More Like this

1. The COMBS Survey – III. The chemodynamical origins of metal-poor bulge stars

   https://doi.org/10.1093/mnras/stab2878  

   Lucey, Madeline; Hawkins, Keith; Ness, Melissa; Nelson, Tyler; Debattista, Victor P; Luna, Alice; Bensby, Thomas; Freeman, Kenneth C; Kobayashi, Chiaki (November 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The characteristics of the stellar populations in the Galactic bulge inform and constrain the Milky Way’s formation and evolution. The metal-poor population is particularly important in light of cosmological simulations, which predict that some of the oldest stars in the Galaxy now reside in its centre. The metal-poor bulge appears to consist of multiple stellar populations that require dynamical analyses to disentangle. In this work, we undertake a detailed chemodynamical study of the metal-poor stars in the inner Galaxy. Using R ∼ 20 000 VLT/GIRAFFE spectra of 319 metal-poor (−2.55 dex ≤ [Fe/H] ≤ 0.83 dex, with $$\overline{\rm {[Fe/H]}}$$ = −0.84 dex) stars, we perform stellar parameter analysis and report 12 elemental abundances (C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Zn, Ba, and Ce) with precisions of ≈0.10 dex. Based on kinematic and spatial properties, we categorize the stars into four groups, associated with the following Galactic structures: the inner bulge, the outer bulge, the halo, and the disc. We find evidence that the inner and outer bulge population is more chemically complex (i.e. higher chemical dimensionality and less correlated abundances) than the halo population. This result suggests that the older bulge population was enriched by a larger diversity of nucleosynthetic events. We also find one inner bulge star with a [Ca/Mg] ratio consistent with theoretical pair-instability supernova yields and two stars that have chemistry consistent with globular cluster stars. 

   more » « less
2. On the Hunt for the Origins of the Orphan–Chenab Stream: Detailed Element Abundances with APOGEE and Gaia

   https://doi.org/10.3847/1538-4357/acb698  

   Hawkins, Keith; Price-Whelan, Adrian M.; Sheffield, Allyson A.; Subrahimovic, Aidan Z.; Beaton, Rachael L.; Belokurov, Vasily; Erkal, Denis; Koposov, Sergey E.; Lane, Richard R.; Laporte, Chervin F. P.; et al (May 2023, The Astrophysical Journal)

   Abstract Stellar streams in the Galactic halo are useful probes of the assembly of galaxies like the Milky Way. Many tidal stellar streams that have been found in recent years are accompanied by a known progenitor globular cluster or dwarf galaxy. However, the Orphan–Chenab (OC) stream is one case where a relatively narrow stream of stars has been found without a known progenitor. In an effort to find the parent of the OC stream, we use astrometry from the early third data release of ESA’s Gaia mission (Gaia EDR3) and radial velocity information from the Sloan Digital Sky Survey (SDSS)-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to find up to 13 stars that are likely members of the OC stream. We use the APOGEE survey to study the chemical nature (for up to 10 stars) of the OC stream in theα(O, Mg, Ca, Si, Ti, and S), odd-Z(Al, K, and V), Fe-peak (Fe, Ni, Mn, Co, and Cr), and neutron-capture (Ce) elemental groups. We find that the stars that make up the OC stream are not consistent with a monometallic population and have a median metallicity of −1.92 dex with a dispersion of 0.28 dex. Our results also indicate that the α elements are depleted compared to the known Milky Way populations and that its [Mg/Al] abundance ratio is not consistent with second-generation stars from globular clusters. The detailed chemical pattern of these stars, namely the [α/Fe]–[Fe/H] plane and the metallicity distribution, indicates that the OC stream progenitor is very likely to be a dwarf spheroidal galaxy with a mass of ∼10⁶M_⊙. 

   more » « less
3. In-situ versus accreted Milky Way globular clusters: a new classification method and implications for cluster formation

   https://doi.org/10.1093/mnras/stad3920  

   Belokurov, Vasily; Kravtsov, Andrey (January 2024, Monthly Notices of the Royal Astronomical Society)

   NA (Ed.)

   ABSTRACT We present a new scheme for the classification of the in-situ and accreted globular clusters (GCs). The scheme uses total energy E and z-component of the orbital angular momentum and is calibrated using the [Al/Fe] abundance ratio. We demonstrate that this classification results in two GC populations with distinct spatial, kinematic, and chemical abundance distributions. The in-situ GCs are distributed within the central 10 kpc of the Galaxy in a flattened configuration aligned with the Milky Way (MW) disc, while the accreted GCs have a wide distribution of distances and a spatial distribution close to spherical. In-situ and accreted GCs have different $$\rm [Fe/H]$$ distributions with the well-known bimodality present only in the metallicity distribution of the in-situ GCs. Furthermore, the accreted and in-situ GCs are well separated in the plane of $$\rm [Al/Fe]-[Mg/Fe]$$ abundance ratios and follow distinct sequences in the age–$$\rm [Fe/H]$$ plane. The in-situ GCs in our classification show a clear disc spin-up signature – the increase of median Vϕ at metallicities −1.3 < [Fe/H] < −1 similar to the spin-up in the in-situ field stars. This signature signals the MW’s disc formation, which occurred ≈11.7−12.7 Gyr ago (or at z ≈ 3.1−5.3) according to in-situ GC ages. In-situ GCs with metallicities of $$\rm [Fe/H]\gtrsim -1.3$$ were thus born in the MW disc, while lower metallicity in-situ GCs were born during early, turbulent, pre-disc stages of the evolution of the Galaxy and are part of its Aurora stellar component. 

   more » « less
4. HALO7D. III. Chemical Abundances of Milky Way Halo Stars from Medium-resolution Spectra

   https://doi.org/10.3847/1538-4357/acd33d  

   McKinnon, Kevin A.; Cunningham, Emily C.; Rockosi, Constance M.; Guhathakurta, Puragra; Escala, Ivanna; Kirby, Evan N.; Deason, Alis J. (July 2023, The Astrophysical Journal)

   Abstract The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions (HALO7D) survey measures the kinematics and chemical properties of stars in the Milky Way (MW) stellar halo to learn about the formation of our Galaxy. HALO7D consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope–measured proper motions of MW halo main-sequence turnoff stars in the four Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields. HALO7D consists of deep pencil beams, making it complementary to other contemporary wide-field surveys. We present the [Fe/H] and [α/Fe] abundances for 113 HALO7D stars in the Galactocentric radial range of ∼10–40 kpc along four separate pointings. Using the full 7D chemodynamical data (3D positions, 3D velocities, and abundances) of HALO7D, we measure the velocity anisotropy,β, of the halo velocity ellipsoid for each field and for different metallicity-binned subsamples. We find that two of the four fields have stars on very radial orbits, while the remaining two have stars on more isotropic orbits. Separating the stars into high-, mid-, and low-[Fe/H] bins at −2.2 and −1.1 dex for each field separately, we find differences in the anisotropies between the fields and between the bins; some fields appear dominated by radial orbits in all bins, while other fields show variation between the [Fe/H] bins. These chemodynamical differences are evidence that the HALO7D fields have different fractional contributions from the progenitors that built up the MW stellar halo. Our results highlight the additional information available on smaller spatial scales compared to results from a spherical average of the stellar halo. 

   more » « less
5. Dwarf galaxy archaeology from chemical abundances and star-formation histories

   https://doi.org/10.1093/mnras/stad2985  

   Johnson, James W.; Conroy, Charlie; Johnson, Benjamin D.; Peter, Annika H. G.; Cargile, Phillip A.; Bonaca, Ana; Naidu, Rohan P.; Woody, Turner; Ting, Yuan-Sen; Han, Jiwon Jesse; et al (October 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We model the stellar abundances and ages of two disrupted dwarf galaxies in the Milky Way stellar halo: Gaia-Sausage Enceladus (GSE) and Wukong/LMS-1. Using a statistically robust likelihood function, we fit one-zone models of galactic chemical evolution with exponential infall histories to both systems, deriving e-folding time-scales of τin = 1.01 ± 0.13 Gyr for GSE and $$\tau _\text{in} = 3.08^{+3.19}_{-1.16}$$ Gyr for Wukong/LMS-1. GSE formed stars for $$\tau _\text{tot} = 5.40^{+0.32}_{-0.31}$$ Gyr, sustaining star formation for ∼1.5–2 Gyr after its first infall into the Milky Way ∼10 Gyr ago. Our fit suggests that star formation lasted for $$\tau _\text{tot} = 3.36^{+0.55}_{-0.47}$$ Gyr in Wukong/LMS-1, though our sample does not contain any age measurements. The differences in evolutionary parameters between the two are qualitatively consistent with trends with stellar mass M⋆ predicted by simulations and semi-analytic models of galaxy formation. Our inferred values of the outflow mass-loading factor reasonably match $$\eta \propto M_\star ^{-1/3}$$ as predicted by galactic wind models. Our fitting method is based only on Poisson sampling from an evolutionary track and requires no binning of the data. We demonstrate its accuracy by testing against mock data, showing that it accurately recovers the input model across a broad range of sample sizes (20 ≤ N ≤ 2000) and measurement uncertainties (0.01 ≤ σ[α/Fe], σ[Fe/H] ≤ 0.5; $$0.02 \le \sigma _{\log _{10}(\text{age})} \le 1$$). Due to the generic nature of our derivation, this likelihood function should be applicable to one-zone models of any parametrization and easily extensible to other astrophysical models which predict tracks in some observed space. 

   more » « less