The R-Process Alliance: 2MASS J09544277+5246414, the Most Actinide-enhanced R-II Star Known

Holmbeck, Erika M.; Beers, Timothy C.; Roederer, Ian U.; Placco, Vinicius M.; Hansen, Terese T.; Sakari, Charli M.; Sneden, Christopher; Liu, Chao; Lee, Young Sun; Cowan, J. J.; Frebel, Anna

doi:10.3847/2041-8213/aac722

Cited by 82 publications

(105 citation statements)

References 43 publications

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“…Actinide abundances in metal-poor stars could hold the key to disentangle the hydrodynamical environment(s) of the r-process. The existence of stars with super-and sub-solar actinide yields (Schatz et al 2002;Roederer et al 2009a;Holmbeck et al 2018;Ji & Frebel 2018) possibly indicate varying r-process conditions. The fact that all currently known actinide-boost stars are very metal-poor ([Fe/H] −2.0) is another interesting finding.…”

Section: Actinidesmentioning

confidence: 99%

Neutron Star Mergers Might Not Be the Only Source of r-process Elements in the Milky Way

Côté

Eichler

Arcones

et al. 2019

ApJ

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Probing the origin of r-process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H] > −1 in the disk of the Milky Way cannot be reproduced while accounting for the delaytime distribution (DTD) of coalescence times (∝ t −1 ) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions (∝ t −1.5 ) or power laws combined with a strong burst of mergers before the onset of Type Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this extra site could be responsible for roughly 50 % of the Eu production in the early universe, before the onset of Type Ia supernovae. Rare classes of supernovae could be this additional r-process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern.

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Section: Actinidesmentioning

confidence: 99%

Neutron Star Mergers Might Not Be the Only Source of r-process Elements in the Milky Way

Côté

Eichler

Arcones

et al. 2019

ApJ

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“…However, the conditions in the ejecta differ considerably (i.e., electron fraction, entropy, and ejecta velocity), which leaves an imprint on the compositions. Hansen et al (2018) and the recently discovered actinide-boost star J09544277+5246414 (Holmbeck et al 2018). In addition, we also include the Reticulum II star with known Th yield, DESJ033523-540407 (Ji & Frebel 2018).…”

Section: Actinide and Lanthanide Yields In Our Modelsmentioning

confidence: 99%

“…Even before these observations, theoretical calculations have shown that actinide production in the r-process can vary depending on the conditions and the nuclear mass model (e.g., Goriely & Clerbaux 1999). An overview on actinide-boost stars has been given in Roederer et al (2009), with more recent discoveries by Mashonkina et al (2014) and Holmbeck et al (2018). Ji & Frebel (2018), on the other hand, have measured the Th abundance of DES J033523−540407, a star in the r-process-enriched ultra-faint dwarf galaxy Reticulum II, and their result suggests that this star might belong to a different, actinide-deficient category of stars.…”

Section: Introductionmentioning

confidence: 99%

Probing the Production of Actinides under Different r-process Conditions

Eichler

Sayar

Arcones

et al. 2019

ApJ

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Several extremely metal-poor stars are known to have an enhanced thorium abundance. These actinide-boost stars have likely inherited material from an r-process that operated under different conditions than the r-process that is reflected in most other metal-poor stars with no actinide enhancement.In this article, we explore the sensitivity of actinide production in r-process calculations on the hydrodynamical conditions as well as on the nuclear physics. We find that the initial electron fraction Y e is the most important factor determining the actinide yields and that the abundance ratios between long-lived actinides and lanthanides like europium can vary for different conditions in our calculations. In our setup, conditions with high entropies systematically lead to lower actinide abundances relative to other r-process elements. Furthermore, actinide-enhanced ejecta can be distinguished from the "regular" composition also in other ways, most notably in the second r-process peak abundances.

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“…There are two main reasons for this: 1) The enhancement in r-process elements ensures that spectral lines from a wide variety of r-process elements are sufficiently strong to be measured, while the (relative) lack of metal lines (compared to more metal-rich stars) reduces the severe blending typically seen in the blue spectral region; and 2) These stars are selected to have little-to-no contamination from the slow (s-) process, simplifying comparisons with models of r-process yields. If the enhancement in radioactive elements like Th and U is sufficiently high, cosmo-chronometric ages can also be determined (see, e.g., Holmbeck et al 2018a and references therein).…”

Section: Introductionmentioning

confidence: 99%

The R-Process Alliance: First Release from the Northern Search for r-process-enhanced Metal-poor Stars in the Galactic Halo

Sakari

Placco

Farrell

et al. 2018

ApJ

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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 sakaricm@u.washington.edu 2 Sakari et al.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 <3D> 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 carbonenhanced metal-poor stars have been discovered, 3 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, α, and Fe-peak elements match those of typical Milky Way halo stars, except for one r-I star which has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UV W space velocities for these stars which are consistent with membership in the Milky Way halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.

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The R-Process Alliance: 2MASS J09544277+5246414, the Most Actinide-enhanced R-II Star Known

Cited by 82 publications

References 43 publications

Neutron Star Mergers Might Not Be the Only Source of r-process Elements in the Milky Way

Neutron Star Mergers Might Not Be the Only Source of r-process Elements in the Milky Way

Probing the Production of Actinides under Different r-process Conditions

The R-Process Alliance: First Release from the Northern Search for r-process-enhanced Metal-poor Stars in the Galactic Halo

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