Massive star rotating

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. 

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... 

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.

Fig. 1. Abundance gradient of N/O predicted by models adopting stellar yields where rotation is not taken into account (as model 7 of [3] - thin solid line) and the same models computed with MM02 yields ([2] - thick solid line). A model where we increased only the amount of primary N in massive stars for metallicities below Z=10-B overlaps with the thick solid line shown here [1], This shows that the N/O gradient along the MW disk is affected mainly by the amount of nitrogen production in low and intermediate mass stars and not the primary N in massive stars. For the abundance data see [3] and references therein - asterisks are B stars (see Cunha, this conference).

In the massive range, the yields of Meynet Maeder predict some primary N production. In [2] we showed that models for the MW computed with this new set of yields lead to a plateau in log(N/0), due to massive stars with initial rotational velocities of 300 kmsec 1, at log(N/0) —4. This value is below the value of —2.2 dex observed in some DLAs and hence we suggested that in these systems both, massive and intermediate mass stars, would be responsible for the N enrichment. This is at variance with recent claims that massive stars were the only ones to enrich systems which show a log(N/0) —2.2. 

The conceptually simplest way of forming a black hole at the heart of a massive star, thereby setting up the conditions of the hypernova model, is to begin by repudiating the traditional explosion model detonated by neutrinos. The iron core then collapses without remission in the space of one second. A black hole prospers, pulling down the rest of the stellar edifice. This may be a common occurrence for stars of 35 to 40 Mq. However, uncertainties remain concerning convection, mass loss and mixing due to rotation, not to mention the explosion mechanism itself. 

The analysis of nucleosynthesis in hypernovae suggests a possible classification scheme of supernova explosions [111]. In this scheme, core collapse in stars with initial main sequence masses Mms < 25 — 30M leads to the formation of neutron stars, while more massive stars end up with the formation of black holes. Whether or not the collapse of such massive stars is associated with powerful hypernovae ( Hypernova branch ) or faint supernovae ( Faint SN branch ) can depend on additional ( hidden ) physical parameters, such as the presupernova rotation, magnetic fields. [39], or the GRB progenitor being a massive binary system component [145, 117]. The need for other parameters determining the outcome of the core collapse also follows from the continuous distribution of C+O cores of massive stars before the collapse, as inferred from observations, and strong discontinuity between masses of compact remnants (the mass gap between neutron stars and black holes) [28]2. 

By applying the concept of rotationally induced mixing as it has been developed for massive stars during the last years without alteration to a 3 Mq TP-AGB model sequence, Langer et al. (1999) obtained conditions which appear favourable for the development of the s-process, i.e. a 13C-rich layer which produces a considerable neutron flux later-on. 

It is crucial to realize that for T —> 1 Eq. (5.31) gives vcrit —> 0. Therefore, if we assume a star to evolve towards the Eddington limit (T — 1), no matter what its rotation rate may be, it will arrive at critical rotation well before T = 1 is actually reached. Therefore, one may rather speak of the D-limit instead of the Eddington limit. Note that in this simplified approach gravity darkening is neglected however, Maeder (1999) found this to not change our conclusions for the most massive stars qualitatively. 

Let us now describe several effects which may happen when massive stars approach the 12-limit,. Striking qualitative features can be derived from simplified models of rotating, mass losing 60 Mq stars (Langer 1998). 

As shown by Friend Abbott (1986), rotating massive stars have an enhanced radiation driven stellar wind mass loss well before they actually hit the Q-limit, with an enhancement factor... 

---