Supergiant star

Key words 10 pm observations - nebulae proto-planetary - stars drcumsteUar matter - stars supergiants... 

Abstract. We present initial results from our study of mixing in M Supergiants. C, N and O abundances are measured in five stars. N/C and N/O ratios indicate extensive mixing in excess of the standard models and in support of the rotational models. 

Massive stars play an important role in numerous astrophysical contexts that range from the understanding of starburst environments to the chemical evolution in the early Universe. It is therefore crucial that their evolution be fully and consistently understood. A variety of observations of hot stars reveal discrepancies with the standard evolutionary models (see [1] for review) He and N excesses have been observed in O and B main sequence stars and large depletions of B accompanied by N enhancements are seen in B stars and A-F supergiants [2,3,4,5], All of these suggest the presence of excess-mixing, and have led to the development of a new generation of evolutionary models which incorporate rotation (full reviews in [1], [6], [7]). 

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. 

Massive stars play a key role in the spectral evolution of galaxies, they are also the progenitors of Wo I f Rayet (WR) stars, supernovae and y-ray bursts. They are the main agents of nucleosynthesis driving the chemical evolution of galaxies. The relative numbers of the various kinds of massive stars (blue, red supergiants, WR stars), their properties and nucleosynthesis very much depend on mass loss and rotation, as well as on the interaction of these two effects. 

The four general types of stars (main sequence, white dwarfs, giants and supergiants) provide a classification based on the fundamental observable properties but also suggest an evolution of stars. Astrochemically, the cooler giants and supergiants have many more atomic and molecular species that are the products of the nuclear fusion processes responsible for powering the stars. The nuclear fusion processes allow for the formation of more of the elements in the Periodic Table, especially the heavier elements that dominate life on Earth - principally carbon. 

Low (<1 solar mass) Middle (5-10 solar masses) High (>20 solar masses) Protostar — pre-main sequence — main sequence — red giant — planetary nebula — white dwarf — black dwarf Protostar - main sequence — red giant — planetary nebula or supernova —> white dwarf or neutron star Protostar — main sequence —> supergiant — supernova — neutron star... 

Herzprung-Russell diagram A graphical plot of stellar intensity versus photosphere temperature showing that observed stars fall into classes main sequence, red giants, supergiants and white dwarfs. 

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).

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. 

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