Stars evolved

During the evolution of many stars, mass loss during the giant branch phase modifies a star s physical properties. Studies of the stellar envelopes that result from this mass loss process indicate that evolved stars are a significant contributor of processed material to the interstellar medium. Since this processed material is eventually incorporated into the next generation of stars, an understanding of evolved stars and their mass loss mechanisms is key to understanding the entire star formation process. 

The morphology and extent of circumstellar envelopes is determined by the dynamics of the mass loss process and by the stellar and interstellar radiation field. Since the envelope s density structure is approximately known, the time exposure of the interstellar radiation field can be estimated, and the distribution of molecular photoproducts can be measured. Stellar envelopes represent an unparalleled astrochemistry and radiative transfer laboratory. For this reason, evolved stars have been prime targets for the detection and study of rare molecules in the overall investigation of interstellar chemistry. 

FIGURE 9 Spectrally Integrated CO 2-1 emission Image of a portion of the Ophiuchus star formation region. The intensity scale Is in K + km seo . Symbols identify the positions of the young stellar objects in the region, some of which possess molecular outflows. 

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During the red giant phase of stellar evolution, free neutrons are generated by reactions such as C(a,n) and Ne(a,n) Mg. (The (ot,n) notation signifies a nuclear reaction where an alpha particle combines with the first nucleus and a neutron is ejected to form the second nucleus.) The neutrons, having no charge, can interact with nuclei of any mass at the existing temperatures and can in principle build up the elements to Bi, the heaviest stable element. The steady source of neutrons in the interiors of stable, evolved stars produces what is known as the "s process," the buildup of heavy elements by the slow interaction with a low flux of neutrons. The more rapid "r process" occurs in... 

Abstract. In this contribution we present the results based on high-resolution spectra of 45 clump stars of the Galactic field. The main atmospheric parameters and abundances of 12C, 13C, N, O and other mixing sensitive chemical elements were investigated. Elemental ratios in the sample of field stars are compared to the results available for evolved stars in open clusters and to the theoretical prediction of extra mixing in stellar interiors. 

The next feature that deserves attention in Fig. 2, is bimodal frequency distribution of lithium line strengths for MO - M4 giants. Here we get some hint at the existence of a gap in the distribution of the lithium abundances of the evolved stars. The results of a more precise abundance analysis of 25 M giants by Luck and... 

Finally, I would like to stress that the bulk and quality of the lithium abundance data for red giants need to be considerably raised for an adequate statistical analysis. The main task of the present report was to provoke some interest in the problem concerning the distribution of lithium abundances in evolved stars. 

But among the more luminous and more evolved stars, the story becomes more... 

Infrared spectra of evolved stars are generally dominated by the radiation from their circumstellar shells. M stars are characterized by the 10 pm emission feature from silicate dust grains, while C stars by the 11 pm SiC band. However, some C stars have been found to show the 10 pm feature indicating the oxygen-rich property of their circumstellar dust (Willems and de Jong 1986, Little-Marenin 1986). 

The interpretation for these peculiar objects has not been established. There are two possibilities though. One is that we are observing the transition stage of an evolved star from M- to C-type with a... 

However, the hot evolved stars do not form a homogeneous group. The most prominent division is into Central Stars of Planetary Nebulae differences exist in spectral type and chemical composition (as deduced from medium and high resolution spectra). Consequently, a unique progenitor for all kinds of hot evolved stars appears to be very unlikely and accurate stellar parameters are needed to compare these stars to predictions of stellar evolution theory. 

This paper summarizes the results of analyses of highly evolved stars with spectral type B or hotter, namely sdB, sdOB and sdO types, CSPN and extremely helium-rich stars. It does not consider white dwarfs since their chemical surface composition is apparently governed by diffusion processes and accretion of interstellar material (Wesemael, 1979 Vauclair et al., 1979 Wesemael and Truran, 1982) and is not linked to their past evolution. Section 2 deals with the positions of the hot evolved stars in the (log Te -log g) plane and their helium to hydrogen ratios. Metal abundances are considered in section 3 and comparisons of stellar evolution calculations with the available data are performed in section 4. 

Figure 1 summarizes the results of all available fine analyses of hot evolved stars. It is important to note that most of these stars are not known to be members of binary systems. We find that all sdB and sdOB stars are drastically depleted in helium. A well-defined border line at Te = 4200OK separates these stars from the classical or "compact sdOs which cluster at Teff 50000K, have surface gravities log g > 5 and are systematically enriched in helium (y = NHe/(Njj+Njje)... 

Abstract A molecular line search in the range between 85 and 89 GHz has been performed in the circumstellar envelopes of 11 evolved stars. Emissions of 29SiO J=2-l,28SiO J=2-l, HCN J=l-0, H13CN J=l-0, HC5 N J=33-32, HCO+ J=l-0 transitions and other transitions of C2 H, C4 H, and C3 N have been observed in 11 stars. We have detected the ground state 29SiO J=2-l maser in several stars. We have also detected HCN emission in VY CMa. A narrow H13CN spike feature near the central velocity has been found in the spectrum of CRL 2688. 

THE V-STOKES PARAMETER AS A MANIFESTATION OF ENVELOPE ACTIVITY FOR COOL, BRIGHT, EVOLVED STARS... 

The Y-stokes Parameter as a Manifestation of Envelope Activity for Cool, Bright, Evolved Stars... 

Carbon rich giant stars are highly evolved stars that have a greater concentration of carbon than oxygen in their atmospheres. This superficial enrichment is the consequence of nuclear reactions that take place in their interiors and of the efficient convection that transports the nucleosynthesis products to the most external layers. Carbon stars have effective temperatures in the range 2,000-3,000 K and are some 10,000 times brighter than the Sun. They often present important mass loss... 

A small percentage of the27 A1 is created instead in the hydrogen-burning shells of evolved stars, where the 2 Mg + 2H —27 A1 reaction can occur. At the time of the supernova explosion, this shell burning can have almost doubled the27A1 abundance locally in the hydrogen convective shell that has been exposed at the stellar surface. 

A related question is this Which types of stars or supernova explosions produce the extinct radioactive nuclei thatare found in the solar system Here the reader is referred to the entry for each specific isotope. But this much must be appreciated first. Some radioactivities appear to be made primarily by the thermonuclear explosions of white-dwarf stars, called Type la supernovae. Others are created primarily in massive stars whose cores collapse to become neutron stars to initiate an explosive ejection (Type II supernovae). Type II supernovae occur three to five times more frequendy than do Type la supernovae. Some radioactive nuclei are made within differing portions of each event, some prior to the ejection, but some during the heat of the ejection process. And still other radioactive nuclei are created within evolved stars that do not become supernovae (red giants). This diversity of origin renders uncertain the identity of those extinct radioactivities that are to be attributed specifically to that supernova thatis supposed to have triggered the formation of the solar system. In recent scientific... 

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