The temporal evolution of neutron-capture elements in the Galactic discs (original) (raw)
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Observational nuclear astrophysics: neutron-capture element abundances in old, metal-poor stars
Journal of Physics G: Nuclear and Particle Physics, 2014
The chemical abundances of metal-poor stars provide a great deal of information regarding the individual nucleosynthetic processes that created the observed elements and the overall process of chemical enrichment of the galaxy since the formation of the first stars. Here we review the abundance patterns of the neutron-capture elements (Z ≥ 38) in those metal-poor stars and our current understanding of the conditions and sites of their production at early times. We also review the relative contributions of these different processes to the build-up of these elements within the galaxy over time, and outline outstanding questions and uncertainties that complicate the interpretation of the abundance patterns observed in metal-poor stars. It is anticipated that future observations of large samples of metal-poor stars will help discriminate between different proposed neutron-capture element production sites and better trace the chemical evolution of the galaxy.
Astronomy Reports, 2007
We derived Sr, Y, Zr, and Ce abundances for a sample of 74 cool dwarfs and subgiants with iron abundances, [Fe/H], between 0.25 and −2.43. These estimates were obtained using synthetic spectra, assuming local thermodynamic equilibrium (LTE) for Y, Zr, and Ce, allowing for non-LTE conditions for Sr. We used high-resolution (λ/Δλ 40 000 and 60 000) spectra with signal-to-noise ratios between 50 and 200. We find that the Zr/Y, Sr/Y, and Sr/Zr ratios for the halo stars are the same in a wide metallicity range (−2.43 ≤ [Fe/H] ≤ −0.90), within the errors, indicating a common origin for these elements at the epoch of halo formation. The Zr/Y ratios for thick-disk stars quickly decrease with increasing Ba abundance, indicating a lower rate of production of Zr compared to Y during active thick-disk formation. The thickdisk and halo stars display an increase in the [Zr/Ba] ratio with decreasing Ba abundance and a correlation of the Zr and Eu overabundances relative to Ba. The evolutionary behavior of the abundance ratios found for the thick-disk and halo stars does not agree with current models for the Galaxy's chemical evolution. The abundance ratios of Y and Zr to Fe and Ba for thin-disk stars, as well as the abundance ratios within each group, are, on average, solar, though we note a slight decrease of Zr/Ba and Zr/Y with increasing Ba abundance. These results provide evidence for a dominance of asymptotic-giant-branch stars in the enrichment of the interstellar medium in heavy elements during the thin-disk epoch, in agreement with the predictions of the nucleosynthesis theory for the main s-process component.
Neutron‐Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal‐poor Giants
The Astrophysical Journal, 2000
New abundances for neutron-capture (n-capture) elements in a large sample of metal-poor giants from the Bond survey are presented. The spectra were acquired with the KPNO 4-m echelle and coudé feed spectrographs, and have been analyzed using LTE fine-analysis techniques with both line analysis and spectral synthesis. Abundances of eight n-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, Dy) in 43 stars have been derived from blue (λλ4070-4710Å, R∼20,000, S/N ratio∼100-200) echelle spectra and red (λλ6100-6180Å, R∼22,000, S/N ratio∼100-200) coudé spectra, and the abundance of Ba only has been derived from the red spectra for an additional 27 stars.
arXiv (Cornell University), 2023
Based on high-resolution spectra obtained with Magellan/MIKE, we present a chemo-dynamical analysis for 27 near mainsequence turnoff metal-poor stars, including 20 stars analyzed for the first time. The sample spans a range in [Fe/H] from −2.5 to −3.6, with 44% having [Fe/H]< −2.9. We derived chemical abundances for 17 elements, including strontium and barium. We derive Li abundances for the sample, which are in good agreement with the "Spite Plateau" value. A dozen of stars are carbon-enhanced, i.e., [C/Fe] > 0.7. The lighter elements (< 30) generally agree well with those of other low-metallicity halo stars. This broadly indicates chemically homogeneous gas at the earliest times. Of the neutron-capture elements, we only detected strontium and barium. We used the [Sr/Ba] vs. [Ba/Fe] diagram to classify metal-poor stars into five populations based on their observed ratios. We find HE 0232−3755 to be a likely main-process star, and HE 2214−6127 and HE 2332−3039 to be limited-stars. CS30302-145, HE 2045−5057, and CD −24°17504 plausibly originated in long-disrupted early dwarf galaxies as evidenced by their [Sr/Ba] and [Ba/Fe] ratios. We also find that the derived [Sr/H] and [Ba/H] values for CD −24°17504 are not inconsistent with the predicted yields of the-process in massive rotating low-metallicity stars models. Further theoretical explorations will be helpful to better understand the earliest mechanisms and time scales of heavy element production for comparison with these and other observational abundance data. Finally, we investigate the orbital histories of our sample stars. Most display halo-like kinematics although three stars (CS 29504-018, HE 0223−2814, and HE 2133−0421) appear to be disk-like in nature. This confirms the extragalactic origin for CS 30302-145, HE 2045−5057, and, in particular, CD −24°17504 which likely originated from a small accreted stellar system as one of the oldest stars.
Observations of neutron-capture elements in the early galaxy
Nuclear Physics A, 2003
Neutron-capture elements in low metallicity Galactic halo stars vary widely both in overall contents and detailed abundance patterns. This review discusses recent observational results on the n-capture elements, discussing the implications for early Galactic nucleosynthesis of: (a) the star-to-star "bulk" variations in the n-capture/Fe abundance ratios; (b) the distinct signature of rapid n-capture synthesis events in many (most?) of the lowest metallicity stars; (c) the existence of metal-poor stars heavily enriched in the products of slow n-capture synthesis reactions; and (d) the now-routine detection of radioactive thorium (and even uranium in one and possibly two cases) in the spectra of metal-poor stars. * The authors all thank our colleagues who have contributed to the results presented here.
Nucleosynthesis: Stellar and Solar Abundances and Atomic Data
2006
Abundance observations indicate the presence of often surprisingly large amounts of neutron capture (i.e., sand r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy-the progenitors of the halo stars-responsible for neutron-capture synthesis. Comparisons of abundance trends can be used to understand the chemical evolution of the Galaxy and the nature of heavy element nucleosynthesis. In addition age determinations, based upon long-lived radioactive nuclei abundances, can now be obtained. These stellar abundance determinations depend critically upon atomic data. Improved laboratory transition probabilities have been recently obtained for a number of elements. These new gf values have been used to greatly refine the abundances of neutron-capture elemental abundances in the solar photosphere and in very metal-poor Galactic halo stars. The newly determined stellar abundances are surprisingly consistent with a (relative) Solar System r-process pattern, and are also consistent with abundance predictions expected from such neutron-capture nucleosynthesis.
Astronomy & Astrophysics
We present an automated statistical method that uses medium-resolution spectroscopic observations of a set of stars to select those that show evidence of possessing significant amounts of neutron-capture elements. Our tool was tested against a sample of ∼70 000 F- and G-type stars distributed among 215 plates from the Galactic Understanding and Exploration (SEGUE) survey, including 13 that were directed at stellar Galaxy clusters. Focusing on five spectral lines of europium in the visible window, our procedure ranked the stars by their likelihood of having enhanced content of this atomic species and identifies the objects that exhibit signs of being rich in neutron-capture elements as those scoring in the upper 2.5%. We find that several of the cluster plates contain relatively large numbers of stars with significant absorption around at least three of the five selected lines. The most prominent is the globular cluster M 3, where we measured a fraction of stars that are potentially ...
The Astrophysical Journal, 2005
Elemental abundance measurements have been obtained for a sample of 18 very metal-poor stars using spectra obtained with the Subaru Telescope High Dispersion Spectrograph. Seventeen stars, among which 16 are newly analyzed in the present work, were selected from candidate metal-poor stars identified in the HK survey of Beers and colleagues. The metallicity range covered by our sample is −3.1 [Fe/H] −2.4. The abundances of carbon, α-elements, and iron-peak elements determined for these stars confirm the trends found by previous work. One exception is the large overabundance of Mg, Al and Sc found in BS 16934-002, a giant with [Fe/H] = −2.8. Interestingly, this is the most metal-rich star (by about 1 dex in [Fe/H]) known with such large overabundances in these elements. Furthermore, BS 16934-002 does not share the large over-abundances of carbon that are associated with the two other, otherwise similar, extremely metal-poor stars CS 22949-037 and CS 29498-043. By combining our new results with those of previous studies, we investigate the distribution of neutron-capture elements in very metal-poor stars, focusing on the production of the light neutron-capture elements (e.g., Sr, Y, and Zr). Large scatter is found in the abundance ratios between the light and heavy neutron-capture elements (e.g., Sr/Ba, Y/Eu) for stars with low abundances of heavy neutron-capture elements.
Abundance Analysis of HE 2148−1247, A Star with Extremely Enhanced Neutron Capture Elements
The Astrophysical Journal, 2003
Abundances for 27 elements in the very metal poor dwarf star HE 2148-1247 are presented, including many of the neutron capture elements. We establish that HE 2148-1247 is a very highly s-process enhanced star with anomalously high Eu as well, Eu/H ∼half Solar, demonstrating the large addition of heavy nuclei at [Fe/H] = −2.3 dex. Ba and La are enhanced by a somewhat larger factor and reach the solar abundance, while Pb significantly exceeds it, thus demonstrating the addition of substantial s-process material. Ba/Eu is ten times the solar r-process ratio but much less than that of the s-process, indicating a substantial r-process addition as well. C and N are also very highly enhanced. We have found that HE 2148-1247 is a radial velocity variable; it is probably a small amplitude long period binary. The C, N and the s-process element enhancements thus presumably were produced through mass transfer from a former AGB binary companion. The large enhancement of heavy r-nuclides also requires an additional source as this is far above any inventory in the ISM at such low [Fe/H]. We consider that the s-process material was added by mass transfer of a more massive companion during its thermally pulsating AGB phase and ending up as a white dwarf. We further hypothesize that accretion onto the white dwarf from the envelope of the star caused accretion induced collapse of the white dwarf, forming a neutron star, which then produced heavy r-nuclides and again contaminated its companion. This mechanism in a binary system can thus enhance the envelope of the lower mass star in s and r-process material sequentially. Through analysis of the neutron capture element abundances taken from the literature for a large sample of very metal poor stars, we demonstrate, as exemplified by HE 2148-1247, that mass transfer in a suitable binary can be very efficient in enhancing the heavy elements in a star; it appears to be capable of enhancing 1 Based on observations obtained at the W.M.
Neutron Capture Elements ins‐Process–Rich, Very Metal‐Poor Stars
The Astrophysical Journal, 2001
We report abundance estimates for neutron-capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion AGB stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z = 38) to Pb (Z = 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron-capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a re-analysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure τ is estimated to be about 0.7mb −1 , significantly larger than that which best fits solar-system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed