Evolution of massive stars

The rotation models predict significant effects on the properties and the evolution of the massive stars. They alter the ratio of red to blue supergiants and hence the nature of SNII progenitors they affect the properties, formation and evolution of Wolf-Rayet stars they result in the enrichment of He and C in the ISM while the abundance of O decreases they produce higher He and a-element yields from SNII via larger He cores. Many of these effects are metallicity dependent. With such far ranging impact, the effects of rotation and mass loss on the evolution of massive stars should be thoroughly understood. 

Further burning stages evolution of massive stars... 

Prantzos N, Doom C, Amould M, De Loore C (1986) Nucleosynthesis and evolution of massive stars with mass loss and overshooting. Astrophys J 304 695-712... 

III. THE EVOLUTION OF MASSIVE STARS AND THE DYNAMICS OF STELLAR WINDS... 

The observed terminal velocities of O-stars provide interesting information about the evolution of massive stars. This is shown in Fig. 5, which is taken from Garmany and Conti (1985) and displays vM vs. Tef for a sample of O-stars (luminosity classes between V and III) in the Galaxy, LMC and SMC. Two striking facts can be read off from Fig. 5 ... 

From the distribution of their heliocentric distances we estimate that the WR star cencus is practically complete out to d 3 kpc. Considering a volume around the Sun with d < 2. 5 kpc. we count 44 WR stars in the ratio NyVN NWC NWO - 15 28 1. If we accept that the WN and WC phases are consecutive phases in the evolution of massive stars, then this implies that in the solar neighbourhood the average WC phase lifetime is twice as long as the WN phase lifetime. More specifically, we count in the range 7. 5 <... 

They were either formed with the progenitor from the interstellar medium, or they were created in it as an adjunct to helium-burning and mixed to the surface. In the latter event, some enrichment of the elements would be expected, although calculations of the evolution of massive stars have indicated that no products from the helium burning core should be convected to the surface (Lamb et al. 1977). 

These consideration will lead us deeper understanding of the mixing, mass loss, and explosion during the evolution of massive stars. 

The most abundant product of the evolution of massive stars is oxygen, in particular—the third most abundant isotope in the Universe and the most abundant metal. Massive stars are also the main source of most heavy elements up to atomic mass number A 80, of some of the rare proton-rich nuclei, and of the r-process nuclei from barium to uranium. In the following, we will briefly review the burning stages and nuclear processes that characterize the evolution of massive stars and the resulting core collapse supemovae. 

An extensive discussion of basic physics of core collapse supernovae can be found in [12] the evolution of massive stars, core collapse, formation of stellar remnants and supernova nucleosynthesis are reviewed in [163] a recent concise discussion of problems and prospects for core collapse supernovae can be found in [105]. 

The formation of elements is a long and complicated path of diverse nuclear reactions, which took place during the evolution of massive stars (>8 times the solar mass). The raw material is always hydrogen, which was formed in addition to the smaller amounts of deuterium and helium shortly after the Big Bang. During the lifetime... 

Neutron absorption processes occur at different times and places in the course of the evolution of massive stars. The S-process (slow neutron capture) occurs in the He burning region (state of a red giant). The R-process (rapid neutron capture) occurs during the super nova explosion, either within a short distance of the forming neutron star or in the shell where He burning took place prior to the blast when the shock wave hits this area. 

The Evolution of Massive Stars and the Concomitant Non-explosive and Explosive Nucleosynthesis... 

Summary. These lectures are concerned with some aspects of the evolution of massive stars and of the concomitant nucleosynthesis. They complement other lectures in this volume. Special emphasis is put on the production of the nuclides heavier than iron by the r- and p-processes. 

It is quite important to stress that the true stellar structure is certainly much more complicated than sketched in Fig. 1, even when effects like deviations from spherical symmetry (induced by rotation or certain mechanisms of transport of matter) are neglected. This spherically symmetric picture of a star may break down, especially during the advanced stages of the evolution of massive stars, and would lead to a dramatic growing of the complication of the stellar structure and evolution (e.g. [2,3], and references therein). This increased complexity is demonstrated by multi-dimensional simulations of the structure of massive stars. The consideration of rotation of course brings additional difficulties. Steady mass loss from a star may also affect its evolution in various ways. Finally binarity may lead to specific evolutionary patterns resulting for the largest part from episodic mass transfers from one component to the other. 

In the following we present a very brief account of the nucleosynthesis developing during the various non-explosive stages in the evolution of massive stars depicted in Fig. 1. 

Some progress in the description of convection in advanced stages of the evolution of massive stars, and in particular in their O-Ne layers. The possibility of survival at the explosion stage of at least a fraction of pre-explosively produced p-nuclides remains an uncertain issue. 

Supernova events as well as pre-supernova stages of stars play important roles in cosmochemistry. However, a number of aspects of astrophysics of both pre-supernova and supernova stages of evolution of massive stars remain unclear, as emphasized by Marcel Arnould (this volume). A major challenge comes from the uncertainties in the rates of some nuclear reactions and of weak interaction processes. Multi-dimensional simulations, especially of the late presupernova phases, are expected to be particularly helpful to understand these processes. Similarly, considerable efforts are still required for successful simulations of supernova explosions based on less uncertain pre-supernova models. Here again, the multi-dimensional treatment of a variety of physical effects, including rotation, magnetic fields, instabilities of different origins, and the transport of neutrinos needs careful inclusion. 

Part II deals with explosive nucleosynthesis that plays a critical role in cosmochemistry. The lectures by Kamales Kar provide essential background material on weak-interaction rates for stellar evolution, supernovae and r-process nucleosynthesis. He also discusses in detail the solar neutrino problem. Massive stars, their evolution and nuclear reaction rates from the point of view of astronomers and nuclear physicists are discussed by Alak Ray. His lectures also describe the various stages of hydrostatic nuclear fuel burning with illustrative examples of how the reactions are computed. He also discussed core-collapse (thermonuclear vs. core-collapse) and supernovae in brief. The lectures by Marcel Arnould address the phenomena of evolution of massive stars and the concomitant non-explosive and explosive nucleosynthesis. He highlights a number of important problems that are yet unresolved but crucial for our understanding of Galactic chemical evolution. The p-process nucleosynthesis attributed to the production of proton-rich elements, a topic of great importance but yet less explored is also discussed in his lectures. 

The elements of the iron peak group provide information on the late evolution of massive stars by nuclear statistical equilibrium processes. Any isotopic anomalies found in these elements would reflect variations in the quasi-equilibrium conditions of stellar interiors, provided that the isotopic... 

Figure 4 shows the isotopic anomalies of the iron peak elements predicted by the multi-zone mixing model as compared with the average excesses as observed in Ca-Al-rich inclusions. The match between the two data sets is impressive, except for Fe and Zn. In the case of Fe no significant anomalies have been measured, but the multi-zone mixing model only predicts a Fe excess of approximately 1 part in 10", which is at the limit of present mass spectrometric capability. In the case of Zn, the excess in Zn is approximately an order of magnitude less than that expected. This can be explained in terms of the volatility of Zn with respect to the other iron peak elements, as it would be the last of these elements to condense from the expelled stellar material. The correlation between anomalies in neutron-rich isotopes in the iron peak elements can therefore be explained in terms of the nuclear statistical equilibrium processes, which took place at a late stage in the evolution of massive stars. 

The final evolution of massive stars (see e.g. the review given by Woosley, Heger, and Weaver, 2002 [366]) becomes more violent than that for low massive stars. A cosmic catastrophe occurs. The gas of the Fe core becomes electron degenerate. As soon as it exceeds about the mass of 1.4 solar masses (this is called Chandrasekhar mass), i.e. the core exceeds the size of the Earth, gravity is too strong to... 

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