Stardust grains that originated in ancient stars and supernovae are recovered from meteorites and carry the detailed composition of their astronomical sites of origin. We present evidence that the ...majority of large ( m-sized) meteoritic silicon carbide (SiC) grains formed in C-rich asymptotic giant branch (AGB) stars that were more metal-rich than the Sun. In the framework of the slow neutron captures (the s process) that occur in AGB stars, the lower-than-solar Sr/ Sr isotopic ratios measured in the large SiC grains can only be accompanied by Ce/Y elemental ratios that are also lower than solar and predominately observed in metal-rich barium stars-the binary companions of AGB stars. Such an origin suggests that these large grains represent the material from high-metallicity AGB stars needed to explain the s-process nucleosynthesis variations observed in bulk meteorites. In the outflows of metal-rich, C-rich AGB stars, SiC grains are predicted to be small ( 0.2 m); large ( m-sized) SiC grains can grow if the number of dust seeds is 2-3 orders of magnitude lower than the standard value of 10−13 times the number of H atoms. We therefore predict that with increasing metallicity, the number of dust seeds might decrease, resulting in the production of larger SiC grains.
ABSTRACT
Aluminium-26 is a radioactive isotope which can be synthesized within asymptotic giant branch (AGB) stars, primarily through hot bottom burning. Studies exploring 26Al production within AGB ...stars typically focus on single-stars; however, observations show that low- and intermediate-mass stars commonly exist in binaries. We use the binary population synthesis code binary_c to explore the impact of binary evolution on 26Al yields at solar metallicity both within individual AGB stars and a low/intermediate-mass stellar population. We find the key stellar structural condition achieving most 26Al overproduction is for stars to enter the thermally pulsing AGB (TP-AGB) phase with small cores relative to their total masses, allowing those stars to spend abnormally long times on the TP-AGB compared to single-stars of identical mass. Our population with a binary fraction of 0.75 has an 26Al weighted population yield increase of 25 per cent compared to our population of only single-stars. Stellar-models calculated from the Mt Stromlo/Monash Stellar Structure Program, which we use to test our results from binary_c and closely examine the interior structure of the overproducing stars, support our binary_c results only when the stellar envelope gains mass after core-He depletion. Stars which gain mass before core-He depletion still overproduce 26Al, but to a lesser extent. This introduces some physical uncertainty into our conclusions as 55 per cent of our 26Al overproducing stars gain envelope mass through stellar wind accretion onto pre-AGB objects. Our work highlights the need to consider binary influence on the production of 26Al.
We present CN and CH indices and Ca ii triplet metallicities for 34 giant stars and chemical abundances for 33 elements in 14 giants in the globular cluster M2. Assuming that the programme stars are ...cluster members, our analysis reveals (i) an extreme variation in CN and CH line strengths, (ii) a metallicity dispersion with a dominant peak at Fe/H ≈ −1.7 and smaller peaks at −1.5 and −1.0, (iii) star-to-star abundance variations and correlations for the light elements O, Na, Al and Si and (iv) a large (and possibly bimodal) distribution in the abundances of all elements produced mainly via the s-process in Solar system material. Following Roederer, Marino & Sneden, we define two groups of stars, ‘r + s’ and ‘r-only’, and subtract the average abundances of the latter from the former group to obtain an ‘s-process residual’. This s-process residual is remarkably similar to that found in M22 and in M4 despite the range in metallicity covered by these three systems. With recent studies identifying a double subgiant branch in M2 and a dispersion in Sr and Ba abundances, our spectroscopic analysis confirms that this globular cluster has experienced a complex formation history with similarities to M22, NGC 1851 and ω Centauri.
Abstract
Thermohaline mixing is a favoured mechanism for the so-called ‘extra mixing’ on the red giant branch of low-mass stars. The mixing is triggered by the molecular weight inversion created ...above the hydrogen shell during first dredge-up when helium-3 burns via 3He(3He,2p)4He. The standard 1D diffusive mixing scheme cannot simultaneously match carbon and lithium abundances to NGC 6397 red giants. We investigate two modifications to the standard scheme: (1) an advective two-stream mixing algorithm and (2) modifications to the standard 1D thermohaline mixing formalism. We cannot simultaneously match carbon and lithium abundances using our two-stream mixing approach. However, we develop a modified diffusive scheme with an explicit temperature dependence that can simultaneously fit carbon and lithium abundances to NGC 6397 stars. Our modified diffusive scheme induces mixing that is faster than the standard theory predicts in the hotter part of the thermohaline region and mixing that is slower in the cooler part. Our results infer that the extra mixing mechanism needs further investigation and more observations are required, particularly for stars in different clusters spanning a range in metallicity.
We present updated calculations of stellar evolutionary sequences and detailed nucleosynthesis predictions for the brightest asymptotic giant branch (AGB) stars in the Galaxy with masses between 5 M ...sub(middot in circle) and 9 M sub(middot in circle), with an initial metallicity of Z = 0.02 (Fe/H = 0.14). In our previous studies we used the Vassiliadis & Wood mass-loss rate, which stays low until the pulsation period reaches 500 days after which point a superwind begins. Vassiliadis & Wood noted that for stars over 2.5 M sub(middot in circle) the superwind should be delayed until P approximately 750 days at 5 M sub(middot in circle). We calculate evolutionary sequences where we delay the onset of the superwind to pulsation periods of P approximately 700-800 days in models of M = 5, 6, and 7 M sub(middot in circle). Post-processing nucleosynthesis calculations show that the 6 and 7 M sub(middot in circle) models produce the most Rb, with Rb/Fe approximately 1 dex, close to the average of most of the Galactic Rb-rich stars (Rb/Fe approximately 1.4 + or - 0.8 dex). Changing the rate of the super(22)Ne + alpha reactions results in variations of Rb/Fe as large as 0.5 dex in models with a delayed superwind. The largest enrichment in heavy elements is found for models that adopt the NACRE rate of the super(22)Ne( alpha , n) super(25)Mg reaction. Using this rate allows us to best match the composition of most of the Rb-rich stars. A synthetic evolution algorithm is then used to remove the remaining envelope resulting in final Rb/Fe of approximately 1.4 dex although with C/O ratios > 1. We conclude that delaying the superwind may account for the large Rb overabundances observed in the brightest metal-rich AGB stars.
The Ubiquity of the Rapid Neutron-capture Process Roederer, Ian U; Cowan, John J; Karakas, Amanda I ...
Astrophysical journal/The Astrophysical journal,
12/2010, Letnik:
724, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
To better characterize the abundance patterns produced by the
r
-process, we have derived new abundances or upper limits for the heavy elements zinc (Zn,
Z
= 30), yttrium (Y,
Z
= 39), ...lanthanum (La,
Z
= 57), europium (Eu,
Z
= 63), and lead (Pb,
Z
= 82). Our sample of 161 metal-poor stars includes new measurements from 88 high-resolution and high signal-to-noise spectra obtained with the Tull Spectrograph on the 2.7 m Smith Telescope at the McDonald Observatory, and other abundances are adopted from the literature. We use models of the
s
-process in asymptotic giant branch stars to characterize the high Pb/Eu ratios produced in the
s
-process at low metallicity, and our new observations then allow us to identify a sample of stars with no detectable
s
-process material. In these stars, we find no significant increase in the Pb/Eu ratios with increasing metallicity. This suggests that
s
-process material was not widely dispersed until the overall Galactic metallicity grew considerably, perhaps even as high as Fe/H =−1.4, in contrast with earlier studies that suggested a much lower mean metallicity. We identify a dispersion of at least 0.5 dex in La/Eu in metal-poor stars with Eu/Fe <+0.6 attributable to the
r
-process, suggesting that there is no unique “pure”
r
-process elemental ratio among pairs of rare earth elements. We confirm earlier detections of an anti-correlation between Y/Eu and Eu/Fe bookended by stars strongly enriched in the
r
-process (e.g., CS 22892–052) and those with deficiencies of the heavy elements (e.g., HD 122563). We can reproduce the range of Y/Eu ratios using simulations of high-entropy neutrino winds of core-collapse supernovae that include charged-particle and neutron-capture components of
r
-process nucleosynthesis. The heavy element abundance patterns in most metal-poor stars do not resemble that of CS 22892–052, but the presence of heavy elements such as Ba in nearly all metal-poor stars without
s
-process enrichment suggests that the
r
-process is a common phenomenon.
Nucleosynthesis in Primordial Hypernovae Grimmett, J J; Heger, Alexander; Karakas, Amanda I ...
Monthly Notices of the Royal Astronomical Society,
09/2018
Journal Article
The dimensions of Fanaroff–Riley class I jets and the stellar densities at galactic centres imply that there will be numerous interactions between the jet and stellar winds. These may give rise to ...the observed diffuse and ‘knotty’ structure of the jets in the X-ray, and can also mass load the jets. We performed modelling of internal entrainment from stars intercepted by Centaurus A's jet, using stellar evolution- and wind codes. From photometry and a code-synthesized population of 12 Gyr (Z = 0.004), 3 Gyr (Z = 0.008) and 0–60 Myr (Z = 0.02) stars, appropriate for the parent elliptical NGC 5128, the total number of stars in the jet is ∼8 × 108. Our model is energetically capable of producing the observed X-ray emission, even without young stars. We also reproduce the radio through X-ray spectrum of the jet, albeit in a downstream region with distinctly fewer young stars, and recover the mean X-ray spectral index. We derive an internal entrainment rate of ∼2.3 × 10−3 M⊙ yr−1 which implies substantial jet deceleration. Our absolute nucleosynthetic yields for the Asymptotic Giant Branch stellar population in the jet show the highest amounts for 4He, 16O, 12C, 14N and 20Ne. If some of the events at ≥55 EeV detected by the Pierre Auger Observatory originate from internal entrainment in Centaurus A, we predict that their composition will be largely intermediate-mass nuclei with 16O, 12C and 14N the key isotopes.
The aim of Galactic archaeology is to recover the history of our Galaxy through the information encoded in stars. An unprobed assumption of this field is that the chemical composition of a star is an ...immutable marker of the gas from which it formed. It is vital to test this assumption on open clusters, a group of stars formed from the same gas. Previous investigations have shown that unevolved stars in clusters are chemically homogeneous within the typical uncertainties of these analyses, i.e., 15% of the elemental abundances. Our strictly differential analysis on five members of the Pleiades allows us to reach precisions of 5% for most elements and to unveil chemical anomalies within the cluster that could be explained by planet engulfment events. These results reveal that the evolution of planetary systems may alter the chemical composition of stars, challenging our capability of tagging them to their native environments, and also paving the way for the study of planetary architectures and their evolution, through the chemical pattern of their host stars.
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