due to the universal Fermi interaction. With neutrino energy losses included, stars of mass 0 5-0.8 M/sub sub solar / undergo a single thermal pulse before becoming white dwarfs, while more massive stars have stable helium-burning shells. In the absenbce of neutrino energy losses, carbon burning occurs in the core for M approx equal 0.8 M/sub sub solar /. No thermal-instability in t he helium shell precedes the onset of carbon buming. At 0.7 M/sub sub solar / carbon fails to ignite and a thermal pulse occurs in the helium shell, while at 0.5 M/sub sub solar / neither carbon burning nor thermal instability is found. The ebluae is in the spectra of the nuclei of advanced planetary nebulae, and of some white dwarfs. A possible relationship with the hydrogen-deficient carbon stars is also discussed. (auth)

Presentation and Interpretation of High Resolution Infrared Spectra of Late-Type Stars

Current interest in stellar evolution is concentrated on the life of a star after it has left the main sequence. Of particular interest are the red giant or supergiant periods during the hydrogen and helium shell burning phases. Convective mixing during these stages can mix nuclear processed material to the surface where it may be viewed by spectroscopic methods. It is imperative that this rare chance to view processed material be exploited fully to increase our knowledge of stellar evolution.The observation and interpretation of cool star spectra has its own particular set of problems and advantages. A particular difficulty is the formation of molecules at the low temperatures which occur in the atmospheres of late stars. Not only must the particularly complex spectra of molecules be dealt with but the problem of chemical equilibrium in the atmosphere must be solved accurately before quantitative analysis may be performed. The formation of molecules, however, has one advantage in that it very dramatically separates those stars with carbon to oxygen ratios greater than one from those with ratios less than one. It is the very high dissociation energy of 11.1 eV for the CO molecule which performs this separation. If carbon is less abundant than oxygen all of the carbon is tied up in CO and only oxides are formed in the stellar atmosphere which produce typical M star spectra. If, however, carbon is more abundant than oxygen then carbon compounds such as C2 are formed in place of the oxides and a carbon star spectrum is formed. One of the great advantages of infrared stellar spectra is that it is the only ground based technique for observing CO in stellar atmospheres.

Mixing Between the Core and the Envelope in Stars with Deep Convection Zones

The extensive surface convection zone of a red giant star becomes deeper and deeper, as the star evolves. In some cases it reaches the bottom of the hydrogen-rich envelope and even penetrates into the stellar core. From the standpoint of entropy distribution in the star, the structure of the convective envelope and the mechanism of its penetration into the core are summarized. Possibilities of the penetration are discussed for stars of masses in the range of 1–60 M⊙ over their entire lifetimes of evolution. Effects of uncertainty in the mixing length theory of convection, neutrino loss and thermal instability of helium-burning shell are studied also. When the convection penetrates into the core, material of the core is brought up to the stellar surface and the star will be a peculiar star. It is concluded that there are possibilities to interpret the origin of peculiar stars with luminosities fainter than 7 × 104L⊙, but that the more luminous ones do not originate from the penetration of surface convection.

FG Sagittae: Observational Evidence of a Thermal Pulse?

A stellar remnant which has lost its hydrogen-rich envelope could be a hot dense star that is still burning helium in a shell near the surface. If there is a thermal pulse in the helium-burning shell, the star will expand, increase in luminosity, and become cooler. Such a star would resemble qualitatively the observed behavior of FG Sagittae, and it is urged that the future behavior of this star be watched closely because it may be the first observational evidence of a thermal pulse.

A Preliminary Investigation of Period Changes For W Virginis Stars in Globular Clusters

Investigations in recent years have shown that there may be two mechanisms which place stars in the W Virginis instability region (Kraft, 1972). The variables with periods less than 8 days seem to be in the stage of ‘above horizontal branch’ evolution discussed by Strom et al. (1970). The longer period group apparently results when thermal instabilities in the helium burning shell of an asymptotic red giant branch star cause it to loop to the left in the HR diagram. This longer period group has been investigated by Schwarzschild and Härm (1970) and Mengel (1972). The present study has been undertaken to see if there are any notable differences between the period changes of variables belonging to the two groups.

The Evolutionary Status of Population II Cepheids

Schwarzschild and Härm (1970, hereafter abbreviated SH) have suggested that the presence of Cepheids in globular clusters can be explained by the occurrence of ‘flashes’ in the thermally unstable helium-burning shells of post-horizontal-branch, or ‘asymptotic-branch’ stars. These flashes in the deep interior of the star produce large changes in the energy flux at the base of the envelope, which in turn cause the position of the model in the Mbol — logTe diagram to move. For most values of the total stellar mass and envelope composition studied by SH, the models simply moved up and down the asymptotic branch in response to a helium shell flash. However in a few cases, characterized by low total mass and a relatively high value of the envelope helium abundance, the models left the asymptotic branch and described loops extending into the region of pulsational instability. The Cepheids of Population II were thus tentatively identified by SH as asymptotic-branch stars that had recently undergone a helium shell flash and whose tracks were as a result describing a loop extending into the instability strip.

A Simplified Model for Oscillatory Secular Modes

A star is considered to consist of two zones, one of which contains a nuclear source. The interaction between the two zones can lead to slow oscillations during which the star remains near hydrostatic equilibium. The model permits a discussion of the roles played by thermonuclear reactions, radiative heat transport, and hydrostatic adjustment to heat input. Applications are made to stars on the main sequence and to stars with helium-burning shells. (auth)

Thermal Instability of the Helium-Burning Shell in Massive Stars

Nonlinear numerical calculations of stellar evolution at high mass show that thermal instability develops temporarily in the helium-burning shell, shortly after the ignition of shell helium. Manifestation of the instability is an irregular flickering of very small amplitude. There appear to be no observable consequences of the phenomenon.-

Thermal pulses in He shell-burning stars.

view Abstract Citations (16) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Thermal pulses in helium shell-burning stars. II. Wood, P. R. ; Faulkner, D. J. Abstract The shell-burning evolution of pure-helium stars in the mass range 0.5-1.0 . is studied, both including and neglecting neutrino energy losses due to the universal Fermi interaction. With neutrino energy losses included, stars of mass 0.5-0.8 undergo a single thermal pulse before becoming white dwarfs, while more massive stars have stable helium-burning shells. In the absence of neutrino energy losses, carbon burning occurs in the core for 0.8 . No thermal instability in the helium shell precedes the onset of carbon burning. At 0.7 carbon fails to ignite and a thermal pulse occurs in the helium shell, while at 0.5 neither carbon burning nor thermal instability is found. The significance of these results for the central stars of planetary nebulae is discussed, and a possible explanation is given for carbon features in the spectra of the nuclei of advanced planetary nebulae, and of some white dwarfs. A possible relationship with the hydrogendeficient carbon stars is also discussed. Subject headings: hydrogen-deficient stars - instabilities - interiors, stellar - planetary nebulae Publication: The Astrophysical Journal Pub Date: April 1973 DOI: 10.1086/152037 Bibcode: 1973ApJ...181..147W full text sources ADS | data products SIMBAD (2)

Oscillatory Thermal Instabilities at the Onset of Helium Shell Burning.

view Abstract Citations (28) References (4) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Oscillatory Thermal Instabilities at the Onset of Helium Shell Burning. Härm, R. ; Schwarzschild, M. Abstract The onset of thermal instability has been investigated in detail for globular-cluster stars in the evolutionary phase during which the helium-burning shell is just forming. The investigation shows that (a) the main unstable phase ("second red-giant phase" or "asymptotic branch") is preceded by a short and inconsequential preliminary unstable phase during which the instability is weak and of an oscillatory character, (b) the main unstable phase generally commences not with a simple exponential instability (real eigenvalue), but with an oscillatory instability (complex eigenvalue), (c) this oscillatory instability appears to be caused by an interaction between the driving layer containing the helium shell source and a reacting layer below the driving layer, and (d) the oscillatory instability gives way (after a quite limited time span) to a simple exponential instability which causes the repetitive helium-shell flashes described in earlier investigations. Publication: The Astrophysical Journal Pub Date: March 1972 DOI: 10.1086/151360 Bibcode: 1972ApJ...172..403H full text sources ADS |

Thermal Stability of the Helium-Burning Shell in Stars of 15 Solar Masses

Thermal Stability of the Helium-Burning Shell in Stars of 15 Solar Masses