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Register a new userNucleosynthesis in early rotating massive stars and chemical composition of CEMP stars
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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.
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Stellar parameters of 25 planet-hosting stars and abundances of Li, C, O, Na, Mg, Al, S, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Zn, Y, Zr, Ba, Ce, Pr, Nd, Sm and Eu, were studied based on homogeneous high resolution spectra and uniform techniques. The iron abundance [Fe/H] and key elements (Li, C, O, Mg, Si) indicative of the planet formation, as well as the dependencies of [El/Fe] on $T_{cond}$, were analyzed. The iron abundances determined in our sample stars with detected massive planets range within -0.3<[Fe/H]<0.4. The behaviour of [C/Fe], [O/Fe], [Mg/Fe] and [Si/Fe] relative to [Fe/H] is consistent with the Galactic Chemical Evolution trends. The mean values of C/O and [C/O] are <C/O>= 0.48 +/-0.07 and <[C/O]>=-0.07 +/-0.07, which are slightly lower than solar ones. The Mg/Si ratios range from 0.83 to 0.95 for four stars in our sample and from 1.0 to 1.86 for the remaining 21 stars. Various slopes of [El/Fe] vs. Tcond were found. The dependencies of the planetary mass on metallicity, the lithium abundance, the C/O and Mg/Si ratios, and also on the [El/Fe]-Tcond slopes were considered.
In order to get a broader view of the s-process nucleosynthesis we study the abundance distribution of heavy elements of 35 barium stars and 24 CEMP-stars, including nine CEMP-s stars and 15 CEMP-r/s stars. The similar distribution of [Pb/hs] between CEMP-s and CEMP-r/s stars indicate that the s-process material of both CEMP-s and CEMP-r/s stars should have a uniform origin, i.e. mass transfer from their predominant AGB companions. For the CEMP-r/s stars, we found that the r-process should provide similar proportional contributes to the second s-peak and the third s-peak elements, and also be responsible for the higher overabundance of heavy elements than those in CEMP-s stars. Which hints that the r-process origin of CEMP-r/s stars should be closely linked to the main r-process. The fact that some small $r$ values exist for both barium and CEMP-s stars, implies that the single exposure event of the s-process nucleosynthesis should be general in a wide metallicity range of our Galaxy. Based on the relation between $C_{r}$ and $C_{s}$, we suggest that the origin of r-elements for CEMP-r/s stars have more sources. A common scenario is that the formation of the binary system was triggered by only one or a few supernova. In addition, accretion-induced collapse(AIC) or SN 1.5 should be the supplementary scenario, especially for these whose pre-AGB companion with higher mass and smaller orbit radius, which support the higher values of both $C_{r}$ and $C_{s}$.
We present a dense model grid with tailored input chemical composition appropriate for the Large Magellanic Cloud. We use a one-dimensional hydrodynamic stellar evolution code, which accounts for rotation, transport of angular momentum by magnetic fields, and stellar wind mass loss to compute our detailed models. We calculate stellar evolution models with initial masses of 70-500 Msun and with initial surface rotational velocities of 0-550 km/s, covering the core-hydrogen burning phase of evolution. We find our rapid rotators to be strongly influenced by rotationally induced mixing of helium, with quasi-chemically homogeneous evolution occurring for the fastest rotating models. Above 160 Msun, homogeneous evolution is also established through mass loss, producing pure helium stars at core hydrogen exhaustion independent of the initial rotation rate. Surface nitrogen enrichment is also found for slower rotators, even for stars that lose only a small fraction of their initial mass. For models above 150 MZAMS, and for models in the whole considered mass range later on, we find a considerable envelope inflation due to the proximity of these models to their Eddington limit. This leads to a maximum zero-age main sequence surface temperature of 56000 K, at 180 Msun, and to an evolution of stars in the mass range 50-100 Msun to the regime of luminous blue variables in the HR diagram with high internal Eddington factors. Inflation also leads to decreasing surface temperatures during the chemically homogeneous evolution of stars above 180 Msun. The cool surface temperatures due to the envelope inflation in our models lead to an enhanced mass loss, which prevents stars at LMC metallicity from evolving into pair-instability supernovae. The corresponding spin-down will also prevent very massive LMC stars to produce long-duration gamma-ray bursts, which might, however, originate from lower masses.
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.
The aim of this work is to shed some light on the problem of the formation of carbon stars of R-type from a detailed study of their chemical composition. We use high-resolution and high signal-to-noise optical spectra of 23 R-type stars selected from the Hipparcos catalogue. The chemical analysis is made using spectral synthesis in LTE and state-of-the-art carbon-rich spherical model atmospheres. We derive their CNO content (including the carbon isotopic ratio), average metallicity, lithium, and light (Sr, Y, Zr) and heavy (Ba, La, Nd, Sm) s-element abundances. The observed properties of the stars (galactic distribution, kinematics, binarity, photometry and luminosity) are also discussed. Our analysis shows that late-R stars are carbon stars with identical chemical and observational characteristics than the normal (N-type) AGB carbon stars. We confirm the results of the sole previous abundance analysis of early-R stars by Dominy (1984, ApJS, 55, 27), namely: they are carbon stars with near solar metallicity showing enhanced nitrogen, low carbon isotopic ratios and no s-element enhancements. In addition, we have found that early-R stars have Li abundances larger than expected for post RGB tip giants. We also find that a significant number (aprox. 40 %) of the early-R stars in our sample are wrongly classified, being probably classical CH stars and normal K giants. In consequence, we suggest that the number of true R stars is considerably lower than previously believed. We briefly discuss the different scenarios proposed for the formation of early-R stars. The mixing of carbon during an anomalous He-flash is favoured, although no physical mechanism able to trigger that mixing has been found yet. The origin of these stars still remains a mystery.
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