We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (
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Abstract 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 km s−1, which results in a dynamical mass ofM ⊙and a mass-to-light ratio ofM/L V =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 linemore » -
ABSTRACT 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 alsomore »
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Abstract 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. Themore » -
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,more »Free, publicly-accessible full text available May 6, 2023
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Abstract We demonstrate that using up to seven stellar abundance ratios can place observational constraints on the star formation histories (SFHs) of Local Group dSphs, using Sculptor dSph as a test case. We use a one-zone chemical evolution model to fit the overall abundance patterns of
α elements (which probe the core-collapse supernovae that occur shortly after star formation),s -process elements (which probe AGB nucleosynthesis at intermediate delay times), and iron-peak elements (which probe delayed Type Ia supernovae). Our best-fit model indicates that Sculptor dSph has an ancient SFH, consistent with previous estimates from deep photometry. However, we derive a total star formation duration of ∼0.9 Gyr, which is shorter than photometrically derived SFHs. We explore the effect of various model assumptions on our measurement and find that modifications to these assumptions still produce relatively short SFHs of duration ≲1.4 Gyr. Our model is also able to compare sets of predicted nucleosynthetic yields for supernovae and AGB stars, and can provide insight into the nucleosynthesis of individual elements in Sculptor dSph. We find that observed [Mn/Fe] and [Ni/Fe] trends are most consistent with sub-M ChType Ia supernova models, and that a combination of “prompt” (delay times similar to core-collapse supernovae) and “delayed” (minimum delaymore » -
Abstract The astrophysical origins of r -process elements remain elusive. Neutron star mergers (NSMs) and special classes of core-collapse supernovae (rCCSNe) are leading candidates. Due to these channels’ distinct characteristic timescales (rCCSNe: prompt, NSMs: delayed), measuring r -process enrichment in galaxies of similar mass but differing star formation durations might prove informative. Two recently discovered disrupted dwarfs in the Milky Way’s stellar halo, Kraken and Gaia-Sausage Enceladus (GSE), afford precisely this opportunity: Both have M ⋆ ≈ 10 8 M ⊙ but differing star formation durations of ≈2 Gyr and ≈3.6 Gyr. Here we present R ≈ 50,000 Magellan/MIKE spectroscopy for 31 stars from these systems, detecting the r -process element Eu in all stars. Stars from both systems have similar [Mg/H] ≈ −1, but Kraken has a median [Eu/Mg] ≈ −0.1 while GSE has an elevated [Eu/Mg] ≈ 0.2. With simple models, we argue NSM enrichment must be delayed by 500–1000 Myr to produce this difference. rCCSNe must also contribute, especially at early epochs, otherwise stars formed during the delay period would be Eu free. In this picture, rCCSNe account for ≈50% of the Eu in Kraken, ≈25% in GSE, and ≈15% in dwarfs with extended star formation durationsmore »