Evolution of intermediate- and low-mass stars

But from the hydrostatic equation (5.8), for a fixed included mass m, there is a reduction in pressure, 

For a perfect gas with P a pT, the temperature must therefore go up, which leads to instability, provided that the temperature sensitivity of the reaction rate is 

A further effect during evolution up the AGB is mass loss through stellar winds, at an increasing rate as the star increases in luminosity and radius and becomes unstable to pulsations which drive a super-wind in the case of intermediate-mass stars. For stars with an initial mass below some limit, which may be of order 6 M , the wind evaporates the hydrogen-rich envelope before the CO core has reached the Chandrasekhar limiting mass (see Section 5.4.3), the increase in luminosity ceases and the star contracts at constant luminosity, eventually becoming a white dwarf (Figs. 5.15, 5.19). A computed relation between initial stellar mass and the final white-dwarf mass is shown in Fig. 5.21. 

The ejected material forms a cool molecular and dusty envelope which initially veils the star from optical observations as it goes through the stages of Mira variable followed by OH-infirared star or infrared carbon star later the star becomes hot enough to ionize part or all of the expanding gas-dust envelope forming a planetary nebula. 

The star continues to burn hydrogen in a thin shell until the bluest point on its evolutionary track, at which point H-buming is switched off and both the H-rich envelope and the He-rich layer contract rapidly. At this point, in most cases, it is 

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Figure 5.15 gives an overview of stellar evolution in the HR diagram. Both intermediate- and low-mass stars end their lives as white dwarfs after having expelled a substantial amount of mass in winds and planetary nebulae, the basic reason being the formation of a degenerate CO core that is not massive enough... 

White dwarfs are the remnant of the evolution of low and intermediate (<5 Mq) mass stars, formed by core contraction during the last stages of nuclear burning. Since they reach densities in excess of 10 g cm and typically have radii of order 0.01 R they were expected, by simple theory, to be able to possess very strong surface fields. Any weak field remnant in the core at the time of contraction, should the field be able to survive this stage of evolution, would be amplified by flux-freezing to very large values, of... 

Open clusters (OCs) are important tools both for stellar and for galactic astrophysics, as tests of stellar evolution theory for low and intermediate mass stars and as tracers of the Galactic disk properties. Since old OCs allow us to probe the lifetime of the Milky Way disk, up to about 10 Gyr ago, they can be used to study the disk evolution with time, and in particular its chemical history. 

Although Ba and heavier elements seem to fit the solar r-process pattern, lighter elements show wide varieties [5]. In particular, a large dispersion has been found in [Sr/Ba] at low metallicity[l], suggesting that lighter elements such as Sr does not come from a universal process, which produces Ba and Eu, but from weak r-process. An inhomogeneous chemical evolution model suggests that the dispersions in [Sr/Ba] are well-explained, when weak r-process produces 60% of Sr but only 1% of Ba in metal-poor stars. Furthermore, intermediate mass elements such as Pd must provide clues to understand the weak r-process yield. 

Stages of stellar evolution Low- and intermediate-mass stars... 

Recent modeling based on the lifetimes of stars, their IMF, the star formation rate as a function of time, and nucleosynthesis processes have succeeded in matching reasonably well the abundances of the elements in the solar system and in the galaxy as a whole (e.g. Timmes et al., 1995). These models are still very primitive and do not include nucleosynthesis in low and intermediate-mass stars. But the general agreement between model predictions and observations indicates that we understand the basic principles of galactic chemical evolution. 

Theoretical modeling provides strong evidence that most presolar silicon carbide grains come from 1.5 to 3 M stars. As discussed in Chapter 3, stellar modeling of the evolution of the CNO isotopes in the envelopes of these stars makes clear predictions about the 12C/13C, 14N/15N, 170/160,180/160 ratios as a star evolves. For example, in the envelopes of low- to intermediate-mass stars of solar composition, the 12C/13C ratio drops to 40 (from a starting value of 89), and 14N/15N increases by a factor of six as carbon and nitrogen processed by... 

Stellar Structure, Nucleosynthesis and Evolution of Low and Intermediate-mass Stars... 

In these lecture notes, we are primarily interested in the changes to the surface composition of low and intermediate mass stars during the AGB. Before we get to the AGB stage, it is necessary that we first examine the evolution and nucleosynthesis that occurs prior to the AGB and we cover this material in Sect. 3. AGB evolution is discussed next in Sect. 4, and the nucleosynthesis in Sect. 5. We briefly review the s-process in Sect. 6. We begin with some important preliminaries. 

The book has been organized into three parts to address the major issues in cosmochemistry. Part I of the book deals with stellar structure, nucleosynthesis and evolution of low and intermediate-mass stars. The lectures by Simon Jeffery outline stellar evolution with discussion on the basic equations, elementary solutions and numerical methods. Amanda Karakas s lectures discuss nucleosynthesis of low and intermediate-mass stars covering nucleosynthesis prior to the Asymptotic Giant Branch (AGB) phase, evolution during the AGB, nucleosynthesis during the AGB phase, evolution after the AGB and massive AGB stars. The slow neutron-capture process and yields from AGB stars are also discussed in detail by Karakas. The lectures by S Giridhar provide some necessary background on stellar classification. 

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