Abstract
To reach a deeper understanding of the origin of elements in the periodic table, we construct Galactic chemical evolution (GCE) models for all stable elements from C (
A
= 12) to U (
A
= ...238) from first principles, i.e., using theoretical nucleosynthesis yields and event rates of all chemical enrichment sources. This enables us to predict the origin of elements as a function of time and environment. In the solar neighborhood, we find that stars with initial masses of
M
> 30
M
⊙
can become failed supernovae if there is a significant contribution from hypernovae (HNe) at
M
∼ 20–50
M
⊙
. The contribution to GCE from super-asymptotic giant branch (AGB) stars (with
M
∼ 8–10
M
⊙
at solar metallicity) is negligible, unless hybrid white dwarfs from low-mass super-AGB stars explode as so-called Type Iax supernovae, or high-mass super-AGB stars explode as electron-capture supernovae (ECSNe). Among neutron-capture elements, the observed abundances of the second (Ba) and third (Pb) peak elements are well reproduced with our updated yields of the slow neutron-capture process (s-process) from AGB stars. The first peak elements (Sr, Y, Zr) are sufficiently produced by ECSNe together with AGB stars. Neutron star mergers can produce rapid neutron-capture process (r-process) elements up to Th and U, but the timescales are too long to explain observations at low metallicities. The observed evolutionary trends, such as for Eu, can well be explained if ∼3% of 25–50
M
⊙
HNe are magneto-rotational supernovae producing r-process elements. Along with the solar neighborhood, we also predict the evolutionary trends in the halo, bulge, and thick disk for future comparison with Galactic archeology surveys.
ABSTRACT We present new theoretical stellar yields and surface abundances for three grids of metal-rich asymptotic giant branch (AGB) models. Post-processing nucleosynthesis results are presented for ...stellar models with initial masses between 1 M☉ and 7.5 M☉ for Z = 0.007, and 1 M☉ and 8 M☉ for Z = 0.014 (solar) and Z = 0.03. We include stellar surface abundances as a function of thermal pulse on the AGB for elements from C to Bi and for a selection of isotopic ratios for elements up to Fe and Ni (e.g., / ), which can be obtained from observations of molecules in stars and from the laboratory analysis of meteoritic stardust grains. Ratios of elemental abundances of He/H, C/O, and N/O are also included, which are useful for direct comparison to observations of AGB stars and their progeny, including planetary nebulae. The integrated elemental stellar yields are presented for each model in the grid for hydrogen, helium, and all stable elements from C to Bi. Yields of Li are also included for intermediate-mass models with hot bottom burning. We present the first slow neutron-capture (s-process) yields for super solar metallicity AGB stars with Z = 0.03, and the first complete s-process yields for models more massive than 6 M☉ at all three metallicities.
We present new theoretical stellar evolutionary models of metal-rich asymptotic giant branch (AGB) stars. Stellar models are evolved with initial masses between 1 and 7 M⊙ at Z = 0.007, and 1 and 8 ...M⊙ at Z = 0.014 (solar) and at Z = 0.03. We evolve models with a canonical helium abundance and with helium-enriched compositions (Y = 0.30, 0.35, and 0.40) at Z = 0.014 and 0.03. The efficiency of third dredge-up and the mass range of carbon stars decreases with an increase in metallicity. We predict carbon stars form from initial masses between 1.75 and 7 M⊙ at Z = 0.007 and between 2 and 4.5 M⊙ at solar metallicity. At Z = 0.03, the mass range for C-star production is narrowed to 3.25–4 M⊙. The third dredge-up is reduced when the helium content of the model increases owing to the reduced number of thermal pulses on the AGB. A small increase of ΔY = 0.05 is enough to prevent the formation of C stars at Z = 0.03, depending on the mass-loss rate, whereas at Z = 0.014, an increase of ΔY ≳ 0.1 is required to prevent the formation of C stars. We speculate that the probability of finding C stars in a stellar population depends as much on the helium abundance as on the metallicity. To explain the paucity of C stars in the inner region of M31, we conclude that the observed stars have Y ≳ 0.35 or that the stellar metallicity is higher than Fe/H ≈ 0.1.
Isotope ratios have opened a new window into the study of the details of stellar evolution, supernovae and galactic chemical evolution. We present the evolution of the isotope ratios of elemental ...abundances (from C to Zn) in the solar neighbourhood, bulge, halo and thick disc, using chemical evolution models with updated yields of asymptotic giant branch (AGB) stars and core-collapse supernovae. The evolutionary history of each element is different owing to the effects of the initial progenitor mass and metallicity on element production. In the bulge and thick disc the star formation time-scale is shorter than in the solar neighbourhood, leading to higher α/Fe ratios. Likewise, the smaller contribution from Type Ia supernovae in these regions leads to lower Mn/Fe ratios. Also in the bulge, the abundances of (Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe are higher because of the effect of metallicity on element production from core-collapse supernovae. According to our predictions, it is possible to find metal-rich stars (Fe/H ≳−1) that formed in the early Universe as a result of rapid star formation. The chemical enrichment time-scale of the halo is longer than in the solar neighbourhood, and consequently the ratios of (C, F)/Fe and 12C/13C are higher owing to a significant contribution from low-mass AGB stars. While the α/Fe and Mn/Fe ratios are the same as in the solar neighbourhood, the (Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe ratios are predicted to be lower. Furthermore, we predict that isotope ratios such as 24Mg/25, 26Mg are larger because of the contribution from low-metallicity supernovae. Using isotopic ratios, it is possible to select stars that formed in a system with a low chemical enrichment efficiency such as the satellite galaxies that were accreted on to our own Milky Way Galaxy.
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.
The most metal-rich asymptotic giant branch stars Karakas, Amanda I; Cinquegrana, Giulia; Joyce, Meridith
Monthly notices of the Royal Astronomical Society,
01/2022, Letnik:
509, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We present new stellar evolutionary sequences of very metal-rich stars evolved with the Monash Stellar Structure code and with mesa. The Monash models include masses of 1–8 M⊙ with ...metallicities Z = 0.04 to Z = 0.1 and are evolved from the main sequence to the thermally pulsing asymptotic giant branch (TP-AGB). These are the first Z = 0.1 AGB models in the literature. The mesa models include intermediate-mass models with Z = 0.06 to Z = 0.09 evolved to the onset of the TP phase. Third dredge-up only occurs in intermediate-mass models Z ≤ 0.08. Hot bottom burning shows a weaker dependence on metallicity, with the minimum mass increasing from 4.5 M⊙ for Z = 0.014 to ≈5.5 M⊙ for Z = 0.04, 6 M⊙ for 0.05 ≤ Z ≤ 0.07 and above 6.5 M⊙ for Z ≥ 0.08. The behaviour of the Z = 0.1 models is unusual; most do not experience He-shell instabilities owing to rapid mass-loss on the early part of the AGB. Turning off mass-loss produces He-shell instabilities, however thermal pulses are weak and result in no TDU. The minimum mass for carbon ignition is reduced from 8 M⊙ for Z = 0.04 to 7 M⊙ for Z = 0.1, which implies a reduction in the minimum mass for core-collapse supernovae. mesa models of similarly high metallicity (Z = 0.06–0.09) show the same lowering of the minimum mass for carbon ignition: carbon burning is detected in a 6 M⊙ model at the highest metallicity (Z = 0.09) and in all 7 M⊙ models with Z ≥ 0.06. This demonstrates robustness of the lowered carbon burning threshold across codes.
Theoretical models of stellar evolution predict that most of the lithium inside a star is destroyed as the star becomes a red giant. However, observations reveal that about 1% of red giants are ...peculiarly rich in lithium, often exceeding the amount in the interstellar medium or predicted from the big bang. With only about 150 lithium-rich giants discovered in the past four decades, and no distinguishing properties other than lithium enhancement, the origin of lithium-rich giant stars is one of the oldest problems in stellar astrophysics. Here we report the discovery of 2330 low-mass (1-3 M ) lithium-rich giant stars, which we argue are consistent with internal lithium production that is driven by tidal spin-up by a binary companion. Our sample reveals that most lithium-rich giants have helium-burning cores ( ), and that the frequency of lithium-rich giants rises with increasing stellar metallicity. We find that while planet accretion may explain some lithium-rich giants, it cannot account for the majority that have helium-burning cores. We rule out most other proposed explanations for the origin of lithium-rich giants. Our analysis shows that giants remain lithium-rich for only about two million years. A prediction from this lithium depletion timescale is that most lithium-rich giants with a helium-burning core have a binary companion.
We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor (Fe/H = -1.62 + or - 0.09) star HD 94028. Previous studies revealed that this ...star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., Pb/Fe = +0.79 + or - 0.32) and rapid neutron-capture process (r process; e.g., Eu/Fe = +0.22 + or - 0.12), including unusually large molybdenum (Mo/Fe = +0.97 + or - 0.16) and ruthenium (Ru/Fe = +0.69 + or - 0.17) enhancements. However, this star is not enhanced in carbon (C/Fe = -0.06 + or - 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar As/Ge ratio (+0.99 + or - 0.23) and the enhanced Mo/Fe and Ru/Fe ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar As/Ge ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced As/Ge ratios, hinting that operation of the i process may have been common in the early Galaxy.
Abstract
One-dimensional stellar structure and evolution programs are built using different physical prescriptions and algorithms, which means there can be variations between models’ predictions even ...when using identical input physics. This leads to questions about whether such deviations are physical or numerical; code validation studies are important and necessary tools for studying these questions. We provide the first direct comparison between the Monash stellar evolution program and MESA for a 2
M
⊙
model evolved from the zero-age main sequence to the tip of the thermally pulsing asymptotic giant branch. We compare the internal structure of the two models at six critical evolutionary points and find that they are in excellent agreement with regard to characteristics like central temperature, central density, and the temperature at the base of the convective envelope during the thermally pulsing asymptotic giant branch. The H-exhausted core mass between the models differs by less than 4.2% throughout the entire evolution; the final values vary only by 1.5%. Surface quantities such as luminosity and radius vary by less than 0.2% prior to the asymptotic giant branch. During thermal pulses, the difference extends to 3.4%, largely due to uncertainties in mixing and the treatment of atmospheric boundary conditions. Given that the veteran Monash code is closed-source, the present work provides the first fully open-source computational analog. This increases accessibility to precision modeling on the asymptotic giant branch and lays the groundwork for higher-mass calculations that are performed with MESA but preserve the standards of the Monash code during the asymptotic giant branch.
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