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Abstract R-process-enhanced (RPE) stars are rare and typically metal-poor ([Fe/H]  < −1.0), primarily found in the Milky Way halo system and dwarf galaxies. This study reports the discovery of two relatively bright, highly RPE stars ([Eu/Fe]  >  +0.70) located in the Milky Way disk, with [Fe/H] of −0.34 and −0.80, respectively. These two stars are selected from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope medium-resolution (R ∼ 7500) spectroscopic survey. Follow-up high-resolution (R ∼ 25,000) observations were conducted with the High Optical Resolution Spectrograph installed on the 10.4 m Gran Telescopio Canarias. We perform the determination of elemental abundances and calculate the orbital parameters. We find that they arer-II stars with elemental abundances in agreement with the solarr-process pattern. These two objects are chemically and dynamically consistent with membership in the Galactic disk and exhibit no evidence of being part of accreted systems. 

Abstract Highlyr-process-enhanced (RPE) stars are rare and usually metal poor ([Fe/H] < −1.0), and they mainly populate the Milky Way halo and dwarf galaxies. This study presents the discovery of a relatively bright (V= 12.72), highly RPE (r-II) star ([Eu/Fe] = +1.32, [Ba/Eu] = −0.95), LAMOST J020623.21+494127.9. This star was selected from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope medium-resolution (R∼ 7500) spectroscopic survey; follow-up high-resolution (R∼ 25,000) observations were conducted with the High Optical Resolution Spectrograph installed on the Gran Telescopio Canarias. The stellar parameters (T_eff= 4130 K, $\log \mathrm{g}$= 1.52, [Fe/H] = −0.54,ξ= 1.80 km s⁻¹) have been inferred taking into account nonlocal thermodynamic equilibrium effects. The abundances of [Ce/Fe], [Pr/Fe], and [Nd/Fe] are +0.19, +0.65, and +0.64, respectively, relatively low compared to the Solarr-process pattern normalized to Eu. This star has a high metallicity ([Fe/H] = −0.54) compared to most other highly RPE stars and has the highest measured abundance ratio of Eu to H ([Eu/H] = +0.78). It is classified as a thin-disk star based on its kinematics and does not appear to belong to any known stream or dwarf galaxy. 

Abstract Using proper motions from Gaia Early Data Release 3 (Gaia EDR3) and radial velocities from several surveys, we identify 60 candidate high-velocity stars with a total velocity greater than 75% of the escape velocity that probably originated from the Sagittarius dwarf spheroidal galaxy (Sgr) by orbital analysis. Sgr’s gravity has little effect on the results and the Large Magellanic Cloud’s gravity has a nonnegligible effect on only a few stars. The closest approach of these stars to the Sgr occurred when the Sgr passed its pericenter (∼38.2 Myr ago), which suggests they were tidally stripped from the Sgr. The positions of these stars in the Hertzsprung–Russell diagram and the chemical properties of 19 of them with available [Fe/H] are similar to the Sgr stream member stars. This is consistent with the assumption of their accretion origin. Two of the 60 are hypervelocity stars, which may also be produced by the Hills mechanism. 

Abstract We study the evolution of rapid neutron-capture process (r-process) isotopes in the galaxy. We analyze relative contributions from core-collapse supernovae (CCSNe), neutron star mergers, and collapsars under a range of astrophysical conditions and nuclear input data. Here we show that, although the r-process in each of these sites can lead to a similar (universal) elemental distribution, the detailed isotopic abundances can differ from one site to another. These differences may allow for the identification of which sources contributed to the early evolution of r-process material in the galaxy. Our simulations suggest that the early evolution was dominated by CCSNe and collapsar r-process nucleosynthesis. This conclusion may be testable if the next generation of observatories can deduce isotopic r-process abundances.