Massive stars

More massive stars in the upper part of the main-.sequence diagram (i.e. star.s with masses in the range 1.4-3.5 M ) have a somewhat different hi.story to that considered in the preceding sections. We have seen (p. 6) that such stars consume their hydrogen much more rapidly than do smaller stars and hence spend less... 

Three sources have been proposed to produce fluorine in the Galaxy. The first was suggested by Forestini et al. (1992) and refers to production in low-mass stars during the AGB phase while two others are related to massive stars production in Wolf-Rayet stars (Meynet Arnould 2000) and in type II Supernovae, via the neutrino-induced nucleosynthesis (Woosley et al. 1990). 

This work aims at testing the suggestion of [5] that stellar rotation is faster at lower metallicity by direct measurements, especially in the LMC and SMC, on stars with —3.34 < My < —2.17, i.e. spectral types B0-B6 or masses from 6.7 to 14 M0. This work is complementary to that of [4], which deals with slightly more massive stars. The results are shown on Fig. 1 and commented in the caption. There is an excess of slow rotators in the Galaxy relative to the MCs, but the v sin i distributions of the LMC and the SMC are surprisingly similar. 

The abundance ratios found in the photospheres of our target stars are imprints of the explosions of the first SNe II or even more massive stars. At very low metallicities there is a reasonable hope that the SNe which have polluted the environment were themselves primordial objects. In former papers on the chemical composition of very metal poor stars, some accent was put on trends of abundance ratios with metallicity ( McWilliam et al.[6], Norris et al. [8]). Such was the case for [Mn/Fe], or [Cr/Fe] decreasing with decreasing metallicity, or... 

Spectroscopic observations of globular clusters (GCs) have revealed star-to-star inhomogeneities in the light metals that are not observed in field stars. These light metal anomalies could be interpreted with a self-pollution scenario. But what about heavier (Z > 30) elements Do they also show abundance anomalies Up to now, no model has been developed for the synthesis of n-capture elements in GCs, and the self-pollution models do not explain the origin of their metallicity. In 1988, Truran suggested a test for the self-enrichment scenario [4], which could possibly explain the metallicity and the heavy metal abundances in GCs if self-enrichment occurred in GCs, even the most metal-rich clusters would show both high [a/Fe] ratios and r-process dominated heavy elements patterns, which characterize massive star ejecta as it is seen in the most metal-poor stars. 

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. 

In Talon Charbonnel (1998, TC98), Charbonnel Talon (1999, CT99) and Palacios et al. (2003) we went one step further We included in the models the most complete description currently available for rotation-induced mixing, and we computed self-consistently the transport of the chemicals and that of AM due to wind-driven MC. We used the same input physics than that used with success by the Geneva group to explain several observational patterns of more massive stars (e.g. Maeder Meynet 2000 and Talon Charbonnel 2003 and references therein). 

What happens for cooler (i.e. less massive) stars on the red side of the Li dip As we shall see now, the stellar mass or the effective temperature of the dip is a transition point for stellar structure and evolution. First of all it is a transition as far as the rotation history of the stars is concerned. Indeed the physical processes responsible for surface velocity are different, or at least operate with different timescales on each side of the dip. At the age of the Hyades, the stars hotter than the dip still have their initial velocity while cooler stars have been efficiently spun down (Fig. 1). This behavior is linked to the variation of the thickness of the superficial H-He convection zone which gets rapidly deeper as Teff decreases from 7500 to 6000K (e.g. TC98). Below 6600 K, the stars have a sufficiently deep... 

These complementary observational constraints indicate that another process participates to the transport of AM in solar-type stars, while MC and turbulence are successful in more massive stars. The two most likely candidates are the large-scale magnetic field which could be present in the radiative zone and the internal gravity waves (hereafter 1GW) which are generated by the external convective zone. As we just explained, the observations suggest that the efficiency of this process is finked to the growth of the convective enveiope. This is a characteristics of 1GW. 

While this theory has been shown to be very successful in the case of massive stars (Maeder and Meynet, this volume) and Population 1 low-mass stars (Charbonnel, this volume), full and self-consistent application in the case of globular... 

Rotational Mixing in Massive Stars and Its Many Consequences... 

Abstract. Rotation plays a major role in massive star evolution. Rotation produces a significant mixing and enhances the mass loss. All model outputs are influenced. We show how the chemical yields are modified by rotation. Below 30 M , mixing increases the yields of the a-elements and above 30 M rotational mass loss dominates and enhances the yield in helium. Primary 14 N is produced at very low metallicities. 

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. 

Fig. 1. HR diagram of massive stars with Z=0.02, for rotating stars with r>ini = 300 km/s (continuous lines) and for non-rotating stars (dotted lines). The initial rotation Vini = 300 km/s corresponds to the average observed velocity of about 220 km/s for OB stars.

The various filiation sequences for massive stars can be described. 

The process of star formation in metal free gas might lead to the formation of much more massive stars than at solar metallicity (see e.g. the review [8]). 

We selected three stars with [Fe/H] < — 3, which were known to have [Ba/Fe] — 1, typical for their metallicities, and estimate Eu abundance using Subaru HDS. As shown in Fig. 1, our data add the lowest detections of Eu, at [Fe/H] < —3. The three stars and most others are located between the 50% confidence lines for this case. However, if Eu comes from more massive stars, these stars are located outside the 90% confidence region. We suggest, therefore, the r-process site is most likely to be SNe from low-mass progenitors such as 8 — 10M0 stars. 

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