arXiv subjects

Arthur Choplin

Publications and source records attributed to Arthur Choplin.

24 records · Page 2Linked to original sources

arXiv2017

Effects of axions on Population III stars

Following the renewed interest for axions as a dark matter component, we revisit the effects of energy loss by axion emission on the evolution of the first generation of stars. These stars with zero metallicity are supposed to be massive, more compact, and hotter than subsequent generations. It is hence important to extend previous studies restricted to solar metallicity stars. Our analysis first compares the evolution of solar metallicity 8, 10 and 12 M$_\odot$ stars to previous work. We then calculate the evolution of 8 zero metallicity stars with and without axion losses and with masses ranging from 20 to 150 M$_\odot$. For the solar metallicity models, we confirm the disappearance of the blue loop phase for a value of the axion-photon coupling, $g_{aγ}=10^{-10} {\rm GeV}^{-1}$. Regarding the Pop. III stars, we show that for $g_{aγ}=10^{-10} {\rm GeV}^{-1}$, their evolution is not much affected by axion losses except within the range of masses 80$-$130 M$_\odot$. Such stars show significant differences both in their tracks within the $T_c$-$ρ_c$ diagram and in their central composition (in particular $^{20}$Ne and $^{24}$Mg). We discuss the origin of these modifications from the stellar physics point of view, and their potential observational signatures.

astro-ph.SR↗

arXiv2017

Pre-supernova mixing in CEMP-no source stars

Context. CEMP-no stars are long-lived low-mass stars with a very low iron content, overabundances of carbon and no or minor signs for the presence of s- or r-elements. Although their origin is still a matter of debate, they are often considered as being made of a material ejected by a previous stellar generation (source stars). Aims. We place constraints on the source stars from the observed abundance data of CEMP-no stars. Methods. We computed source star models of 20, 32, and 60 M$_{\odot}$ at Z = 10$^{-5}$ with and without fast rotation. For each model we also computed a case with a late mixing event occurring between the hydrogen and helium-burning shell $\sim$ 200 yr before the end of the evolution. This creates a partially CNO-processed zone in the source star. We use the 12C/13C and C/N ratios observed on CEMP-no stars to put constraints on the possible source stars (mass, late mixing or not). Then, we inspect more closely the abundance data of six CEMP-no stars and select their preferred source star(s). Results. Four out of the six CEMP-no stars studied cannot be explained without the late mixing process in the source star. Two of them show nucleosynthetic signatures of a progressive mixing (due e.g. to rotation) in the source star. We also show that a 20 M$_{\odot}$ source star is preferred compared to one of 60 M$_{\odot}$ and that likely only the outer layers of the source stars were expelled to reproduce the observed 12C/13C. Conclusions. The results suggest that (1) a late mixing process could operate in some source stars, (2) a progressive mixing, possibly achieved by fast rotation, is at work in several source stars, (3) $\sim$ 20 M$_{\odot}$ source stars are preferred compared to $\sim$ 60 M$_{\odot}$ ones, and (4) the source star might have preferentially experienced a low energetic supernova with large fallback.

astro-ph.SR↗

arXiv2016

Constraints on CEMP-no progenitors from nuclear astrophysics

The CEMP-no stars are long-lived small mass stars presenting a very low iron content and overabundances of carbon with no sign or only very weak signs for the presence of s- or r-elements. Although the origin of that abundance pattern is still a matter of debate, it was very likely inherited from a previous massive star, that we shall call here the source star. We rely on a recent classification of CEMP-no stars arguing that some of them are made of a material processed by hydrogen burning that was enriched in products of helium burning during the nuclear life of the source star. We examine the possibility of forming CEMP-no stars with this material. We study the nucleosynthesis of the CNO cycle and the Ne-Na Mg-Al chains in a hydrogen burning single zone while injecting the helium burning products $^{12}$C, $^{16}$O, $^{22}$Ne and $^{26}$Mg. We investigate the impact of changing the density and temperature, as well as the injection rate. The nuclear reaction rates involving the creation and destruction of $^{27}$Al are also examined. $^{14}$N, $^{23}$Na, $^{24}$Mg and $^{27}$Al are formed when injecting $^{12}$C, $^{16}$O, $^{22}$Ne and $^{26}$Mg in the hydrogen burning zone. The $^{12}$C/$^{13}$C ratio is constant under various conditions in the hydrogen burning zone. The predicted [Al/Fe] ratio varies up to $\sim 2$ dex depending on the prescription used for the reaction rates involving $^{27}$Al. The experiments we carried out support the view that some CEMP-no stars are made of a material processed by hydrogen burning, coming from a massive star experiencing mild-to-strong rotational mixing. During its burning, this material was likely enriched in helium burning products. No material coming from the carbon-oxygen rich core of the source star should be added to form the daughter star, otherwise the $^{12}$C/$^{13}$C ratio would be largely above the observed range of values.

astro-ph.SR↗

arXiv2016

Nucleosynthesis in the first massive stars

The nucleosynthesis in the first massive stars may be constrained by observing the surface composition of long-lived very iron-poor stars born around 10 billion years ago from material enriched by their ejecta. Many interesting clues on physical processes having occurred in the first stars can be obtained just based on nuclear aspects. Two facts are particularly clear, 1) in these first massive stars, mixing must have occurred between the H-burning and the He-burning zone during their nuclear lifetimes; 2) only the outer layers of these massive stars have enriched the material from which the very iron-poor stars, observed today in the halo, have formed. These two basic requirements can be obtained by rotating stellar models at very low metallicity. In the present paper, we discuss the arguments supporting this view and illustrates the sensitivity of the results concerning the [Mg/Al] ratio on the rate of the reaction Na(23)(p,γ)Mg(24).

astro-ph.SR↗

arXiv2015

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.

astro-ph.SR↗

arXiv2015

Clues about the first stars from CEMP-no stars

The material used to form the CEMP-no stars presents signatures of material processed by the CNO cycle and by He-burning from a previous stellar generation called the source stars. In order to reproduce the relative abundance ratios like for instance C/N or $^{12}$C/$^{13}$C, some mixing between the two burning regions must have occured in the source stars and only the outer layers of the stars, with modest amount coming from the CO core, must have been expelled either through stellar winds or at the time of the (faint) supernova event. With new models at low metallicity including rotational mixing, we shall discuss how the variety of abundances observed for CEMP-no stars can be reproduced.

astro-ph.SR↗