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With the most trans-iron elements detected of any star outside the solar system, HD 222925 represents the most complete chemical inventory among metal-poor stars enhanced with elements made by the rapid neutron capture (“r”) process. As such, HD 222925 may be a new “template” for the observationalr-process, where before the (much higher-metallicity) solarr-process residuals were used. In this work, we test under which conditions a single site accounts for the entire elementalr-process abundance pattern of HD 222925. We found that several of our tests—with the single exception of the black hole–neutron star merger case—challenge the single-site assumption by producing an ejecta distribution that is highly constrained, in disagreement with simulation predictions. However, we found that ejecta distributions that are more in line with simulations can be obtained under the condition that the nuclear data near the secondr-process peak are changed. Therefore, for HD 222925 to be a canonicalr-process template likely as a product of a single astrophysical source, the nuclear data need to be reevaluated. The new elemental abundance pattern of HD 222925—including the abundances obtained from space-based, ultraviolet (UV) data—call for a deeper understanding of both astrophysicalr-process sites and nuclear data. Similar UV observations of additionalr-process–enhanced stars will be required to determine whether the elemental abundance pattern of HD 222925 is indeed a canonical template (or an outlier) for ther-process at low metallicity.

 

Understanding local stellar kinematic substructures in the solar neighbourhood helps build a complete picture of the formation of the Milky Way, as well as an empirical phase space distribution of dark matter that would inform detection experiments. We apply the clustering algorithm hdbscan on the Gaia early third data release to identify a list of stable clusters in velocity space and action-angle space by taking into account the measurement uncertainties and studying the stability of the clustering results. We find 1405 (497) stars in 23 (6) robust clusters in velocity space (action-angle space) that are consistently not associated with noise. We discuss the kinematic properties of these structures and study whether many of the small clusters belong to a similar larger cluster based on their chemical abundances. They are attributed to the known structures: the Gaia Sausage-Enceladus, the Helmi Stream, and globular cluster NGC 3201 are found in both spaces, while NGC 104 and the thick disc (Sequoia) are identified in velocity space (action-angle space). Although we do not identify any new structures, we find that the hdbscan member selection of already known structures is unstable to input kinematics of the stars when resampled within their uncertainties. We therefore present the stable subset of local kinematic structures, which are consistently identified by the clustering algorithm, and emphasize the need to take into account error propagation during both the manual and automated identification of stellar structures, both for existing ones as well as future discoveries.

 

Abstract The ultrafaint dwarf galaxy Reticulum II was enriched by a single rare and prolific r -process event. The r -process content of Reticulum II thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find 72 − 12 + 10 % of the stars are r -process-enhanced, with a mean [ Ba / H ] = − 1.68 ± 0.07 and unresolved intrinsic dispersion σ [Ba/H] <0.20. The homogeneous r -process abundances imply that Ret II’s metals are well mixed by the time the r -enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r -process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r -enhanced stars in Ret II formed in the absence of substantial pristine gas accretion, perhaps indicating that ≈70% of Ret II stars formed after reionization. 

Abstract We have developed a chemodynamical approach to assign 36,010 metal-poor SkyMapper stars to various Galactic stellar populations. Using two independent techniques (velocity and action space behavior), Gaia EDR3 astrometry, and photometric metallicities, we selected stars with the characteristics of the “metal-weak” thick-disk population by minimizing contamination by the canonical thick disk or other Galactic structures. This sample comprises 7127 stars, spans a metallicity range of −3.50 < [Fe/H] < −0.8, and has a systematic rotational velocity of 〈 V ϕ 〉 = 154 km s −1 that lags that of the thick disk. Orbital eccentricities have intermediate values between typical thick-disk and halo values. The scale length is h R = 2.48 − 0.05 + 0.05 kpc, and the scale height is h Z = 1.68 − 0.15 + 0.19 kpc. The metallicity distribution function is well fit by an exponential with a slope of Δ log N / Δ [ Fe / H ] = 1.13 ± 0.06 . Overall, we find a significant metal-poor component consisting of 261 SkyMapper stars with [Fe/H] < −2.0. While our sample contains only 11 stars with [Fe/H] ≲ −3.0, investigating the JINAbase compilation of metal-poor stars reveals another 18 such stars (five have [Fe/H] < −4.0) that kinematically belong to our sample. These distinct spatial, kinematic, and chemical characteristics strongly suggest that this metal-poor, phase-mixed kinematic sample represents an independent disk component with an accretion origin in which a massive dwarf galaxy radially plunged into the early Galactic disk. Going forward, we propose to call the metal-weak thick-disk population the Atari disk, given its likely accretion origin, and in reference to it sharing space with the Galactic thin and thick disks. 

We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R∼ 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of 〈[Fe/H]〉 = −2.62 ± 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of −143.5 ± 1.2 km s⁻¹and a velocity dispersion of${\sigma }_{\mathrm{rv}}={2.5}_{-0.8}^{+1.3}$km s⁻¹, which results in a dynamical mass of$M_{1/2}(r_h)=8_{-4}^{+12}\times {10}^5$M_⊙and a mass-to-light ratio ofM/L_V=${440}_{-250}^{+650}$M_⊙/L_⊙. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80M_⊙/L_⊙). However, we do not resolve a metallicity dispersion (σ_[Fe/H]< 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass–metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in line with its orbital parameters. Intriguingly, Grus I has among the lowest central densities (${\rho }_{1/2}\sim {3.5}_{-2.1}^{+5.7}\times {10}^7$M_⊙kpc⁻³) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.

 

We present a high-resolution (R ∼ 35 000), high signal-to-noise (S/N = 350) Magellan/MIKE spectrum of the bright extremely metal-poor star 2MASS J1808−5104. We find [Fe/H] = −4.01 (spectroscopic LTE stellar parameters), [Fe/H] = −3.8 (photometric stellar parameters), and [Fe/H] = −3.7 (spectroscopic NLTE stellar parameters). We measured a carbon-to-iron ratio of [C/Fe] = 0.38 from the CH G-band. J1808−5104 is thus not carbon-enhanced, contrary to many other stars with similarly low-iron abundances. We also determine, for the first time, a barium abundance ([Ba/Fe] = −0.78), and obtain a significantly reduced upper limit for the nitrogen abundance ([N/Fe] < −0.2). For its [Ba/Fe] abundance, J1808−5104 has a lower [Sr/Ba] ratio compared to other stars, consistent with behaviour of stars in ultra-faint dwarf galaxies. We also fit the abundance pattern of J1808−5104 with nucleosynthesis yields from a grid of Population III supernova models. There is a good fit to the abundance pattern that suggests J1808−5104 originated from gas enriched by a single massive supernova with a high explosion energy of E = 10 × 1051 erg and a progenitor stellar mass of M = 29.5 M⊙. Interestingly, J1808−5104 is a member of the Galactic thin disc, as confirmed by our detailed kinematic analysis and calculated stellar actions and velocities. Finally, we also established the orbital history of J1808−5104 using our time-dependent Galactic potential the ORIENT. J1808−5104 appears to have a stable quasi-circular orbit and been largely confined to the thin disc. This unique orbital history, the star’s very old age (∼13.5 Gyr), and the low [C/Fe] and [Sr/Ba] ratios suggest that J1808−5104 may have formed at the earliest epoch of the hierarchical assembly of the Milky Way, and it is most likely associated with the primordial thin disc.

 

The Milky Way has accreted many ultra-faint dwarf galaxies (UFDs), and stars from these galaxies can be found throughout our Galaxy today. Studying these stars provides insight into galaxy formation and early chemical enrichment, but identifying them is difficult. Clustering stellar dynamics in 4D phase space (E,L_z,J_r,J_z) is one method of identifying accreted structure that is currently being utilized in the search for accreted UFDs. We produce 32 simulated stellar halos using particle tagging with the Caterpillar simulation suite and thoroughly test the abilities of different clustering algorithms to recover tidally disrupted UFD remnants. We perform over 10,000 clustering runs, testing seven clustering algorithms, roughly twenty hyperparameter choices per algorithm, and six different types of data sets each with up to 32 simulated samples. Of the seven algorithms, HDBSCAN most consistently balances UFD recovery rates and cluster realness rates. We find that, even in highly idealized cases, the vast majority of clusters found by clustering algorithms do not correspond to real accreted UFD remnants and we can generally only recover 6% of UFDs remnants at best. These results focus exclusively on groups of stars from UFDs, which have weak dynamic signatures compared to the background of other stars. The recoverable UFD remnants are those that accreted recently,z_accretion≲ 0.5. Based on these results, we make recommendations to help guide the search for dynamically linked clusters of UFD stars in observational data. We find that real clusters generally have higher median energy andJ_r, providing a way to help identify real versus fake clusters. We also recommend incorporating chemical tagging as a way to improve clustering results.

 

Abstract Little is known about the origin of the fastest stars in the Galaxy. Our understanding of the chemical evolution history of the Milky Way and surrounding dwarf galaxies allows us to use the chemical composition of a star to investigate its origin and to say whether it was formed in situ or was accreted. However, the fastest stars, the hypervelocity stars, are young and massive and their chemical composition has not yet been analyzed. Though it is difficult to analyze the chemical composition of a massive young star, we are well versed in the analysis of late-type stars. We have used high-resolution ARCES/3.5 m Apache Point Observatory, MIKE/Magellan spectra to study the chemical details of 15 late-type hypervelocity star candidates. With Gaia EDR3 astrometry and spectroscopically determined radial velocities we found total velocities with a range of 274–520 km s −1 and mean value of 381 km s −1 . Therefore, our sample stars are not fast enough to be classified as hypervelocity stars, and are what is known as extreme-velocity stars. Our sample has a wide iron abundance range of −2.5 ≤ [Fe/H] ≤ −0.9. Their chemistry indicates that at least 50% of them are accreted extragalactic stars, with iron-peak elements consistent with prior enrichment by sub-Chandrasekhar mass Type Ia supernovae. Without indication of binary companions, their chemical abundances and orbital parameters indicate that they are the accelerated tidal debris of disrupted dwarf galaxies.