The Most Massive Stars

The Eddington limit is not only important in determining the upper mass limit of stars, but it is likely to induce strong mass loss once a massive star comes close to it. I.e., the Eddington limit may shape the initial-final mass relation for the most massive star, with severe consequences for nucleosynthesis of may species. 

Does the Eddington limit apply in the stellar interior  

The Eddington limit, in brief, is a limit to hydrostatic stability defined by the condition that the outwards directed force induced by the radiation momentum balances the inwards directed gravity. We want to emphasise that, as a stability limit, the Eddington limit applies only to hydrostatic situations. This does not mean that radiation forces are unimportant in hydrodynamic situations (e.g stellar pulsations) however, a stability limit makes of course no sense in an unstable situation. 

T = L/Ledd, and by replacing ree by the true flux-mean opacity coefficient re one gets 

25) can be generalised to be evaluated in the stellar interior, i.e. T(r) = 4rcG M(r) an(i some authors used the condition T(r) 1 as stability criterion. However, this is wrong since for T(r) — 1 convection must set in. This can be seen for the case of LB Vs, where the equation of state of gas and radiation applies as P = Pgas + Prad = f pT + f T4, even when we neglect ionization which would reduce the adiabatic temperature gradient Vad and make convection more likely. With P = -Pgas/P, we have 

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Other representative data for stars with different initial masses are given in Table 7.1. Numbers in brackets refer to Z = 0.001, Y = 0.24, others to Z = 0.02, Y = 0.28, i.e. near solar. The luminosities of the most massive stars are quite insensitive to Z. 

Fig. 4.4. All-sky map in the light of the 1.809 MeV gamma-ray hne from radioactive aluminium-26. The galactic distribution of aluminium-26, based on data from the COMPTEL (Compton Telescope) experiment aboard the GRO (Gamma-Ray Observatory), suggests that this isotope is dispersed across the Galaxy by the most massive stars, Wolf-Rayet stars and supernovas. Al is formed by the reaction Mg -b p — A1 -b y. This radioactive isotope has a lifetime of about million years and is ejected into space before it begins to decay.

Once a star has expended its supply of energy, it will contract to a glowing white ember called a white dwarf. The elements produced in the interior of a star depend on the size of the star. Small stars do not have sufficient mass to produce the temperatures required to create the heaviest elements. The most massive stars, though, may go through a series of rapid contractions in their final stages. These massive stars have the ability to generate the temperatures and pressures necessary to produce the heaviest elements such as thorium and uranium. The final fate of these massive stars is a cataclysmic explosion called a supernova. It is in this manner that scientists believe all the naturally occurring elements in the universe... 

In the stable cores of the most massive stars, 42He2+ and other elements may fuse to produce heavier elements up to nonradioactive 5226Fe26+ (Faure, 1998, 13-18 Wallerstein et al 1997 Burbidge et al., 1957 Delsemme, 1998, 44). Radioactive 5628Ni28+ can form through the following helium fusion reaction ... 

It s dead and gone, but I loved her as a man loves a woman. When I was on Earth, I could see Betelgeuse with the naked eye. Remember it was over 400 times the Sun s diameter. Although rare, the most massive stars can evolve into these stars called supergiants. ... 

H.-Th. Janka, L. Scheck, K. Kifonidis, et al. Supernova Asymmetries and Pulsar Kicks - Views on Controversial Issues. In The Fate of the Most Massive Stars, Proc. Eta Carinae Science Symposium (Jackson Hole, May 2004) (2004), in press. Preprint astro-ph/0408439... 

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. 

For M/Mq > 7, the star maintains a radiative envelope and increasingly massive convective core. For the most massive stars, the convective core can comprise over 50% of the mass of the star. In this mass range,... 

Supernova An endpoint of stellar evolution for the most massive stars, an explosion triggered by the gravitational collapse of the stellar core following the exhaustion of fuel for nuclear burning. The collapsed stellar core, depending on its final mass, can become either a black hole or a neutron star (and some of the latter may be observable as pulsars). 

Virtually all models for the ultimate energy source of GRBs involve an endpoint of stellar evolution, particularly of the most massive stars. Thus it has been proposed that the burst rate must be proportional to the overall cosmic star formation rate. This view is supported by the fact that the typical redshifts (z 1) associated with GRB host galaxies correspond to an epoch of early active star formation in the Universe. Burst counterparts also tend to be... 

Redons forming massive stars have been studied extensively by radio observations of Hn regions and masers, far-infrared measurements of the total luminosity, and millimeter mapping of the surrounding molecular gas and dust. These techniques are effective for examining the properties of the most massive stars and the molecular clouds from which they form however, there is little information on the low mass stars which form in the vicinity of massive stars. 

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