Helium Shell Research Articles

Studies of evolved stars. V - Nucleosynthesis in hot-bottom convective envelopes

view Abstract Citations (157) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Studies of evolved stars. V. Nucleosynthesis in hot-bottom convective envelopes. Scalo, J. M. ; Despain, K. H. ; Ulrich, R. K. Abstract Convective envelopes fitted to the cores of luminous double shell models have been found to develop high base temperatures, ( ) >c 106 K, during the period between shell flashes. We show that these "hotbottom" envelopes are the result of the increasing influence of radiation pressure on the radiative and adiabatic temperature gradients. Lower limits on the luminosity (Mbol -3.4) and total mass ( l' o) required for the phenomenon are derived; these values depend sensitively on the treatment of the radiative atmosphere and the ionization zones. Light-element nucleosynthesis in hot-bottom envelopes is examined using a convective diffusion model. Large surface 7Li enhancements are produced by the 7Be transport mechanism if the base temperature Tb exceeds 30 x 106 K. Sufficient time before carbon ignition exists for CN processing if Tb > 50 x 106 K, while full CNO processing and hydrogen depletion may occur if Tb > 75 x 106 K. Comparison with observations indicates that nucleosynthesis in hot-bottom envelopes is not the primary cause of the abundance anomalies observed in peculiar red giants, except possibly for the large 13C/12C ratios and lithium enhancements found in some stars. In particular, mixing of triple-a carbon and s-process elements initiated by the helium shell flashes seems to be required in order to satisfy constraints imposed by the observed line and band strengths and luminosities. Subject headings: abundances, stellar - convection - interiors, stellar - late-type stars - nucleosynthesis Publication: The Astrophysical Journal Pub Date: March 1975 DOI: 10.1086/153471 Bibcode: 1975ApJ...196..805S Keywords: Late Stars; Nuclear Fusion; Stellar Evolution; Astronomical Models; Astrophysics; Convection; Giant Stars; Astrophysics full text sources ADS | data products SIMBAD (5) Related Materials (4) Part 1: 1973ApJ...183..151S Part 2: 1973ApJ...184..801S Part 3: 1974ApJ...192..507U Part 4: 1974ApJ...194..361S

Advanced evolution in globular clusters. II - The ultraviolet-bright stars in omega Centauri

view Abstract Citations (41) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Advanced evolution in globular clusters. II. The ultraviolet-bright stars in Omega Centauri. Norris, J. Abstract The results of a spectrographic and photometric survey of stars which occupy the region above the horizontal branch and to the left of the giant branch (the so-called UV-bright region) in the color-magnitude diagram of the globular cluster omega Centauri are presented. There is no large scatter in the apparent metal abundances of the cool stars ROA 164 and 342 and the red giant ROA 159. Helium lines are present in the spectra of ROA 3596 and 5701. A mean mass of about 0.58 solar mass (internal s.e.) is obtained for the UV-bright stars, indicating that they have the same mass as the blue horizontal-branch stars. The distribution of the UV-bright stars in the theoretical H-R diagram indicates that these objects can occupy any position in the UV-bright region. This implies that if they are to be explained by loops resulting from helium shell flashes and/or by final evolution from the asymptotic branch or the giant branch, the loops must be long and occur at a range in luminosities, and/or the final evolution from the giant branches must occur at a range of luminosities. Publication: The Astrophysical Journal Pub Date: November 1974 DOI: 10.1086/153228 Bibcode: 1974ApJ...194..109N Keywords: Abundance; Globular Clusters; Radial Velocity; Stellar Evolution; Stellar Spectrophotometry; Ultraviolet Spectra; Color-Magnitude Diagram; Cool Stars; Galactic Evolution; Giant Stars; Hubble Diagram; Milky Way Galaxy; Stellar Atmospheres; Stellar Luminosity; Stellar Mass; Ubv Spectra; Astronomy full text sources ADS | data products SIMBAD (37)

Asymptotic Giant-Branch Evolution of a 0.6 m, Star

view Abstract Citations (170) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Asymptotic Giant-Branch Evolution of a 0.6 m, Star Gingold, Robert A. Abstract The evolution of a double shell source 0.6 M0 Population II star has been followed in detail through the asymptotic-branch phase into the region of the R-R diagram occupied by the nuclei of planetary nebula. No attempt has been made to suppress helium shell flashes. During the total of 14 relaxation oscillation cycles encountered, no convective mixing took place between the helium shell and the hydrogen-rich layers. A small amount of the products of hydrogen burning was mixed to the surface by an extensive convective zone that developed shortly after major flash peaks. Toward the end of the asymptotic-branch evolution the star spent a considerable time in the region of the H-R diagram occupied by RV Tauri-type stars in globular clusters. Subjects headings: interiors, stellar - population II stars - stellar evolution - variable stars Publication: The Astrophysical Journal Pub Date: October 1974 DOI: 10.1086/153144 Bibcode: 1974ApJ...193..177G full text sources ADS | data products SIMBAD (2)

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)

Carbon and eruptive stars : surface enrichment of lithium, carbon, nitrogen, and C13 by deep mixing.

view Abstract Citations (163) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Carbon and eruptive stars: surface enrichment of lithium, carbon, nitrogen, and 13C by deep mixing. Sackmann, I. -J. ; Smith, R. L. ; Despain, K. H. Abstract It appears possible to form carbon stars from stars of both low ( 1 M0) and intermediate (5 M0) masses and from both population types. A single helium shell flash followed by deep mixing suffices for the former case, but on the order of 100 such flashes are required for the latter. No more than the order of 100 flashes are possible for the former and 1000 for the latter. The Cameron-Fowler mechanism for the production of high lithium abundances in late-type stars is shown to work satisfactorily, producing 7Li/H up to about 10- in a single flash. This value is nearly independent of mass and population type. Eruptive stars may result when the deep convective envelopes reach into helium-burning iayers. Large amounts of energy can be liberated in a very short time; e.g., the binding energy of the envelope, 1048 ergs, can be exceeded in 106 sec. Many of the gross features of the R Coronae Borealis stars can be understood by erupting carbon star models. For example, observed surface abundances of H, He, 7Li, 12C/16O, 14N/12C, and 12C/13C can be accounted for. For an unrepeated deep mixing, an upper limit can be placed on the mass of a carbon star, namely, 2 M0.. For short-duration mixing, 7Li/H reaches its peak value (7Li/H 12C/16O > 1 or <1, depending on the mass, 10- < 14N/12C < 1, and S < 12C/13C < 100; for intermediate-duration mixing, 7Li/H < 10-8, 12C/16O < 1, 1 < 14N/12C < 40, and 2.4 < 12C/13C < 5; for long-duration mixing, 7Li/H = 0, 12C/16O > 1, 14N/12C = 40, and 12C/13C = 3.2. These abundance ratios agree quite well with the observations in the oxygen and carbon stars, S, Ba ii, C, and CH, ordering these spectral types in a sequence determined by the duration of deep mixing. Repeated deep mixing can explain the observations known for S, Ba II, and C stars when they are considered to be intermediate-mass objects. S stars would be interpreted as those with few flashes, Ba ii stars as those with an intermediate number of flashes, and C stars super-rich in 7Li as those with a very large number of flashes. Subject headings: Ba ii stars - carbon stars - interiors, stellar - nuclear reactions - R Coronae Borealis stars Publication: The Astrophysical Journal Pub Date: February 1974 DOI: 10.1086/152666 Bibcode: 1974ApJ...187..555S full text sources ADS | data products SIMBAD (9)

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.

OBSERVATIONS OF NOVA IV CEPHEI 1971

ABSTRACT I.Photometry

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.

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 |

Termination of Shell Burning of Helium in Solar-Mass Stars

view Abstract Citations (8) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Termination of Shell Burning of Helium in Solar-Mass Stars Kutter, G. S. Abstract The evolution of a 1.04 o star, composed initially of a 1 M0 12C core and a 0.04 M0 4He-rich envelope, is followed through the termina] phase of 4He shell burning to the white-dwarf stage. During the phase of maximum 4He burning, the phofon luminosity (-- 10 6L0) exceeds by a factor of 2 the critical luminosity above which mass loss occurs by momentum transfer from the radiation to the gas. This represents a possible mechanism for the ejection of planetary nebulae from highly evolved stars in the approximate range 1 0-1.4 . Qualitative agreement with the observational data of the precursor stars and the nuclei of planetary nebulae tends to support this connection. Helium shell burning is extinguished when the time scale of photon diffusion from the shell source to the surface becomes of the order of the time scale of nuclear energy production, u/ ( [3a + tt12C(a, )16O] the shell source loses energy more rapidly than it can replenish by nuclear burning and contraction. No thermal instabilities have been found, probably because of the large contribution of radiation to the equation of state in and above the shell source (0.30 < p < 0.65). Publication: The Astrophysical Journal Pub Date: February 1971 DOI: 10.1086/150822 Bibcode: 1971ApJ...164..115K full text sources ADS |

Late Evolution of Solar-Mass Stars

view Abstract Citations (8) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Late Evolution of Solar-Mass Stars Vila, S. C. Abstract The evolution of a 1 M0 star, initially composed of a carbon core and a helium envelope, has been studied by detailed model calculation on the two assumptions of energy losses due to photons and neutrinos and to photons alone. In the evolution which takes no account of neutrinos we have found (a) a phase of quiet helium shell burning, (b) a phase during which thermal oscillations occur, and (c) a phase of central carbon burning. The assumption of energy loss due to neutrinos suppresses both the thermal oscillations and the central carbon burning. The star develops an extended envelope, and the luminosity reaches values comparable to the critical value for mass ejection, but no mass loss is apparent from these models. The star evolves into a white dwarf in both cases, but the core composition and the mass in the remaining helium envelope differ in the two cases. Publication: The Astrophysical Journal Pub Date: November 1970 DOI: 10.1086/150692 Bibcode: 1970ApJ...162..605V full text sources ADS |

On the Evolutionary Phase of Cepheids in Globular Clusters

view Abstract Citations (69) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Evolutionary Phase of Cepheids in Globular Clusters Schwarzschild, M. ; Härm, R. Abstract The beginning of the helium shell burning phase has been computed through the first flash cycle for each of nine globular-cluster stars (differing in mass and initial composition). While seven of thesestarsremained on the red-giant branch throughout the cycle, two stars did not. The latter passed, after each flash, through a loop in the H-R diagram reaching well into the strip occupied by Population II Cepheids. It is suggested that the occurrence of these evolutionary loops might be the reason for the occurrence of Cepheids in globular clusters. Publication: The Astrophysical Journal Pub Date: April 1970 DOI: 10.1086/150433 Bibcode: 1970ApJ...160..341S full text sources ADS |

A polytropic model for the helium shell flash

A model consisting of two polytropes is constructed, to represent a helium core of a star during the helium shell flash occurring at the onset of helium burning in a degenerate core. The maximum temperature reached during the flash can be predicted as a function of core mass and mass inside the helium burning shell. This temperature will generally be too low for the production of neutrons out of 14N. Some additional results on the helium shell flash in a star of 1.3 M| are also presented. Since the first numerical results on stellar evolution through the helium flash were published by Schwarzschild and H~irm (1962), this part of stellar evolution computa- tions has been known for its numerical difficulties. These are caused by a short evo- lutionary timescale in the central part of the star in contrast to slow changes in the rest of the star. Following the evolution timestep by timestep one has to choose a certain chemical composition and mass for the stellar model and for another set of these parameters the computations must be repeated. This was done by Schwarz- schild and Hfirm in several papers (H/irm and Schwarzschild, 1964, 1966). A com- pletely different approach, which avoids these time-consuming computations, is the use of polytropic models. These were used by Sugimoto (1964), to calculate models for the helium flash as a function of core mass. Including the weak interaction neutrino processes leads to a different kind of helium flash, burning in a shell around the center (Thomas, 1965). This introduces the mass inside this shell as a new parameter, which enters the helium flash computations. The polytropic model presented here provides an answer to the question, how changes in this mass value change the behavior of the flash. But first some results using corrected neutrino rates are given, as obtained with the method described in Thomas (1967) (referred to as paper I). These cover the phase from the beginning of the flash until shortly after the top of the flash, which is defined by the maximum rate of nuclear energy generation. Because energy generation depends on density, too, this is not identical with the maximum temperature reached during the flash. The stellar model has a total mass of 1.3 M@ and a chemical composition, X=0.9, Z=0.001, the sequence leading to the flash models started at model 303 of paper I, the plasma neutrino rates being decreased by a factor of 4.

Explosive Ignition of Carbon in Stars of Intermediate Mass

Hoyle and Fowler1 have suggested that type I supernovae are the result of an explosion following the ignition of degenerate nuclear fuel in an evolved star. The purpose of this communication is to place this idea within the context of current stellar evolutionary calculations and to examine the development of the instability. The growth of carbon–oxygen core as a result of helium shell burning in stars of intermediate mass has been studied by Hayashi, Hoshi and Sugimoto2,3, Kippenhahn, Thomas and Weigert4, and Weigert5. For a carbon–oxygen core of mass Mc ∼ 0.7 M⊙, these authors find that inclusion of energy loss by neutrino emission prevents the carbon flash from occurring, and that a cold degenerate core of potentially explosive carbon and oxygen slowly grows. If energy loss by neutrino emission is included, a carbon star cannot ignite by the 12C + 12C reaction if its mass is less than about 1 M⊙ (unpublished results of T. Murai, D. Sugimoto, R. Hoshi and C. Hayashi, and of G. Beaudet and E. E. Salpeter). This implies that all stars which have a less massive carbon–oxygen core at the onset of the helium shell burning stage will not ignite carbon burning by the comparatively gentle carbon flash. In what follows I consider how this ignition might occur.

Three Stars with Helium Shells.

view Abstract Citations (4) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Three Stars with Helium Shells. Merrill, Paul W. Abstract In the B-type stars HD 172694, HD 184279b and HD 195407 absorption lines from metastable levels of the neutral helium atom indicate a cloud of gas at high excitation far above the usual reversing layer. Observations over many years show that all three stars have variable spectra. Many details are given. These helium shells are probably similar to the commoner type showing numerous metallic lines, but differ in having higher excitation. Publication: The Astrophysical Journal Pub Date: January 1952 DOI: 10.1086/145508 Bibcode: 1952ApJ...115...47M full text sources ADS | data products SIMBAD (3)