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Register a new users-Process in Low Metallicity Stars. III. Individual analysis of CEMP-s and CEMP-s/r with AGB models
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We provide an individual analysis of 94 carbon enhanced metal-poor stars showing an s-process enrichment (CEMP-s) collected from the literature. The s-process enhancement observed in these stars is ascribed to mass transfer by stellar winds in a binary system from a more massive companion evolving faster toward the asymptotic giant branch (AGB) phase. The theoretical AGB nucleosynthesis models have been presented in Paper I. Several CEMP-s stars show an enhancement in both s and r-process elements (CEMP-s/r). In order to explain the peculiar abundances observed in CEMP-s/r stars, we assume that the molecular cloud from which CEMP-s formed was previously enriched in r-elements by Supernovae pollution. A general discussion and the method adopted in order to interpret the observations have been provided in Paper II. We present in this paper a detailed study of spectroscopic observations of individual stars. We consider all elements from carbon to bismuth, with particular attention to the three s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios [hs/ls] and [Pb/hs]. The presence of an initial r-process contribution may be typically evaluated by the [La/Eu] ratio. We found possible agreements between theoretical predictions and spectroscopic data. In general, the observed [Na/Fe] (and [Mg/Fe]) provide information on the AGB initial mass, while [hs/ls] and [Pb/hs] are mainly indicators of the s-process efficiency. A range of 13C-pocket strengths is required to interpret the observations. However, major discrepancies between models and observations exist. We highlight star by star the agreements and the main problems encountered and, when possible, we suggest potential indications for further studies. These discrepancies provide starting points of debate for unsolved problems ...
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We provide an updated discussion of the sample of CEMP-s and CEMP-s/r stars collected from the literature. Observations are compared with the theoretical nucleosynthesis models of asymptotic giant branch (AGB) stars presented by Bisterzo et al. (2010, 2011, 2012), in the light of the most recent spectroscopic results.
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 $C_{s}$.
Origin of enhanced abundance of heavy elements observed in the surface chemical composition of carbon-enhanced metal-poor (CEMP) stars still remain poorly understood. Here, we present detailed abundance analysis of seven CEMP stars based on high resolution (R${sim}$ 50,000) spectra that reveal enough evidence of Asymptotic Giant Branch (AGB) stars being possible progenitors for these objects. For the objects HE0110$-$0406, HE1425$-$2052 and HE1428$-$1950, we present for the first time a detailed abundance analysis. Our sample is found to consists of one metal-poor ([Fe/H]$<$$-1.0$) and six very metal-poor ([Fe/H]$<$$-2.0$) stars with enhanced carbon and neutron-capture elements. We have critically analysed the observed abundance ratios of [O/Fe], [Sr/Ba] and [hs/ls] and examined the possibility of AGB stars being possible progenitors. The abundance of oxygen estimated in the programme stars are characteristics of AGB progenitors except for HE1429$-$0551 and HE1447$+$0102. The estimated values of [Sr/Ba] and [hs/ls] ratios also support AGB stars as possible progenitors. The locations of the programme stars in the absolute carbon abundance A(C) vs. [Fe/H] diagram along with the Group I objects hint at binary nature of the object. We have studied the chemical enrichment histories of the programme stars based on abundance ratios [Mg/C], [Sc/Mn] and [C/Cr]. Using [C/N] and $^{12}$C/$^{13}$C ratios we have examined if any internal mixing had modified their surface chemical compositions. Kinematic analysis shows that the objects HE 0110$-$0406 and HE1447$+$0102 are thick disk objects and the remaining five objects belong to the halo population of the Galaxy.
CEMP-$r/s$ stars are metal-poor stars with enhanced abundances of carbon and heavy elements associated with the slow ($s$-) and rapid ($r$-) neutron-capture process. It is believed that carbon and $s$-elements were accreted from the wind of an AGB primary star, a scenario that is generally accepted to explain the formation of CEMP stars that are only enhanced in $s$-elements (CEMP-$s$ stars). The origin of $r$-element-enrichment in CEMP-$r/s$ stars is debated and many formation scenarios have been put forward. We aim to determine the likelihood of the scenarios proposed to explain the formation of CEMP-$r/s$ stars. We calculate the frequency of CEMP-$r/s$ stars among CEMP-$s$ stars for a variety of scenarios, and we compare it with that determined from an observed sample of CEMP-$r/s$ stars collected from the literature. The theoretical frequency of CEMP-$r/s$ stars predicted in most scenarios underestimates the observed ratio by at least a factor of 5. If the enrichments in $s$- and $r$-elements are independent, the model ratio of CEMP-$r/s$ to CEMP-$s$ stars is about 22%, that is approximately consistent with the lowest estimate of the observed ratio. However, this model predicts that about one third of all carbon-normal stars have [Ba/Fe] and [Eu/Fe] higher than 1, and that 40% of all CEMP stars have [Ba/Eu]$le0$. Stars with these properties are at least ten times rarer in our observed sample. The $intermediate$ or $i$-process, which is supposedly active in some circumstances during the AGB phase, could provide an explanation of the origin of CEMP-$r/s$ stars, similar to that of CEMP-$s$ stars, in the context of wind mass accretion in binary systems. Further calculations of the nucleosynthesis of the $i$-process and of the detailed evolution of late AGB stars are needed to investigate if this scenario predicts a CEMP-$r/s$ star frequency consistent with the observations.
Among Carbon-Enhanced Metal-Poor (CEMP) stars, some are found to be enriched in s-process elements (CEMP-s), in r-process elements (CEMP-r) or in both s- and r-process elements (CEMP-rs). The origin of the abundance differences between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that the i-process, whose site still remains to be identified, could better reproduce CEMP-rs abundances than the s-process. We analyze high-resolution spectra of 25 metal-poor stars, observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope, La Palma, or with the UVES/VLT and HIRES/KECK spectrographs. We propose a new, robust classification method for CEMP-s and CEMP-rs stars using eight heavy element abundances. The abundance profiles of CEMP-s and CEMP-rs stars are derived and there appears to be an abundance continuum between the two stellar classes. CEMP-rs stars present most of the characteristics of extrinsic stars such as CEMP-s, CH, Barium and extrinsic S stars, with an even larger binarity rate among CEMP-rs stars than among CEMP-s stars. Stellar evolutionary tracks of an enhanced carbon composition (consistent with our abundance determinations) are necessary to explain the position of CEMP-s and CEMP-rs stars in the HR diagram using Gaia DR2 parallaxes; they are found to lie mostly on the RGB. CEMP-rs stars can be explained as being polluted by a low-mass, low-metallicity TP-AGB companion experiencing i-process nucleosynthesis after proton ingestion during its first convective thermal pulses. The global fitting of our i-process models to CEMP-rs stars is as good as the one of our s-process models to CEMP-s stars. As such, CEMP-rs stars could be renamed as CEMP-sr stars, since they represent a particular manifestation of the s-process at low-metallicities. For these objects a call for an exotic i-process site may not necessarily be required anymore.
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