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1. The Milky Way tomography with Subaru Hyper Suprime-Cam. I. Halo substructures

   https://doi.org/10.1093/pasj/psae003  

   Suzuki, Yoshihisa ; Chiba, Masashi ; Komiyama, Yutaka ; Hayashi, Kohei ; Tanaka, Masayuki ; Fukushima, Tetsuya ; Carlsten, Scott G. ; Tokiwa, Akira ; Qiu, Tian ; Takada, Masahiro ( January 2024 , Publications of the Astronomical Society of Japan)

   We analyze the photometric data in the Wide layer of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) over ∼1200 deg2 to uncover new halo substructures beyond the distance, D⊙ ∼ 30 kpc, from the Sun. For this purpose, we develop an isochrone filter for an old, metal-poor stellar system to extract the faint main-sequence stars at a range of distances. With this method, we detect not only the previously discovered substructures such as the Orphan Stream, but also a new overdensity toward Boötes at about D⊙ ∼ 60 kpc and a new stream-like feature toward Pisces at around D⊙ ∼ 60 kpc. It has been suggested that a small-scale overdensity exists in this direction of Pisces (the so-called Pisces Overdensity), but our results show that the overdensity is widely spread with a tidally elongated feature. Combining our results with the ongoing Hyper Suprime-Cam narrow-band survey and the near-future spectroscopic survey with Prime Focus Spectrograph (PFS) will allow us to place strong constraints on the origin of these halo substructures.

    

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2. RR Lyrae Mid-infrared Period–Luminosity–Metallicity and Period–Wesenheit–Metallicity Relations Based on Gaia DR3 Parallaxes

   https://doi.org/10.3847/1538-4357/acb20a  

   Mullen, Joseph P. ; Marengo, Massimo ; Martínez-Vázquez, Clara E. ; Chaboyer, Brian ; Bono, Giuseppe ; Braga, Vittorio F. ; Dall’Ora, Massimo ; D’Orazi, Valentina ; Fabrizio, Michele ; Monelli, Matteo ; et al ( March 2023 , The Astrophysical Journal)

   We present new empirical infrared period–luminosity–metallicity (PLZ) and period–Wesenheit–metallicity (PWZ) relations for RR Lyae based on the latest Gaia Early Data Release 3 (EDR3) parallaxes. The relations are provided in the Wide-field Infrared Survey Explorer (WISE) W1 and W2 bands, as well as in the W(W1,V− W1) and W(W2,V− W2) Wesenheit magnitudes. The relations are calibrated using a very large sample of Galactic halo field RR Lyrae stars with homogeneous spectroscopic [Fe/H] abundances (over 1000 stars in the W1 band), covering a broad range of metallicities (−2.5 ≲ [Fe/H] ≲ 0.0). We test the performance of our PLZ and PWZ relations by determining the distance moduli of both galactic and extragalactic stellar associations: the Sculptor dwarf spheroidal galaxy in the Local Group (finding${\overline{\mu }}_0=19.47\pm 0.06$), the Galactic globular clusters M4 (${\overline{\mu }}_0=11.16\pm 0.05$), and the Reticulum globular cluster in the Large Magellanic Cloud (${\overline{\mu }}_0=18.23\pm 0.06$). The distance moduli determined through all our relations are internally self-consistent (within ≲0.05 mag) but are systematically smaller (by ∼2–3σ) than previous literature measurements taken from a variety of methods/anchors. However, a comparison with similar recent RR Lyrae empirical relations anchored with EDR3 likewise shows, to varying extents, a systematically smaller distance modulus for PLZ/PWZ RR Lyrae relations.

    

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3. Exploring the origin of the distance bimodality of stars in the periphery of the Small Magellanic Cloud with APOGEE and Gaia

   https://doi.org/10.1093/mnras/stae373  

   Almeida, Andres ; Majewski, Steven R. ; Nidever, David L. ; Olsen, Knut A. G. ; Monachesi, Antonela ; Kallivayalil, Nitya ; Hasselquist, Sten ; Choi, Yumi ; Povick, Joshua T. ; Wilson, John C. ; et al ( February 2024 , Monthly Notices of the Royal Astronomical Society)

   The Magellanic Cloud system represents a unique laboratory for study of both interacting dwarf galaxies and the ongoing process of the formation of the Milky Way and its halo. We focus on one aspect of this complex, three-body interaction – the dynamical perturbation of the Small Magellanic Cloud (SMC) by the Large Magellanic Cloud (LMC), and specifically potential tidal effects on the SMC’s eastern side. Using Gaia astrometry and the precise radial velocities (RVs) and multielement chemical abundances from Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) Data Release 17, we explore the well-known distance bimodality on the eastern side of the SMC. Through estimated stellar distances, proper motions, and RVs, we characterize the kinematics of the two populations in the bimodality and compare their properties with those of SMC populations elsewhere. Moreover, while all regions explored by APOGEE seem to show a single chemical enrichment history, the metallicity distribution function (MDF), of the ‘far’ stars on the eastern periphery of the SMC is found to resemble that for the more metal-poor fields of the western periphery, whereas the MDF for the ‘near’ stars on the eastern periphery resembles that for stars in the SMC Centre. The closer eastern periphery stars also show RVs (corrected for SMC rotation and bulk motion) that are, on average, approaching us relative to all other SMC populations sampled. We interpret these trends as evidence that the near stars on the eastern side of the SMC represent material pulled out of the central SMC as part of its tidal interaction with the LMC.

    

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4. RR Lyrae stars as probes of the outer Galactic halo: chemical and kinematic analysis of a pilot sample

   https://doi.org/10.1093/mnras/stac3800  

   Medina, Gustavo E. ; Hansen, Camilla J. ; Muñoz, Ricardo R. ; Grebel, Eva K. ; Vivas, A. Katherina ; Carlin, Jeffrey L. ; Martínez-Vázquez, Clara E. ( December 2022 , Monthly Notices of the Royal Astronomical Society)

   We report the spectroscopic analysis of 20 halo ab-type RR Lyrae stars with heliocentric distances between 15 and 165 kpc, conducted using medium-resolution spectra from the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We obtain the systemic line-of-sight velocities of our targets with typical uncertainties of 5–10 km s−1 and compute orbital parameters for a subsample out to 50 kpc from the Galactic centre, including proper motion data from Gaia DR3. The orientation of our stars’ orbits, determined for an isolated Milky Way and for a model perturbed by the Large Magellanic Cloud, appears to suggest an accreted origin for at least half of the sample. In addition, we derive atmospheric parameters and chemical abundance ratios for seven stars beyond 20 kpc. The derived α-abundances of five of these stars follow a Milky Way halo-like trend, while the other two display an underabundance of α-elements for their [Fe/H], indicating an association with accretion events. Furthermore, based on the [Sr/Ba] ratio, we can speculate about the conditions for the formation of a potential chemically peculiar carbon-enhanced metal-poor (CEMP) RR Lyrae star. By analysing the stars’ orbital parameters and abundance ratios, we find hints of association of two of our stars with two massive satellites, namely the Large Magellanic Cloud and Sagittarius. Overall, our results are in line with the suggestion that the accretion of sub-haloes largely contributes to the outer halo stellar populations.

    

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5. The bursty origin of the Milky Way thick disc

   https://doi.org/10.1093/mnras/stab1339  

   Yu, Sijie ; Bullock, James S ; Klein, Courtney ; Stern, Jonathan ; Wetzel, Andrew ; Ma, Xiangcheng ; Moreno, Jorge ; Hafen, Zachary ; Gurvich, Alexander B ; Hopkins, Philip F ; et al ( May 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We investigate thin and thick stellar disc formation in Milky Way-mass galaxies using 12 FIRE-2 cosmological zoom-in simulations. All simulated galaxies experience an early period of bursty star formation that transitions to a late-time steady phase of near-constant star formation. Stars formed during the late-time steady phase have more circular orbits and thin-disc-like morphology at z = 0, while stars born during the bursty phase have more radial orbits and thick-disc structure. The median age of thick-disc stars at z = 0 correlates strongly with this transition time. We also find that galaxies with an earlier transition from bursty to steady star formation have a higher thin-disc fractions at z = 0. Three of our systems have minor mergers with Large Magellanic Cloud-size satellites during the thin-disc phase. These mergers trigger short starbursts but do not destroy the thin disc nor alter broad trends between the star formation transition time and thin/thick-disc properties. If our simulations are representative of the Universe, then stellar archaeological studies of the Milky Way (or M31) provide a window into past star formation modes in the Galaxy. Current age estimates of the Galactic thick disc would suggest that the Milky Way transitioned from bursty to steady phase ∼6.5 Gyr ago; prior to that time the Milky Way likely lacked a recognizable thin disc. 

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