Core Helium Burning Research Articles

Neutron capture nucleosynthesis during core helium burning in massive stars

Neutron-capture nucleosynthesis during core He burning in massive (ZAMS mass = 50-100 solar mass) mass-losing stars, which are identified with Wolf-Rayet stars, is studied in the framework of recent stellar models based on the Roxburgh criterion for convection and on the latest nuclear data available. The nucleosynthesis is followed with the aid of a full nuclear reaction network incorporating up-to-date Maxwellian-averaged neutron-capture cross sections and new density- and temperature-dependent beta-decay rates. Numerical techniques are developed in order to integrate efficiently the set of coupled differential equations of the network. The resulting stellar core and surface abundances are presented, as well as the composition of the stellar winds ejected during the WC phase. Consideration is given to the implications of these results for the composition of OB associations and of the solar system, for the isotopic anomalies in meteorites and in the galactic cosmic rays, as well as for nuclear gamma-ray line astronomy. 114 references.

Intermediate-age core helium burning stars and the distance to the Magellanic Clouds

view Abstract Citations (29) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Intermediate-Age Core Helium-burning Stars and the Distance to the Magellanic Clouds Seidel, E. ; Da Costa, G. S. ; Demarque, Pierre Abstract The luminosities of the core helium burning stars in six Magellanic Cloud intermediate-age star clusters, whose ages have recently been determined from photometry reaching below the main-sequence turnoff, are compared with the predictions of a new grid of theoretical models. It is found that for the so-called 'short' distance moduli consistent results can be achieved with moderate amounts of mass loss from the main sequence to the core helium burning stage for the older clusters, and with rather more substantial mass loss for the younger objects. For the 'long' moduli, however, the results of the comparison give inconsistent results in that the required masses for the clump stars generally exceed the corresponding turnoff masses, in some cases by several tenths of a solar mass. The results support the adoption of smaller distances to these galaxies. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/164961 Bibcode: 1987ApJ...313..192S Keywords: Distance; Gravitational Collapse; Helium; Magellanic Clouds; Star Clusters; Stellar Evolution; Horizontal Branch Stars; Nuclear Fusion; Stellar Cores; Stellar Mass Ejection; Astrophysics; CLUSTERS: GLOBULAR; GALAXIES: MAGELLANIC CLOUDS; STARS: EVOLUTION; STARS: HORIZONTAL-BRANCH full text sources ADS | data products NED (8) SIMBAD (6)

The WR/OB Number Ratio within 2.5 kpc from the Sun

Comparing the observed and theoretical HR diagram for massive stars, it is concluded that at least 34 % of the stars within 2.5 kpc from the sun with initial ZAMS mass larger than 35 MO have not yet been detected. It is argued that the number of core helium burning pop I WR stars within 2.5 kpc from the sun may be as low as 26. If the progenitors of WR stars have initial ZAMS masses larger than 35 MO, one then concludes that the observed number ratio (core helium burning pop I WR stars) / (WR progenitors) within 2.5 kpc from the sun ranges between 0.05 and 0.09.

Hydrostatic nucleosynthesis. I - Core helium and carbon burning.

view Abstract Citations (115) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hydrostatic nucleosynthesis. I - Core helium and carbon burning. Arnett, W. D. ; Thielemann, F. -K. Abstract The reaction network of the present examination of thermonuclear synthesis of nuclei during quasi-hydrostatic stages of the evolution of massive stars is complete up to atomic weight values of 74. The equations for temperature and density evolution are formulated in a way that renders them comparatively insensitive to specific assumptions concerning stellar convection and mixing. The first part of this work presents both the model and results of core He and C burning, for original He core masses of 1.5, 2, 4, 8, and 16 solar masses; subsequent stages, up to Si burning, are considered in a second part. The study of these later core-burning stages yields information on minimum size of reaction networks, which can be used to accurately predict the neutron excess. Publication: The Astrophysical Journal Pub Date: August 1985 DOI: 10.1086/163402 Bibcode: 1985ApJ...295..589A Keywords: Abundance; Nuclear Fusion; Stellar Cores; Stellar Evolution; Stellar Mass; Carbon; Helium; Neon; Oxygen; Polytropic Processes; Silicon; Astrophysics full text sources ADS |

BRIGHT LONG-PERIOD VARIABLES IN THE DIRECTION OF THE MAGELLANIC CLOUDS : FOREGROUND STARS OR SUPERGIANTS ?

ABSTRACT Radial velocities and infrared photometry have been obtained for seven bright long-period variables in the direction of the Magellanic Clouds. Five stars with periods shorter than 250 days were found to be Cloud members. They are probably core-helium burning supergiants of about 15 times the solar mass; there is also a possibility that they are overtone pulsators on the upper asymptotic branch with masses in the 7-9 solar mass range. It had previously been suggested that the bright short-period (100-250 days) LPVs might be foreground Mira variables of low mass in the Galactic halo. The supergiant long-period variables in the Magellanic Clouds exhibit a distinct period-luminosity relation which should be useful for the purpose of extra-Galactic distance determination.

Yellow giants in young clusters. II - A comparison of observation with theory

view Abstract Citations (16) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Yellow giants in young clusters. II. A comparison of observation withtheory. Schmidt, E. G. Abstract Luminosities and effective temperatures are calculated for 17 nonvariable yellow giants and supergiants and nine Cepheids which were previously identified as members of open clusters. These stars are identified as being in the helium core burning stage of evolution. A comparison with the predictions of pulsation theory does not show any disagreement. On the other hand, several comparisons with the theory of stellar evolution produce discrepancies which suggest that effects neglected in the calculations may play an important role in the evolution of these stars. Publication: The Astrophysical Journal Pub Date: December 1984 DOI: 10.1086/162685 Bibcode: 1984ApJ...287..261S Keywords: Cepheid Variables; Giant Stars; Star Clusters; Supergiant Stars; Helium; Hertzsprung-Russell Diagram; Stellar Cores; Stellar Evolution; Stellar Luminosity; Stellar Temperature; Astrophysics full text sources ADS | data products SIMBAD (21) Related Materials (1) Part 1: 1984ApJS...55..455S

Observational constraints from models of close binary evolution

The evolution of a system of 9 MO + 5.4 MO is computed from Zero Age Main Sequence through an early case B of mass exchange, up to the second phase of mass transfer after core helium burning. Both components are calculated simultaneously. The evolution is divided into several physically different phases. The characteristics of the models in each of these phases are transformed into corresponding ‘observable’ quantities. The outlook of the system for photometric observations is discussed, for an idealized case. The influence of the mass of the loser, mli, and the initial mass ratio qi is considered.

OH/IR massers. IV - Evolution, pulsation, and nature of the sources

The evolution, pulsation, and nature of the type II OH/IR stars is analyzed using the extensive data base and trends described in the third paper in this series. Using evolutionary tracks and pulsation theory, the lowest luminosity (M/sub bol/>-5) masers on the asymptotic giant branch (AGB) are shown to be cool, fundamental mode pulsators which are possibly metal rich. Comparison of the positions in the period-luminosity plane of the masers width the long-period variables in the Magellanic Clouds and in the Galactic bulge suggests that in fact all of the redder (>5) and more efficient (>1) masers on the AGB are fundamental mode pulsator. The periods of several of the bluer (<3) and less efficient (<0) AGB masers are consistent with first overtone pulsation; these objects are similar to the classical IRC Miras except bolometrically brighter. As well as the AGB maser sources, there is a class of sources which are core helium burning supergiants, but whether they pulsate in the fundamental or first overtone models or not at all is not clear at present.

Astrophysical properties of red giants in three open clusters older than the hyades

Results from CMT1T2 1T2 broad-band and DDO intermediate-band photometry are presented for G and K giants in the old open clusters NGC. 2482, NGC 3680, and IC 4651. Two independent photometric criteria have been used to separate red field stars from the physical members of the clusters. Recent calibrations of the DDO and CMT1T2 systems have been used to derive reddening, distance moduli, metallicities, effective tempe- ratures, and surface gravities. Rough estimates of masses have also been made. The giants of NGC 2482 and IC 4651 have CN strengths nearly identical to the Hyades giants, while those of NGC 3680 are slightly richer in CN than the nearby K giants. CMT1T2 abundance analysis in NGC 2482 and NGC 3680 yield (Fe/H)MT = - 0.1 ± 0.1 as derived from the iron lines, while abundances derived from the CNO - contaminated (C - M) index are 0.4 dex higher. Both CMT1T2 and DDO data support the conclusion that 1C 4651, with (Fe/H) = + 0.2 ± 0.1, is on the metal- rich side of the distribution of intermediate and old open clusters. Finally, the mass results suggest that the clump stars in NGC 3680 and. IC 4651 could have undergone mass loss before reaching their helium core burning phase of evolution.

Deduction of Planetary Nebulae Properties from Long Period Variable Precursors

Infra-red (JHK) photometry of long period variables (LPV) in the Magellanic Clouds has shown that the LPV's can be divided into core helium burning supergiants and asymptotic giant branch (AGB) stars. Application of the pulsation theory allows masses to be derived for the LPV's while stellar evolution theory allows core masses to be derived for the AGB stars. By considering evolution of the LPV's in the (Mbol,P) diagram, estimates of planetary nebula mass and planetary nebula nucleus mass are derived as a function of initial mass. Spectra of the LPV's suggest that many low mass planetary nebulae in the Magellanic Clouds should be carbon rich while the more massive nebulae may be nitrogen enhanced.

The evolution of massive stars. II - The influence of initial composition and mass loss

We have studied the evolution of 15, 30, and 40 M/sub sun/ stars with compositions Y = 0.28 and Z = 0.01, 0.001, and 0.0002, and of 50 M/sub sun/ stars with Y = 0.28, Z = 0.0002. These models have been evolved from the zero-age main sequence through core helium burning and up to the point of carbon ignition. All models were evolved with and without the inclusion of mass loss. We have used a moderate mass loss rate with different efficiency factors for main-sequence and blue supergiant stars. Our results indicate that all massive stars ignite helium as blue supergiants. Stars with initial masses M/sub i/< or approx. =30 M/sub sun/ evolve redward very slowly and spend less than 1% of their total lifetimes as red supergiants. Above 30 M/sub sun/, the rate of redward evolution increases dramatically. The effect of a reduction in the initial metal content relative to solar abundances is to cause the models to be bluer and, in general, slightly more luminous at comparable stages of evolution. In the absence of mass loss, models with M/sub i/< or =30 M/sub sun/ and Z< or =0.001 do not become red supergiants prior to carbon ignition.more » Mass loss causes all models to evolve redward more rapidly during core helium burning.« less

The evolution of massive stars. I - The influence of mass loss on Population 1 stars

We have studied the evolution of 15, 30, and 40 M/sub sun/ models with Y = 0.28 and Z = 0.02. All models have been evolved through core helium burning up to the point of carbon ignition. All models were evolved with and without the inclusion of mass loss. We have used a moderate mass loss rate with different efficiency factors for main-sequence and blue supergiant stars. Our results indicate that all massive stars ignite helium as blue supergiants. Stars with M/sub i/< or approx. =30 M/sub sun/ evolve redward very slowly and spend less than 1% of their total lifetimes as red supergiants. Above 30 M/sub sun/, the rate of redward evolution increases dramatically. Our 40 M/sub sun/ models spend up to 50% of their core helium-burning lifetimes as red supergiants. The primary effect of mass loss is to increase the rate of redward evolution during helium burning for all masses. The effects of mass loss on the internal structure, lifetimes, and surface abundances of the models are also discussed.

Origin and Evolution of Wolf-Rayet Stars

The larger part of close binary components with initial mass exceeding ∼20 Mo becomes WR stars in the core helium burning stage. Some of the most massive WR stars may be products of evolution of single massive stars with initial masses exceeding ∼50 M0 if the mass loss in the infrared supergiant stage is effective enough. The Ledoux criterion of convective stability seems more promising to explain the observed properties of WR stars.

The elements just beyond iron - Formation during explosive carbon burning

It is pointed out that the details of the synthesis of the elements just beyond iron are unclear. Thus, a reexamination of the role played by neutron-capture reactions during explosive carbon burning is indicated. A description is given of calculations of explosive carbon-burning nucleosynthesis using a complete neutron capture (n-process) computer code with a network extending from Cr through Zr and including not only (n,gamma), (gamma,n) reactions and beta-decay but also (p,n), (n,p), (p,gamma), and (p,alpha) reactions. Initial conditions indicative of the composition of a massive star following core helium burning were selected, and a comparison is made with conditions similar to those used by Howard et al. (1972). It is found that neutron reactions during explosive carbon burning are an important source for the elements just beyond iron.

Stellar Evolution, Mass Loss and Nucleosynthesis on the Asymptotic Giant Branch

In this review, I will be concentrating on problems related to the evolution of stars on the asymptotic giant branch (AGB). AGB stars are defined as stars which have completed core helium burning and have subsequently developed degenerate carbon/oxygen cores surrounded by hydrogen and helium burning shells; such stars have main sequence masses M≤9 M⊙ (Paczynski 1971; Becker and Iben 1980). In the HR diagram most AGB stars sit on the red giant branch. An exception to this rule occurs in Population II systems, where the AGB stars evolve asymptotically to the red giant branch from the blue side as the luminosity increases after completion of core helium burning on the horizontal branch.

Stellar evolution with SMC chemical abundances

Evolutionary computations are presented for massive stars between 20 Mo and 100 Mo with chemical abundances holding for the Small Magellanic Cloud, i.e. X = .76 and Z = .003. Mass loss by stellar wind is taken into account during core hydrogen burning. After core hydrogen burning some models are considered as members of close binary systems and are followed during their Roche lobe overflow stage according an early case B of mass transfer. During the core helium burning stage of the RL0F remnants mass loss rates comparable to WR stars are included in order to study the formation and the evolution of WR stars. Comparison with similar galactic computations (Vanbeveren, Packet, 1978) is made.

Low-Mass Evolution from He Ignition to Beyond the Horizontal Branch

AbstractThe evolution of an 0.6 M⊙ stellar model during core helium burning is presented. Following the off-center ignition of helium in the “core” flash, the star remains on the red giant branch for &gt; 106 years, undergoing twelve additional flashes. After leaving the giant branch, the star evolves on the horizontal branch for 8.15×107 years before returning to the giant branch and undergoing strong helium-shell flashes. The implications for horizontal branch and RR Lyrae stars are discussed.

Low-mass evolution from he ignition to beyond the horizontal branch

The evolution of an 0.6 m⊙ stellar model during core helium burning is presented. Following the off-center ignition of helium in the “core” flash, the star remains on the red giant branch for > 106 years, undergoing twelve additional flashes. After leaving the giant branch, the star evolves on the horizontal branch for 8.15×107 years before re turning to the giant branch and undergoing strong helium-shell flashes. The implications for horizontal branch and RR Lyrae stars are discussed.

The Pulsation Properties of Red Variables

AbstractRecently derived bolometric absolute magnitudes are used to investigate the pulsational properties of some classes of red variables. The Mira variables with periods over 200 days have pulsation constants appropriate to overtone pulsatore but the shorter period 135 day Miras seem to be hotter and less luminous and may be fundamental pulsators. The large amplitude Shapley-Nail variables in the SMC are probably fundamental pulsators with masses in the 5-10 solar range. Bright supergiant red variables in the LMC show evidence of the period-luminosity relation expected if they are in the early stages of core helium burning.

Open Clusters and Stellar Evolution

Several topics relevant to the study of stellar evolution through open clusters are discussed. These include composite color-magnitude diagrams, the need for thorough studies of populous clusters, parameters affecting the core helium burning stage, and the potential importance of initial conditions and dynamical evolution on the cluster color-magnitude diagram.