ABSTRACT Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display enrichments in heavy elements associated with either the s (slow) or the r (rapid) neutron-capture process (e.g., barium ...and europium, respectively), and in some cases they display evidence of both. The abundance patterns of these CEMP-s/r stars, which show both Ba and Eu enrichment, are particularly puzzling, since the s and the r processes require neutron densities that are more than ten orders of magnitude apart and, hence, are thought to occur in very different stellar sites with very different physical conditions. We investigate whether the abundance patterns of CEMP-s/r stars can arise from the nucleosynthesis of the intermediate neutron-capture process (the i process), which is characterized by neutron densities between those of the s and the r processes. Using nuclear network calculations, we study neutron capture nucleosynthesis at different constant neutron densities n ranging from 107-1015 cm−3. With respect to the classical s process resulting from neutron densities on the lowest side of this range, neutron densities on the highest side result in abundance patterns, which show an increased production of heavy s-process and r-process elements, but similar abundances of the light s-process elements. Such high values of n may occur in the thermal pulses of asymptotic giant branch stars due to proton ingestion episodes. Comparison to the surface abundances of 20 CEMP-s/r stars shows that our modeled i-process abundances successfully reproduce observed abundance patterns, which could not be previously explained by s-process nucleosynthesis. Because the i-process models fit the abundances of CEMP-s/r stars so well, we propose that this class should be renamed as CEMP-i.
ABSTRACT
We examine the composition of barium stars in the context of mass transfer from an asymptotic giant branch (AGB) companion. We accrete between 0.01 and 0.5 M⊙ of AGB ejecta on to low-mass ...companions of Fe/H = −0.25 at the ages expected for the end of the lives of AGB stars of 2.5, 3, and 4 M⊙. In each case, we form a star of 2.5 M⊙ that is thought to be a typical barium star mass. We discuss the extent of dilution of accreted material as the star evolves, and describe the impact on the surface abundances. For accretion from a 2.5 M⊙ primary, if the secondary’s initial mass is 2.45 M⊙ or more, accretion takes place when the secondary is undergoing core helium burning. Using data from the sample of De Castro et al., we attempt to fit the observed properties of 74 barium giants using the models we have computed. We find that all but six of these objects are best fit using ejecta from 2.5 M⊙ (32 objects) or 3 M⊙ (36 objects) AGB stars. Higher accretion masses are typically required when accreting from a lower mass companion. We find accretion masses that are broadly consistent with recent hydrodynamical simulations of wind mass transfer, though the accretion efficiency is towards the upper limit found in these simulations. For the 18 stars with reported orbital periods, we find no strong correlations between period and accretion mass.
We revisit the observed frequencies of carbon-enhanced metal-poor (CEMP) stars as a function of the metallicity in the Galaxy, using data from the literature with available high-resolution ...spectroscopy. Our analysis excludes stars exhibiting clear overabundances of neutron-capture elements and takes into account the expected depletion of surface carbon abundance that occurs due to CN processing on the upper red giant branch. This allows for the recovery of the initial carbon abundance of these stars, and thus for an accurate assessment of the frequencies of carbon-enhanced stars. The correction procedure we develop is based on stellar-evolution models and depends on the surface gravity, log g, of a given star. Our analysis indicates that for stars with Fe/H < or =, slant -2.0, 20% exhibit C/Fe > or =, slanted +0.7. This fraction increases to 43% for Fe/H < or =, slant -3.0 and 81% for Fe/H < or =, slant -4.0, which is higher than have been previously inferred without taking the carbon abundance correction into account. These CEMP star frequencies provide important inputs for Galactic and stellar chemical evolution models, as they constrain the evolution of carbon at early times and the possible formation channels for the CEMP-no stars. We also have developed a public online tool with which carbon corrections using our procedure can be easily obtained.
Abstract
Lead (Pb) is predominantly produced by the slow neutron-capture process (s process) in asymptotic giant branch (AGB) stars. In contrast to significantly enhanced Pb abundances predicted by ...low-mass, low-metallicity AGB models, observations of Magellanic post-AGB stars show incompatibly low Pb abundances. Observations of carbon-enhanced metal-poor (CEMP) stars whose s-process enrichments are accompanied by heavy elements traditionally associated with the rapid neutron-capture process (r process) have raised the need for a neutron-capture process operating at neutron densities intermediate to the s and r process: the so-called i process. We study i-process nucleosynthesis with single-zone nuclear-network calculations. Our i-process models can explain the heavy-element abundance patterns measured in Magellanic post-AGB stars including their puzzlingly low Pb abundances. Furthermore, the heavy-element enhancements in the post-AGB and CEMP-i stars, particularly their Pb abundance, allow us to characterize the neutron densities and exposures of the i process that produced the observed abundance patterns. We find that the lower-metallicity CEMP-i stars (
) have heavy-element abundances best matched by models with higher neutron densities and exposures (
τ
> 2.0 mbarn
−1
) compared to the higher-metallicity post-AGB stars (
,
τ
< 1.3 mbarn
−1
). This offers new constraints and insights regarding the properties of i-process sites and demonstrates that the responsible process operates on timescales of the order of a few years or less.
We present models for the slow neutron-capture process (s-process) in asymptotic giant branch stars of metallicity Fe/H = -2.3 and masses 0.9-6 M sub(middot in circle). We encountered different ...regimes of neutron-capture nucleosynthesis listed here increasing in importance as the stellar mass decreases: the super(22)Ne( alpha , n) super(25)Mg reaction activated during the thermal pulses (TPs), the super(13)C( alpha , n) super(16)O reaction activated in radiative conditions during the interpulse periods, and the super(13)C( alpha , n) super(16)O reaction activated during the TPs, also as a result of mild proton-ingestion episodes. The models where the super(13)C burns radiatively (masses Asymptotically = to2 M sub(middot in circle)) produce an overall good match to carbon-enhanced metal-poor (CEMP) stars showing s-process enhancements (CEMP-s), except they produce too much Na and F. On the other hand, none of our models can provide a match to the composition of CEMP stars also showing rapid-process enhancements (CEMP-s/r). The models fail to reproduce the observed Eu abundances, and they also fail to reproduce the correlation between the Eu and Ba abundances. They also cannot match the ratio of heavy-to-light s-process elements observed in many CEMP-s/r stars, which can be more than 10 times higher than in the solar system. To explain the composition of CEMP-s/r stars we need to invoke the existence of a different "s/r" neutron-capture process either with features in between the s- and the r-processes, or generated by superpositions of different neutron-capture processes in the same astrophysical site or in sites linked to each other-for example, in multiple stellar systems.
We examine the effects of thermohaline mixing on the composition of the envelopes of low-metallicity asymptotic giant branch (AGB) stars. We have evolved models of 1, 1.5 and 2 M⊙ from the pre-main ...sequence to the end of the thermally pulsing AGB with thermohaline mixing applied throughout the simulations. In agreement with other authors, we find that thermohaline mixing substantially reduces the abundance of 3He on the upper part of the red giant branch in our lowest mass model. However, the small amount of 3He that remains is enough to drive thermohaline mixing on the AGB. We find that thermohaline mixing is most efficient in the early thermal pulses and its efficiency drops from pulse to pulse. Nitrogen is not substantially affected by the process, but we do see substantial changes in 13C. The 12C/13C ratio is substantially lowered during the early thermal pulses, but the efficacy of the process is seen to diminish rapidly. As the process stops after a few pulses, the 12C/13C ratio is still able to reach values of 103– 104, which is inconsistent with the values measured in carbon-enhanced metal-poor stars. We also note a surprising increase in the 7Li abundance, with reaching values of over 2.5 in the 1.5 M⊙ model. It is thus possible to get stars which are both C and Li rich at the same time. We compare our models to measurements of carbon and lithium in carbon-enhanced metal-poor stars which have not yet reached the giant branch. These models can simultaneously reproduce the observed C and Li abundances of carbon-enhanced metal-poor turn-off stars that are Li rich, but the observed nitrogen abundances still cannot be matched.
The nature of the progenitors of type Ia supernovae (SNe Ia) remains a mystery. Binary systems consisting of a white dwarf (WD) and a main-sequence (MS) donor are potential progenitors of SNe Ia, in ...which a thermonuclear explosion of the WD may occur when its mass reaches the Chandrasekhar limit during accretion of material from a companion star. In the present work, we address theoretical rates and delay times of a specific MS donor channel to SNe Ia, in which a helium (He) star + MS binary produced from a common envelope event subsequently forms a WD + MS system without the He star undergoing mass transfer by Roche lobe overflow. By combining the results of self-consistent binary evolution calculations with population synthesis models, we find that the contribution of SNe Ia in this channel is around 2.0 × 10
−4
yr
−1
. In addition, we find that delay times of SNe Ia in this channel cover a range of about 1.0–2.6 Gyr, and almost all SNe Ia produced in this way (about 97%) have a delay time of ≳1 Gyr. While the rate of SN Ia in this work is about 10% of the overall SN Ia rate, the channel represents a possible contribution to the old population (1–3 Gyr) of observed SNe Ia.
Metal-poor stars in the Galactic halo often show strong enhancements in carbon and/or neutron-capture elements. However, the Galactic bulge is notable for its paucity of these carbon-enhanced ...metal-poor (CEMP) and/or CH-stars, with only two such objects known to date. This begs the question whether the processes that produced their abundance distribution were governed by a comparable nucleosynthesis in similar stellar sites as for their more numerous counterparts in the halo. Recently, two contenders of these classes of stars were discovered in the bulge, at Fe/H = −1.5 and −2.5 dex, both of which show enhancements in C/Fe of 0.4 and 1.4 dex (respectively), Ba/Fe in excess of 1.3 dex, and also elevated nitrogen. The more metal-poor of the stars can be well matched by standard s-process nucleosynthesis in low-mass asymptotic giant branch (AGB) polluters. The other star shows an abnormally high Rb/Fe ratio. Here, we further investigate the origin of the abundance peculiarities in the Rb-rich star by new, detailed measurements of heavy element abundances and by comparing the chemical element ratios of 36 species to several models of neutron-capture nucleosynthesis. The i-process with intermediate neutron densities between those of the slow (s-) and rapid (r)-neutron-capture processes has been previously found to provide good matches of CEMP stars with enhancements in both r- and s-process elements (class CEMP-r/s), rather than invoking a superposition of yields from the respective individual processes. However, the peculiar bulge star is incompatible with a pure i-process from a single ingestion event. Instead, it can, statistically, be better reproduced by more convoluted models accounting for two proton ingestion events, or by an i-process component in combination with s-process nucleosynthesis in low-to-intermediate mass (2–3 M⊙) AGB stars, indicating multiple polluters. Finally, we discuss the impact of mixing during stellar evolution on the observed abundance peculiarities.
A large fraction of stars in binary systems are expected to undergo mass and angular momentum exchange at some point in their evolution, which can drastically alter the chemical and dynamical ...properties and fates of the systems. Interaction by stellar wind is an important process in wide binaries. However, the details of wind mass transfer are still not well understood. We perform three-dimensional hydrodynamical simulations of wind mass transfer in binary systems to explore mass-accretion efficiencies and geometries of mass outflows, for a range of mass ratios from 0.05 to 1.0. In particular, we focus on the case of a free wind, in which some physical mechanism accelerates the expelled wind material balancing the gravity of the mass-losing star with the wind velocity comparable to the orbital velocity of the system. We find that the mass-accretion efficiency and accreted specific angular momentum increase with the mass ratio of the system. For an adiabatic wind, we obtain that the accretion efficiency onto the secondary star varies from about 0.1% to 8% for mass ratios between 0.05 and 1.0.
CEMP-r/s stars are metal-poor stars with enhanced abundances of carbon and heavy elements associated with the slow and rapid neutron-capture process (s- and r-elements, respectively). It is believed ...that carbon and s-elements were accreted in the past from the wind of a primary star in the asymptotic giant branch (AGB) phase of evolution, 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 currently 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 formation 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 formation scenarios underestimates the observed ratio by at least a factor of five. 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 one, and that 40% of all CEMP stars have Ba/Eu ≤ 0. 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.
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