The Early Generations of Rotating Massive Stars and the Origin of Carbon-Enhanced Metal-Poor Stars

The study of the long-dead early generations of massive stars is crucial in order to obtain a complete picture of the chemical evolution of the Universe, hence the origin of the elements. The nature of these stars can be inferred indirectly by investigating the origin of low-mass metal-poor stars observed in our Galaxy, some of which are almost as old as the Universe. The peculiar extremely iron-poor Carbon-Enhanced Metal-Poor (CEMP) stars, whose precise origin is still debated, are thought to have formed with the material ejected by only one or very few previous massive stars. The main aim of this thesis is to explore the physics and the nucleosynthesis of the early generations of massive stars. It is achieved by combining stellar evolution modeling including rotation and full nucleosynthesis with observations of CEMP stars.Comment: PhD thesis. Also available at: https://archive-ouverte.unige.ch/unige:11171

Massive stars: stellar models and stellar yields, impact on Galactic Archaeology

The physics of massive stars depends (at least) on convection, mass loss by stellar winds, rotation, magnetic fields and multiplicity. We briefly discuss the impact of the first three processes on the stellar yields trying to identify some guidelines for future works.Comment: 8 pages, 6 figures, in press for the proceedings of IAU Symposium 334, Rediscovering our Galaxy, C. Chiappini, I. Minchev, E. Starkenburg, M. Valentini ed

Nucleosynthesis in early rotating massive stars and chemical composition of CEMP stars

The first massive stars triggered the onset of chemical evolution by releasing the first metals (elements heavier than helium) in the Universe. The nature of these stars and how the early chemical enrichment took place is still largely unknown. Rotational-induced mixing in the stellar interior can impact the nucleosynthesis during the stellar life of massive stars and lead to stellar ejecta having various chemical compositions. We present low and zero-metallicity 20, 25 and 40 $M_{\odot}$ stellar models with various initial rotation rates and assumptions for the nuclear reactions rates. With increasing initial rotation, the yields of light (from $\sim$ C to Al) and trans-iron elements are boosted. The trans-iron elements (especially elements heavier than Ba) are significantly affected by the nuclear reaction uncertainties. The chemical composition of the observed CEMP (carbon-enhanced metal-poor) stars CS29528-028 and HE0336+0113 are consistent with the chemical composition of the material ejected by a fast rotating 40~$M_{\odot}$ model.Comment: Proceeding of the Nuclear Physics in Astrophysics IX conference, to appear in the Journal of Physics: Conference Serie

Impact of rotation on stellar models

After a brief recall of the main impacts of stellar rotation on the structure and the evolution of stars, four topics are addressed: 1) the links between magnetic fields and rotation; 2) the impact of rotation on the age determination of clusters; 3) the exchanges of angular momentum between the orbit of a planet and the star due to tides; 4) the impact of rotation on the early chemical evolution of the Milky Way and the origin of the Carbon-Enhanced-Metal-Poor stars.Comment: 5 pages, 2 figures, To appear in Astronomische Nachrichten, special issue "Reconstruction the Milky Way's History: Spectroscopic surveys, Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J. Montalban, and M. Steffen, AN 2016 (in press

Massive stars evolution: feedbacks in low-Z environment

Massive stars are the drivers of the chemical evolution of dwarf galaxies. We review here the basics of massive star evolution and the specificities of stellar evolution in low-Z environment. We discuss nucleosynthetic aspects and what observations could constrain our view on the first generations of stars.Comment: 8 pages, 4 figures. Proceedings of IAUS 344 "Dwarf Galaxies: From the Deep Universe to the Present" held in Vienna (Austria) August 20-24 201

Non-standard s-process in massive rotating stars - Yields of 10–150 M⊙ models at Z = 10E−3

Recent studies show that rotation significantly affects the s-process in massive stars. We provide tables of yields for non-rotating and rotating massive stars between 10 and 150 $M_{\odot}$ at $Z=10^{-3}$ ([Fe/H] $=-1.8$). Tables for different mass cuts are provided. The complete s-process is followed during the whole evolution with a network of 737 isotopes, from Hydrogen to Polonium. A grid of stellar models with initial masses of 10, 15, 20, 25, 40, 60, 85, 120 and 150 $M_{\odot}$ and with an initial rotation rate of both 0 or 40$~\%$ of the critical velocity was computed. Three extra models were computed in order to investigate the effect of faster rotation (70$~\%$ of the critical velocity) and of a lower $^{17}$O($\alpha,\gamma$) reaction rate. At the considered metallicity, rotation has a strong impact on the production of s-elements for initial masses between 20 and 60 $M_{\odot}$. In this range, the first s-process peak is boosted by $2-3$ dex if rotation is included. Above 60 $M_{\odot}$, s-element yields of rotating and non-rotating models are similar. Increasing the initial rotation from 40$~\%$ to 70$~\%$ of the critical velocity enhances the production of $40 \lesssim Z \lesssim 60$ elements by $\sim 0.5-1$ dex. Adopting a reasonably lower $^{17}$O($\alpha,\gamma$) rate in the fast rotating model (70$~\%$ of the critical velocity) boosts again the yields of s-elements with $55 \lesssim Z \lesssim 82$ by about 1 dex. In particular, a modest amount of Pb is produced. Together with s-elements, some light elements (particularly fluorine) are strongly overproduced in rotating models.Comment: 16 pages, 14 figures, 4 tables. Accepted for publication in A&

The R-Process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5-455510

We report on the spectroscopic analysis of RAVE J183013.5-455510, an extremely metal-poor star, highly enhanced in CNO, and with discernible contributions from the rapid neutron-capture process. There is no evidence of binarity for this object. At [Fe/H]=-3.57, this is one of the lowest metallicity stars currently observed, with 18 measured abundances of neutron-capture elements. The presence of Ba, La, and Ce abundances above the Solar System r-process predictions suggest that there must have been a non-standard source of r-process elements operating at such low metallicities. One plausible explanation is that this enhancement originates from material ejected at unusually fast velocities in a neutron star merger event. We also explore the possibility that the neutron-capture elements were produced during the evolution and explosion of a rotating massive star. In addition, based on comparisons with yields from zero-metallicity faint supernova, we speculate that RAVE J1830-4555 was formed from a gas cloud pre-enriched by both progenitor types. From analysis based on Gaia DR2 measurements, we show that this star has orbital properties similar to the Galactic metal-weak thick-disk stellar population.Comment: Accepted for publication in Ap

Early Rotating Massive Stars and Abundances of Extremely Metal-Poor Stars

info:eu-repo/semantics/publishe