Neutron-Capture Elements in the Early Galaxy Sneden, Christopher; Cowan, John J.; Gallino, Roberto
Annual review of astronomy and astrophysics,
01/2008, Letnik:
46, Številka:
1
Journal Article
Recenzirano
The content of neutron-capture (trans-iron-peak) elements in the low-metallicity Galactic halo varies widely from star to star. The differences are both in bulk amount of the neutron-capture elements ...with respect to lighter ones and in element-to-element ratios among themselves. Several well-defined abundance distributions have emerged that reveal characteristic rapid and slow neutron-capture nucleosynthesis patterns. In this review we summarize these observed metal-poor star's abundances, contrasting them with the Solar-system values, comparing them to theoretical predictions, using them to assess the types of stars responsible for their specific anomalies, and speculating on the timing and nature of early Galactic nucleosynthesis.
ABSTRACT We have derived new, very accurate abundances of the Fe-group elements Sc through Zn (Z = 21−30) in the bright main-sequence turnoff star HD 84937 based on high-resolution spectra covering ...the visible and ultraviolet spectral regions. New or recent laboratory transition data for 14 species of seven elements have been used. Abundances from more than 600 lines of non-Fe species have been combined with about 550 Fe lines in HD 84937 to yield abundance ratios of high precision. The abundances have been determined from both neutral and ionized transitions, which generally are in agreement with each other. We find no substantial departures from the standard LTE Saha ionization balance in this Fe/H = −2.32 star. Noteworthy among the abundances are Co/Fe = +0.14 and Cu/Fe = −0.83, in agreement with past studies of abundance trends in this and other low-metallicity stars, and which has not been noted previously. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys reveals that they are positively correlated in stars with Fe/H < −2; HD 84937 lies at the high end of this correlation. These trends constrain the synthesis mechanisms of Fe-group elements. We also examine the Galactic chemical evolution abundance trends of the Fe-group elements, including a new nucleosynthesis model with jet-like explosion effects.
This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were ...identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ∼ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have Fe/H < −1.5, 105 have Fe/H < −2.0, and 4 have Fe/H < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ Eu/Fe ≤ +1.0 and Ba/Eu < 0) and 4 new r-II stars (with Eu/Fe > +1.0). Nineteen stars are found to exhibit a "limited-r" signature (Sr/Ba > +0.5, Ba/Eu < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, , and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.
We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor (Fe/H = −2.99) K giant in the LAMOST survey, and ...found to be highly r-process-enhanced (r-II; Eu/Fe = +1.28), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of Th/Eu = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance (U/Fe = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.
Abstract
We have obtained new detailed abundances of the Fe-group elements Sc through Zn (
Z
= 21–30) in three very metal-poor (Fe/H ≈ −3) stars: BD+03
o
740, BD−13
o
3442, and CD−33
o
1173. ...High-resolution ultraviolet
Hubble Space Telescope
/Space Telescope Imaging Spectrograph spectra in the wavelength range 2300–3050 Å were gathered, and complemented by an assortment of optical echelle spectra. The analysis featured recent laboratory atomic data for a number of neutral and ionized species for all Fe-group elements except Cu and Zn. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys indicates that they are positively correlated in stars with Fe/H ≤ −2. The abundances of these elements in BD+03
o
740, BD−13
o
3442, CD−33
o
1173, and HD 84937 (studied in a previous paper of this series) are in accord with these trends and lie at the high end of the correlations. Six elements have detectable neutral and ionized features, and generally their abundances are in reasonable agreement. For Cr we find only minimal abundance disagreement between the neutral (mean of Cr
i
/Fe = +0.01) and ionized species (mean of Cr
ii
/Fe = +0.08), unlike most studies in the past. The prominent exception is Co, for which the neutral species indicates a significant overabundance (mean of Co
i
/H = −2.53), while no such enhancement is seen for the ionized species (mean of Co
ii
/H = −2.93). These new stellar abundances, especially the correlations among Sc, Ti, and V, suggest that models of element production in early high-mass metal-poor stars should be revisited.
Abstract
We have derived new detailed abundances of Mg, Ca, and the Fe-group elements Sc through Zn (
Z
= 21−30) for 37 main-sequence turnoff very metal-poor stars (Fe/H ≲−2.1). We analyzed Keck ...HIRES optical and near-UV high signal-to-noise spectra originally gathered for a Be abundance survey. Using typically ∼400 Fe-group lines with accurate laboratory transition probabilities for each star, we have determined accurate LTE metallicities and abundance ratios for neutral and ionized species of the 10 Fe-group elements as well as
α
elements Mg and Ca. We find good neutral/ion abundance agreement for the six elements that have detectable transitions of both species in our stars in the 3100–5800 Å range. Earlier reports of correlated Sc−Ti−V relative overabundances are confirmed, and appear to slowly increase with decreasing metallicity. To this element trio we add Zn; it also appears to be increasingly overabundant in the lowest-metallicity regimes. Co appears to mimic the behavior of Zn, but issues surrounding its abundance reliability cloud its interpretation.
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.
The Rise of the s-Process in the Galaxy Simmerer, Jennifer; Sneden, Christopher; Cowan, John J ...
The Astrophysical journal,
12/2004, Letnik:
617, Številka:
2
Journal Article
Recenzirano
From newly obtained high-resolution, high signal-to-noise ratio spectra the abundances of the elements La and Eu have been determined over the stellar metallicity range -3 < Fe/H < +0.3 in 159 giant ...and dwarf stars. Lanthanum is predominantly made by the s-process in the solar system, while Eu owes most of its solar system abundance to the r-process. The changing ratio of these elements in stars over a wide metallicity range traces the changing contributions of these two processes to the Galactic abundance mix. Large s-process abundances can be the result of mass transfer from very evolved stars, so to identify these cases we also report carbon abundances in our metal-poor stars. Results indicate that the s-process may be active as early as Fe/H = -2.6, although we also find that some stars as metal-rich as Fe/H = -1 show no strong indication of s-process enrichment. There is a significant spread in the level of s-process enrichment even at solar metallicity.
Abstract
We present new observational benchmarks of rapid neutron-capture process (
r
-process) nucleosynthesis for elements at and between the first (
A
∼ 80) and second (
A
∼ 130) peaks. Our ...analysis is based on archival ultraviolet and optical spectroscopy of eight metal-poor stars with Se (
Z
= 34) or Te (
Z
= 52) detections, whose
r
-process enhancement varies by more than a factor of 30 (−0.22 ≤ Eu/Fe ≤ +1.32). We calculate ratios among the abundances of Se, Sr through Mo (38 ≤
Z
≤ 42), and Te. These benchmarks may offer a new empirical alternative to the predicted solar system
r
-process residual pattern. The Te abundances in these stars correlate more closely with the lighter
r
-process elements than the heavier ones, contradicting and superseding previous findings. The small star-to-star dispersion among the abundances of Se, Sr, Y, Zr, Nb, Mo, and Te (≤0.13 dex, or 26%) matches that observed among the abundances of the lanthanides and third
r
-process-peak elements. The concept of
r
-process universality that is recognized among the lanthanide and third-peak elements in
r
-process-enhanced stars may also apply to Se, Sr, Y, Zr, Nb, Mo, and Te, provided the overall abundances of the lighter
r
-process elements are scaled independently of the heavier ones. The abundance behavior of the elements Ru through Sn (44 ≤
Z
≤ 50) requires further study. Our results suggest that at least one relatively common source in the early Universe produced a consistent abundance pattern among some elements spanning the first and second
r
-process peaks.
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