Stars evolution

Abstract. Rotation plays a major role in massive star evolution. Rotation produces a significant mixing and enhances the mass loss. All model outputs are influenced. We show how the chemical yields are modified by rotation. Below 30 M , mixing increases the yields of the a-elements and above 30 M rotational mass loss dominates and enhances the yield in helium. Primary 14 N is produced at very low metallicities. 

The matter that made up the solar nebula from which the solar system was formed already was the product of stellar birth, aging and death, yet the Sun is 4.5 billion years old and will perhaps live to be 8 billion years but the Universe is thought to be 15 billion years old (15 Gyr) suggesting that perhaps we are only in the second cycle of star evolution. It is possible, however, that the massive clouds of H atoms, formed in the close proximity of the early Universe, rapidly formed super-heavy stars that had much shorter lifetimes and entered the supernova phase quickly. Too much speculation becomes worrying but the presence of different elements in stars and the subsequent understanding of stellar evolution is supported by the observations of atomic and molecular spectra within the light coming from the photosphere of stars. 

The lifetime of the molecular cloud is considered to be a time line running from cloud formation, star evolution and finally dispersion in a period that is several tci. The chemistry of the TMC and, to a good approximation, all molecular clouds must then be propagated over a timescale of at most 20 million years. The model must then investigate the chemistry as a function of the age of the cloud, opening the possibility of early-time chemistry and hence species present in the cloud being diagnostic of the age of the cloud. The model should then expect to produce an estimated lifetime and the appropriate column densities for the known species in the cloud. For TMC-1 the species list and concentrations are shown in Table 5.4. 

Finally we consider the question whether the effect of diquark condensation which occurs in the earlier stages of the compact star evolution (t 100 s) [8, 21, 22] at temperatures T Tc 20 — 50 MeV can be considered as an engine for explosive astrophysical phenomena like supernova explosions due... 

A reference configuration with total baryon number Ni> = 1.51 Nq (where Nq is the total baryon number of the sun) is chosen and the case with (configurations A and B in Fig. 13) and without antineutrinos (/ in Fig. 13) are compared. A mass defect can be calculated between the configurations with trapped antineutrinos and without it at a constant total baryon number and the result is shown in Fig. 14). The mass defect could be interpreted as an energy release if the configurations A, B with antineutrinos are initial states and the configuration / without them is the final state of a protoneutron star evolution. 

Figure 16. Schematic protoquark star evolution corresponding to Fig. 15 plotted in the phase diagram for 2-flavor quark matter...

IRAS observations of RV Tauri stars show that these objects have recently (i.e. within the last 500 years) significantly decreased their mass loss rates from a level near 10 " M yr 1 Jura (1986). These stars may have recently undergone an episode of very rapid mass loss, as one might expect for stars in the last stages of AGB star evolution. Jura speculates that the RV Tauri stars will become planetary nebulae if they evolve to high temperatures to photo-ionize the surrounding circumstellar material before it dissipates. 

THE POINT ON THE THEORETICAL CHANGES OF SURFACE CHEMISTRY DURING MASSIVE STAR EVOLUTION... 

Fig. 6 The "alternative HR-diagram" of massive star evolution. Upper part Surface escape velocity vs. T . Lower part Terminal veolcity vs. eff The position of observed objects (see Fig. 5) is also shown.

Mass loss is an essential property of massive star evolution [1], Recent parametrisations of... 

The Point on the Theoretical Changes of Surface Chemistry During Massive Star Evolution... 

MASSIVE-STAR EVOLUTION AND NUCLEOSYNTHESIS 1.01.4.1 Nucleosynthesis in Massive Stars... 

E. Schatzman. In Star Evolution. (Academic Press, New York and London, 1962), p. 389... 

Keywords neutron stars, evolution, supernovae, cosmic rays... 

Neon, sulfur, and argon are products of the late stages of massive star evolution. 20Ne results from carbon burning, while S and Ar are products of O burning. As they are... 

It seems likely that during the early life of galaxies, the formation of massive stars and consequently super novae events took place much more often than today. As a result, larger amoimts of their ash were distributed to interstellar space. During the above-mentioned period of 10 billion years, the ratio of hydrogen to heavier elements in the interstellar medium was continuously decreased, due to star evolutions and explosions. 

The probability of the formation of higher atomic number elements, like tungsten, by absorption of neutrons and/or protons during a star evolution and subsequent super nova blast is quite low. Therefore, the abundances of higher atomic number elements are considerably smaller, as for the elements which were formed by nuclear fusion reactions in an evoluting star. 

Cosmochemistry. 2. Interstellar matter. 3. Stars—Evolution. I. Title. 

Keywords Stars interiors - Stars evolution - Stars horizontal-branch -Stars AGB and post-AGB - HR and C-M diagrams - Equation of state -Convection - Atomic Processes - Nucleosynthesis... 

The observant reader will realise that the existence of an EHB star with Mh -C 0.01 Mg is inconsistent with the minimum hydrogen mass anticipated assuming conventional mass loss from a red giant (Mh > O.OlMg). Thus the existence of real EHB stars would appear to be impossible assuming standard single-star evolution. 

The question raised by this result is what mechanism can produce an enhanced mass-loss rate close to the red-giant tip Two have been proposed that involve binary star evolution [50,51]. 

Keywords Stars massive - Stars evolution - Stars nucleosynthesis - Solar System composition - r-process - p-process... 

Massive Stars Evolution and Nucleosynthesis 285 3.4 Neon, oxygen, and silicon burning... 

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