Alessandro Chieffi

Explosive Yields of Massive Stars from Z = 0 to Z = Z☉

We present a new and homogeneous set of explosive yields for masses 13, 15, 20, 25, 30, and 35 M? and metallicities Z = 0, 10-6, 10-4, 10-3, 6 ? 10-3, and 2 ? 10-2. A wide network extending up to Mo has been used in all computations. We show that at low metallicities (Z ? 10-4), the final yields do not depend significantly on the initial chemical composition of the models, so a scaled solar distribution may be safely assumed at all metallicities. Moreover, no elements above Zn are produced by any mass in the grid up to a metallicity ~10-3. These yields are available for any choice of the mass cut on request.

The Nucleosynthesis of 26Al and 60Fe in Solar Metallicity Stars Extending in Mass from 11 to 120 M☉: The Hydrostatic and Explosive Contributions

We present the 26Al and 60Fe yields produced by a generation of solar metallicity stars ranging in mass between 11 and 120 M☉. We discuss the production sites of these γ-ray emitters and quantify the relative contributions of the various components. We provide the contributions of the wind, the C convective shell, and the explosive Ne/C burning to the total 26Al yield together with the contributions of the He convective shell, the C convective shell, and the explosive Ne/C burning to the 60Fe yield. We conclude that, at variance with current beliefs, 26Al is mainly produced by the explosive C/Ne burning over most of the mass interval analyzed here, while 60Fe is mainly produced by the C convective shell and the He convective shell. By means of these yields we try to reproduce two quite strong observational constraints related to the abundances of these nuclei in the interstellar medium, i.e., the number of γ1.8 photons per Lyman continuum photon, RGxL, and the 60Fe/26Al γ-ray line flux ratio. RGxL is found to be roughly constant along the Galactic plane (and of the order of 1.25 × 10-11), while the 60Fe/26Al ratio has been recently measured by both RHESSI (0.17 ± 0.05) and SPI (INTEGRAL) (0.11 ± 0.03). We can quite successfully fit simultaneously both ratios for a quite large range of exponents of the power-law initial mass function. We also address the fit to γ2 Velorum, and we find that a quite large range of initial masses, at least from 40 to 60 M☉, do eject an amount of 26Al (through the wind) compatible with the current upper limit quoted for this W-R star: such a result removes a long-standing discrepancy between the models and the observational data.

On the origin of HE 0107-5240, the most iron-deficient star presently known

We show that the puzzling chemical composition observed in the extremely metal-poor star HE 0107-5240 may be naturally explained by the concurrent pollution of at least two supernovae. In the simplest possible model, a supernova of quite low mass (~15 M☉) underwent a normal explosion and ejected ~0.06 M☉ of 56Ni while a second one was massive enough (~35 M☉) to experience a strong fallback that locked in a compact remnant all the carbon-oxygen core. In a more general scenario, the pristine gas clouds were polluted by one or more supernovae of relatively low mass (less than ~25 M☉). The successive explosion of a quite massive star experiencing an extended fallback would have largely raised the abundances of the light elements in its close neighborhood.

Evolution and Nucleosynthesis of Primordial Low-Mass Stars

We discuss in detail the evolutionary properties of low-mass stars (M ≤ 1 M☉) having metallicity lower than Z = 10-6 from the pre-main sequence up to (almost) the end of the early asymptotic giant branch phase. We also discuss the possibility that the large [C/Fe], [N/Fe] observed on the surface of the most iron-poor star currently known, HE 0107-5240, could be attributed to the autopollution induced by the penetration of the He convective shell into the H-rich mantle during the He core flash of a low-mass, very low metallicity star. On the basis of a quite detailed analysis, we conclude that the autopollution scenario cannot be responsible for the observed chemical composition of HE 0107-5240.

Nucleosynthesis of 60Fe in massive stars

We discuss at some extent the production of Fe60 in massive stars in the range between 11 and 120 Msun both in the hydrostatic and explosive stages. We also compare the Fe60/Al26 gamma-ray line flux ratio obtained according to the present calculations to the detected value reported by INTEGRAL/SPI.

Why Do Low-Mass Stars Become Red Giants?

We revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are discussed. We find that while a change in the mean molecular weight is necessary (but not sufficient) for a 1-M⊙ star to become a red giant, this is not the case in a star of 5 M⊙. It therefore seems that there may be more than one way to make a giant.

Presolar Graphite from the Murchison Meteorite: Imprint of Nucleosynthesis and Grain Formation

Presolar graphite is the carrier of Ne-E(L) and most 22Ne in Ne-E(L) had long been attributed to radiogenic decay of 22Na from novae. Of presolar graphite grains with a range of density (1.6-2.2g/cm3), low-density graphite grains extracted from the Murchison meteorite are characterized by 18O excesses and Si isotopic anomalies and are believed to have formed in supenova ejecta. From noble gas analyses of low-density graphite grains, we conclude that 22Ne in the grains is from the in situ decay of 22Na (T1/2=2.6a) produced in the C-burning zone in presupernova stars. The grains also contain Kr that was produced by neutron capture, either in the He-burning zone or the C-burning zone during hydrostatic burning. The 22Ne of a 22Na origin indicates that the grains formed shortly after the explosion. The presence of 22Ne of a 22Na origin and Kr, and the absence of 22Ne of a non-radiogenic origin might give us a further clue for graphite formation in supernova ejecta.

Presupernova evolution and explosion of massive stars

We review our recent progresses on the presupernova evolution of massive stars in the range 11-120 M⊙ of solar metallicity. Special attention will be devoted to the effect of the mass loss rate during the Wolf-Rayet stages in determining the structure and the physical properties of the star prior the supernova explosion. We also discuss the explosive yields and the initial mass-remnant mass relation in the framework of the kinetic bomb induced explosion and hence the contribution of these stars to the global chemical enrichment of the interstellar medium

On the evolution and explosion of massive stars

We review our recent progresses on the presupernova evolution of massive stars in the range 11–120u2009M⊙ of solar metallicity. Special attention will be devoted to the effect of the mass loss rate during the Wolf‐Rayet stages in determining the structure and the physical properties of the star prior the supernova explosion. We also discuss the explosive yields and the initial mass‐remnant mass relation in the framework of the kinetic bomb induced explosion and hence the contribution of these stars to the global chemical enrichment of the interstellar medium

The Evolutionary Properties of Zero Metal Stars in the Mass Range between 4 and 100 M

In this review we will discuss the main evolutionary properties of the zero metallicity stars in the mass interval which extends from the intermediate mass to the massive stars together with their final yields.