Stellar evolution carbon stars

Abstract. We have performed the chemical analysis of extragalactic carbon stars from VLT/UVES spectra. The derived individual abundances of metals and s-elements as well as the well known distance of the selected stars in the Small Magellanic Cloud and the Sagittarius dwarf galaxies permit us to test current models of stellar evolution and nucleosynthesis during the Asymptotic Giant Branch phase in low metallicity environments. 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

Table 6.2 gives an overview of some of the stages of stellar evolution where carbon and/or s-process anomalies occur (see Fig. 3.37). The C/O ratio increases down the series. In addition to the types listed there, there are infrared carbon stars such as IRC +10216,1 proto-planetary nebulae and a whole zoo of peculiar carbon stars, including J stars (strong 13C as in the case of HD 52432 shown in Fig. 1.7) and hydrogen-deficient carbon stars which can be cool, e.g. R Cor Bor, RY Sag and HD 137613 shown in Fig. 1.7, or hot (when they look like extreme helium stars) such stars may have lost their envelopes by binary mass transfer, or they may be born-again AGB stars. 

Fig. 7.7. Stellar duo. The presence of a companion star can considerably perturb a star s evolution. Hence, mass transfer by accretion transforms a rather dull white dwarf into an erupting nova or a type la supernova. As an example, let us follow the life of a star with mass between 4 and 9 Mq and its little sister star with mass between 0.9 and 3 M , separated by a distance of between 1500 and 30000 Rq (where Rq is the solar radius). In childhood, the system is calm. The big star evolves more quickly than the small one, however, a universal feature of stellar evolution. It soon becomes an asymptotic giant, sweeping the companion star with its winds, and then a white dwarf. The oxygen- and carbon-built white dwarf shares an envelope with its partner and together they evolve beneath this cloak as one and the same star. The result is a pair comprising a white dwarf with mass between 0.9 and 1.2 M and a normal star with mass between 0.9 and 3 M , still evolving on the main sequence. The two components are separated by a distance of some 40-400 Rq, corresponding to a period of revolution of 30-800 days. The second star swells up and becomes a red giant. This is a boon for the dwarf. It captures the matter so generously donated. However, it cannot absorb it A tremendous wind is generated and, in the end, a cataclysmic explosion ensues. (After Nomoto et al. 2001.)...

Two subgroups of warmer, luminous supergiants offer candidates for the post-AGB phase of stellar evolution - the group of R CrB stars and hydrogen-deficient carbon stars, and the Population II SRd variables (the 89 Her stars), RV Tauri variables, and the W Virginis stars. 

In this review we wish to discuss how observations of AGB stars can be used to determine the manner in which heavy elements are created during a thermal pulse, and how these heavy elements and carbon are transported to the stellar surface. In particular we wish to study how the periodic hydrogen and helium shell burning above a degenerate carbon-oxygen (C-0) core forms a neutron capture nucleosynthesis site that may eventually account for the observed abundance enhancements at the surfaces of AGB stars. In section II we discuss the nucleosynthesis provided by stellar evolution models (for a general review see [1]). In section III we discuss the isotopic abundances provided by nucleosynthesis reaction network calculations (see [2, 3]). In section IV we discuss how observations of AGB stars can be used to discriminate between the neutron capture nucleosynthesis sources (see [4]). And in section V we note some of the current uncertainty in this work. 

Carbon is the fourth most abundant element in the universe. Its abundance in the Sun is about one-half that of oxygen, butreveals differing ratios to oxygen in other stars and in nebulae. The most abundant isotope of carbon, 12C, is the fourth most abundant nucleus in the universe. The two most abundant, 2H and 4He, are remnants of the Big Bang, whereas l60, the third most abundant, and 12C are created during the evolution of stars. Carbon ranks therefore as one of the great successes of stellar nucleosynthesis. The evolution of stars makes evident why this is so. From the isotopic decomposition of normal carbon one finds that the mass-12 isotope, 12C, is 98.9% of all C isotopes. 

It is important here to call attention to the revised determinations of the oxygen and carbon abundances in the Sun. Allende Prieto et al. (2001) derived an accurate oxygen abundance for the Sun of log s(0) = 8.69 0.05 dex, a value approximately a factor of 2 below that quoted by Anders and Grevesse (1989). Subsequently, Allende Prieto et al. (2002) determined the solar carbon abundance to be log s(C) = 8.39 0.04 dex, and the ratio C/O = 0.5 0.07. The bottom line here is a reduction in the abundances of the two most abundant heavy elements in the Sun, relative to hydrogen and helium, by a factor 2. The implications of these results for stellar evolution, nucleosynthesis, the formation of carbon stars, and galactic chemical evolution remain to be explored. 

Table 5. Surface mass fractions of various isotopes in stellar evolution models in the initial mass range 15M0 < Minitiai < 50M (Langer Henkel 1995) at the pre-SN stage. The pre-SN configuration is also indicated, where RSG means red supergiant and WC stands for Wolf-Rayet star of the carbon sequence. The last column gives the initial abundances used in the stellar evolution calculations.

The evolution of a. star after it leaves the red-giant phase depends to some extent on its mass. If it is not more than about 1.4 M it may contract appreciably again and then enter an oscillatory phase of its life before becoming a white dwarf (p. 7). When core contraction following helium and carbon depletion raises the temperature above I0 K the y-ray.s in the stellar assembly become sufficiently energetic to promote the (endothermic) reaction Ne(y,a) 0. The a-paiticle released can penetrate the coulomb barrier of other neon nuclei to form " Mg in a strongly exothermic reaction ... 

Abstract. AGB stars, in particular those of carbon types, are excellent laboratories to constraint the theory of stellar structure, evolution and nucleosynthesis. Despite the uncertainties still existing in the chemical analysis of these stars, the determination of the abundances of several key species in their atmospheres (lithium, s-elements, carbon and magnesium isotopic ratios etc.) is an useful tool to test these theories and the mixing processes during the AGB phase. This contribution briefly review some recent advances on this subject. 

Abstract. We present the results from our non-LTE investigation for neutral carbon, which was carried out to remove potential systematic errors in stellar abundance analyses. The calculations were performed for late-type stars and give substantial negative non-LTE abundance corrections. When applied to observations of extremely metal-poor stars, which within the LTE framework seem to suggest a possible [C/O] uprise at low metallicities (Akerman et al. 2004), these improvements will have important implications, enabling us to understand if the standard chemical evolution model is adequate, with no need to invoke signatures by Pop. Ill stars for the carbon nucleosynthesis. 

Fig. 5.13. Time evolution of the chemical profile of a 40 Mq star that becomes a Wolf-Rayet star as a result of the outer layers peeling off in stellar winds. The spectrum evolves from type O to type B to a red supergiant (RSG) and then back to a blue supergiant (BSG) and towards increasing effective temperatures ending up well to the left of the main sequence. The chemically modified spectrum evolves from nitrogen-rich late, i.e. relatively cool (WNL), to nitrogen-rich early (WNE) to carbon-rich (WC) in some cases still hotter stars are observed that are oxygen-rich (WO). After Maeder and Meynet (1987).

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