Stellar winds

When metallicity decreases, the opacity decreases and the stars are more compact. For instance the radius of a 20 Mq star on the ZAMS is decreased by about a factor 4 when its metallicity passes from 0.020 in mass fraction to 0. Due to the decrease of the opacities, the radiative driven stellar winds are also weaker [5]. 

Early Be and B Observations primaries, while Standard GCR produce secondaries GCR metallicity always the same, originating in SN ejecta or in Superbubbles (SB) Problem with absence of unstable 59Ni in GCR it becomes stable if directly accelerated in SN or continuously accelerated in SB by stellar winds... 

Cyclical Variability in Stellar Winds Proceedings, 1997. XXII, 415 pages. 1998. 

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).

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. 

Massive stars from 25 to 100 Mo already lose a substantial fraction of their mass in strong stellar winds ranging from 2x10 Mo/y during the H-Buming phase up to 5 x 10 Mo/y in the He-buming phase also known as Wolf-Rayet phase. As a large convective core develops in these stars, fresh nucleosynthetic product are soon exposed on the surface and ejected with the stellar winds (Prantzos et al. 1986). Wolf-Rayet stars may have been the principal source of Al in meteorites (Amould et al. 2000). 

Wolf-Rayet star massive star producing a high stellar wind... 

The different types of grain can be related to specific classes of stellar objects. The very hot and bright, even lavish Wolf-Rayet stars are considered to be one of the most favourable sites for grain formation, for their strong stellar winds are particularly rich in carbon. Matter thrown out by supernovas and cooling very quickly due to its expansion is also an excellent scenario for grain formation. Elements with any affinity for the solid state are likely to be abundantly transformed. 

These nebulas are similar in some respects to the Hll regions. The difference is that here the source of ionisation is an ageing star (white dwarf) in its death throes rather than a strapping young blue star. The fluorescent region is both denser and chemically more complex for it includes those atoms expelled from the envelope of the dying star in the form of a stellar wind. 

All stellar evolution can be summed up by a simple rule the star tries to make itself as small as possible. Its life story is one of contraction, but in a discontinuous manner, with sometimes long pauses during which it maintains its size. There are phases when the outer layers are driven off by radiation pressure (stellar winds, ejection of the envelope) and brief periods when the star violently readjusts itself, but without breaking apart (helium flash, thermal pulses). 

S tellar winds and planetary nebulas also play an important role in the chemical economy of our Galaxy, as they probably do in all the others. In particular, they enhance levels of nitrogen, carbon and heavy elements beyond iron (by the s process). For the main part, newly made elements are produced and launched into circulation by the last gasp of fight stars (generating planetary nebulas), stellar winds and supernova explosions. 

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