We investigate the progenitors of low-inclination retrograde substructures in the Milky Way (MW) halo, which are remnants of accreted dwarf galaxies on retrograde orbits. Our sample consists of halo stars with low orbital inclinations and eccentricities (0 ≤ e ≤ 0.5), constructed by combining spectroscopic data with Gaia astrometry. We identify substructures using metallicity distribution functions (MDFs) in apogalactic distance-orbital phase space. In the low-eccentricity range (0 ≤ e ≤ 0.3), we find four substructures with MDF peaks at [Fe/H] ≈ -1.5, -1.9, -2.1, and -2.3. In the intermediate-eccentricity range (0.3 < e ≤ 0.5), we identify five substructures that span [Fe/H] ≈ -1.5 to -2.3. By combining chemical and dynamical information, we show that substructures with identical MDF peaks in the two eccentricity regions can either form coherent structures or remain dynamically distinct. This shows that MDF similarity alone is insufficient to uniquely identify progenitor systems and must be combined with dynamical information. We find that the retrograde halo was assembled through multiple accretion events rather than a single progenitor. The dominant contribution arises from a primary progenitor whose debris traces a coherent metallicity-energy sequence, consistent with hierarchical tidal stripping and core bifurcation. In addition, we identify independent progenitors that contribute to other substructures. In particular, the components with [Fe/H] ≈ -1.7 are interpreted as a dual-origin population, likely associated with systems accreted at different epochs. These results highlight the complex, multi-progenitor origin of the retrograde stellar halo of the MW.

We combine narrow/medium-band filter photometry from the Southern Photometric Local Universe Survey (S-PLUS) DR4 with ultrabroadband filter photometry from Gaia EDR3 to derive fundamental stellar parameters (T-eff,log g , [Fe/H], and ages) and elemental-abundance ratios ([C/Fe] and [alpha/Fe]) for 5.4 million stars in the Galaxy (4.9 million dwarfs and 0.5 million giants), as well as for over 0.7 million red giant stars in the Large and Small Magellanic Clouds (LMC and SMC). The precisions of the abundance estimates range from 0.05 to 0.10 dex for metallicity in the relatively metal-rich range ([Fe/H] > -1.0) to 0.10-0.30 dex in the metal-poor regime ([Fe/H] < -1.0), 0.10-0.20 dex for [C/Fe], and 0.05 dex for [alpha/Fe]. The stellar parameters for LMC and SMC member stars are somewhat less precise than those from the S-PLUS main survey, primarily because of the effect of high reddening. The use of both metallicity- and carbon-sensitive filters provides unbiased measurements of both [Fe/H] and [C/Fe], of particular importance for very low-metallicity ([Fe/H] < -2.0) stars, where carbon enhancement can lead to systematically high estimates of [Fe/H] when only a single metallicity-sensitive filter is employed. Furthermore, multiple narrowband filters enable metallicity estimates down to [Fe/H] similar to -4.0 with an accuracy of around 0.3 dex, exceeding the precision typically achieved by low/medium-resolution spectroscopy. This extensive photometric data set, combined with the other three data sets in this series, will serve as a valuable legacy resource for Milky Way and Magellanic Clouds studies.

Recent large-scale multi-band photometric surveys now enable elemental-abundance estimates for millions of stars with accuracies approaching those of low- to medium-resolution spectroscopy. Using [Fe/H] and [C/Fe] estimates derived from the Javalambre Photometric Local Universe Survey (J-PLUS) DR3 and the Southern Photometric Local Universe Survey (S-PLUS) DR4, which together cover ∼6,200 deg^2 of the sky, we identify large numbers of carbon-enhanced metal-poor (CEMP) stars in the Milky Way. After applying data-quality cuts and evolutionary corrections to the carbon-abundance estimates, we construct a combined J/S-PLUS sample of ∼6.40 million stars and identify ∼104,900 CEMP candidates, roughly twice the number of CEMP candidates identified from Gaia XP spectra by Lucey et al. We photometrically confirm that the absolute carbon abundance A(C) separates CEMP stars into two primary groups, CEMP-no and CEMP-s stars, consistent with previous spectroscopic studies. We also recover CEMP morphological Groups I-III in the Yoon-Beers diagram, as well as the recently proposed Group IV, and show that it is statistically distinct even in photometric data. A cumulative frequency analysis confirms that the CEMP fraction increases toward lower metallicity and that CEMP-no stars dominate in the most metal-poor regime. By comparing frequencies with and without Group IV stars, we assess their relation to CEMP-no and CEMP-s stars, and examine CEMP distributions across different Galactic components. The resulting catalog provides a substantial sample for future spectroscopic follow-up, in particular to constrain the likely origin(s) of the Group IV stars.

We present a detailed chemical-abundance analysis of an actinide-boost ( log & varepsilon; (Th/Dy) = -0.74) star, LAMOST J122216.85-063345.2 (J1222), a very metal-poor ([Fe/H] = -2.45) halo star with moderate enhancement in rapid neutron-capture process (r-process) elements ([Eu/Fe] = +0.61). From high-resolution spectra (R similar to 55,000) taken with Gemini-S/GHOST, we determine the abundances for 47 elements, including thorium. The abundance pattern of J1222 is consistent with predicted nucleosynthetic yields from neutron star mergers (NSMs) and black hole-neutron star mergers (BH-NSMs) under specific ejecta conditions. Our kinematic analysis of J1222 indicates that it is a member of the I'itoi substructure. A comparative analysis of J1222 and seven other stars from the literature with similar dynamics to the I'itoi substructure exhibits a broad dispersion in r-process enrichment-spanning nonenhancement ([Eu/Fe] <=+0.3), moderate enhancement (+0.3 < [Eu/Fe] <=+0.7), strong enhancement ([Eu/Fe] > +0.7), and actinide-boost stars (including one additional actinide-boost candidate newly recognized to be associated with I'itoi)-suggesting a complex enrichment history shaped by multiple r-process events and inhomogeneous mixing. After exploring several astrophysical scenarios to explain the observed r-process abundances, we find that NSMs and BH-NSMs were likely the main contributors to the enrichment, while magnetorotational supernovae may have played a secondary role in enriching some light r-process element-rich stars in the I'itoi substructure.

We present a new method for identifying Galactic halo substructures accreted from dwarf galaxies by combining metallicity distribution functions (MDFs) with orbital parameters. Using apogalactic distance-orbital phase space, we assume that the MDF peak of a substructure reflects its progenitor's chemical signature. We test this approach with two Galactic potentials (Stackel and McMillan) and find consistent results. Our sample consists of retrograde halo stars with low orbital inclinations and intermediate eccentricities (0.5 < e <= 0.7), drawn from Sloan Digital Sky Survey and Large sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy combined with Gaia DR3 astrometry. We identify four distinct low-inclination retrograde substructures (LRS 1, LRS 2, LRS 3, LRS 4) with MDF peaks at [Fe/H] = -1.5, -1.7, -1.9, and -2.1, respectively; LRS 3 is newly discovered. Further analysis reveals an additional stream (LRS 2B) with [Fe/H] = -2.3 embedded within LRS 2; the remaining LRS 2 stars (LRS 2A) are associated with Sequoia. LRS 1 is likely linked to Thamnos 2 and Arjuna, and LRS 4 is likely linked to I'itoi. Comparison with the ED-2 stream suggests that LRS 2B is chemically distinct, but high-resolution spectroscopy is required to confirm whether they originate from separate progenitors. Our MDF-based approach demonstrates the utility of chemodynamical space for uncovering halo substructures, while highlighting caveats such as metallicity gradients and redshift evolution of the mass-metallicity relation, which may blur the mapping between MDF peaks and progenitors.

Recent advancements in near-infrared (NIR) spectroscopy have opened new opportunities for studying multiple stellar populations in globular clusters (GCs), particularly for newly discovered clusters in the inner Milky Way. While optical spectroscopy has traditionally played a primary role in detailed chemical abundance studies of GCs, the increasing discovery of GCs in highly reddened environments underscores the need for robust NIR spectroscopic methods. To evaluate the utility of high-resolution NIR spectroscopy for studying multiple stellar populations, we observed six stars in M5, a well-studied halo GC, using the recently commissioned IGRINS-2 spectrograph on the Gemini-North telescope. Our chemical abundance measurements in the NIR wavelength range show good agreement with those derived from high-resolution optical spectroscopy, with minor systematic offsets in elements such as Na and Mg. In addition, the measured chemical abundance ratios clearly reproduce the distinctive patterns of multiple stellar populations, including the Na-O anti-correlation. The ability of NIR spectroscopy to measure C, N, and O abundances with high precision further enhances its utility for studying chemical properties of stars and GCs. Our findings demonstrate that IGRINS-2 and similar instruments have significant potential to advance our understanding of GC formation, stellar chemical evolution, and the evolutionary history of the Milky Way.

We report the discovery of a new subclass of carbon-enhanced metal-poor (CEMP) stars, characterized by high absolute carbon abundances ( A (C) > 7.39) and extremely low metallicity ([Fe/H] ≤ –3.1) but notably lacking enhancements in neutron-capture elements, thus falling under the CEMP-no category. This population emerged from a detailed analysis of low-resolution spectroscopic data obtained from the Sloan Digital Sky Survey and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, where the observed frequency trends with the decreasing metallicity of CEMP- s ( s -process-enhanced) and CEMP-no (no neutron-capture enhanced) stars deviated from established expectations. In contrast to earlier findings, we observe a rise in high- A (C) stars below [Fe/H] = −3.1, which we interpret as a distinct group not accounted for in traditional CEMP classifications. Following the Yoon–Beers group classification, we define these stars as Group IV. Statistical modeling confirms their presence as a separate peak in the A (C) distribution, and available radial velocity data suggest that about 30% of Group IV stars may be binaries, indicating possible binary-related formation mechanisms. This discovery challenges the current CEMP-no star formation pathways and implies the existence of alternative or hybrid enrichment scenarios in the early Universe. High-resolution spectroscopic follow-up of Group IV candidates will be crucial for identifying their progenitors and understanding their evolutionary implications.

The origin of very metal-poor (VMP; [Fe/H] <= -2.0) stars on planar orbits has been the subject of great attention since their first discovery. However, prior to the release of the Gaia BP/RP (XP) spectra and large photometric samples such as SkyMapper, SAGES, J-PLUS, and S-PLUS, most studies have been limited due to their small sample sizes or strong selection effects. Here, we crossmatch photometric metallicities derived from Gaia XP synthetic photometry and geometric distances from Bailer-Jones et al., and select 12,000 VMP stars (1604 dwarfs and 10,396 giants) with available high-quality astrometry. After calculating dynamical parameter estimates using AGAMA, we employ the nonnegative matrix factorization technique to the v phi distribution across bins in Zmax (the maximum height above or below the Galactic plane during the stellar orbit). We find three primary populations of the selected VMP stars: halo, disk system, and the Gaia Sausage/Enceladus structure. The fraction of disk-like stars decreases with increasing Zmax (as expected), although it is still similar to 20% for stars with Zmax similar to 3 kpc. Similar results emerge from the application of the Hayden criterion, which separates stellar populations on the basis of their orbital inclination angles relative to the Galactic plane. We argue that such high fractions of disk-like stars indicate that they are an independent component, rather than originating solely from Galactic building blocks or heating by minor mergers. We suggest that most of these VMP stars are members of the hypothesized "primordial" disk.

Context. Evidence suggests that the Milky Way (MW) underwent a major collision with the Gaia-Sausage/Enceladus (GSE) dwarf galaxy around cosmic noon. While GSE has since been fully disrupted, it brought in ex situ stars and dynamically heated in situ stars into the halo. In addition, the gas-rich merger may have triggered a burst of in situ star formation, potentially giving rise to a chemically distinct stellar component. Aims. We investigated the region of phase space where stars formed during the GSE merger likely reside, and retain distinct chemical and dynamical signatures. Methods. Building on our previous investigation of metallicity ([Fe/H]) and vertical angular momentum (L-Z) distributions, we analysed spectroscopic samples from GALAH, APOGEE, SDSS, and LAMOST, combined with Gaia kinematics. We focused on high proper-motion stars as effective tracers of the phase-space volume likely influenced by the GSE merger. To correct for selection effects, we incorporated metallicity estimates derived from SDSS and SMSS photometry. Results. Our analysis reveals that low-alpha stars with GSE-like kinematics exhibit bimodality in [Na/Fe] and [Al/Fe] at -1.0 less than or similar to[Fe/H] less than or similar to -0.4. One group follows the low light-element abundances of GSE stars, while another exhibits enhanced values. These low-alpha, high-Na stars have eccentric orbits but are more confined to the inner MW. Eos overlaps with a high-eccentricity subset of these stars, implying that it constitutes a smaller structure nested within the broader population. After correcting for sampling biases, we estimated a population ratio of approximately 1:10 between the low-alpha, high-Na stars and the GSE debris. Conclusions. These results suggest that the low-alpha, high-Na stars formed in a compact region, likely fuelled by gas from the GSE progenitor, analogous to clumpy star-forming clouds seen in high-redshift galaxies. Such stars may trace the first sparks of more extensive merger-driven starburst activity.

Chemical anomalies in planet-hosting stars (PHSs) are studied in order to assess how the planetary nature and multiplicity affect the atmospheric chemical abundances of their host stars. We employ APOGEE DR17 to select thin-disk stars of the Milky Way, and crossmatch them with the Kepler Input Catalog to identify confirmed PHSs, which results in 227 PHSs with available chemical abundance ratios for six refractory elements. We also examine an ensemble of stars without planet signals, which are equivalent to the selected PHSs in terms of evolutionary stage and stellar parameters, to correct for Galactic chemical evolution effects, and derive the abundance gradient of refractory elements over the condensation temperature for the PHSs. Using the Galactic chemical evolution corrected abundances, we find that our PHSs do not show a significant difference in abundance slope from the stars without planets. However, when we examine the trends of the refractory elements of PHSs, based on the total number of their planets and their planet types, we find that the PHSs with giant planets are more depleted in refractory elements than those with rocky planets. Among the PHSs with rocky planets, the refractory depletion trends are potentially correlated with the terrestrial planets' radii and multiplicity. In the cases of PHSs with giant planets, sub-Jovian PHSs demonstrate more depleted refractory trends than stars hosting Jovian-mass planets, raising questions on different planetary formation processes for Neptune-like and Jupiter-like planets.

RR Lyrae stars play a central role in tracing phase-space structures within the Milky Way because they are easy to identify, are relatively luminous, and are found in large numbers in the Galactic bulge, disc, and halo. In this work, we present a new set of spectroscopic metallicity calibrations that use the equivalent widths of the CaiiK and Balmer H gamma and H delta lines to calculate metallicity values from low-resolution spectra. This builds on an earlier calibration from Layden by extending the range of equivalent widths which map between CaiiK and the Balmer lines. We have developed the software rrlfe to apply this calibration to spectra in a consistent, reproducible, and extensible manner. This software is open-source and available to the community. The calibration can be updated with additional data sets in the future.

We present stellar parameters and abundances of 13 elements for 18 very metal-poor (VMP; [Fe/H] < –2.0) stars, selected as extremely metal-poor (EMP; [Fe/H] < –3.0) candidates from the Sloan Digital Sky Survey and Large sky Area Multi-Object Fiber Spectroscopic Telescope survey. High-resolution spectroscopic observations were performed using GEMINI-N/GRACES. We find 10 EMP stars among our candidates, and we newly identify three carbon-enhanced metal-poor stars with [Ba/Fe] < 0. Although chemical abundances of our VMP/EMP stars generally follow the overall trend of other Galactic halo stars, there are a few exceptions. One Na-rich star ([Na/Fe] = +1.14) with low [Mg/Fe] suggests a possible chemical connection with second-generation stars in a globular cluster. The progenitor of an extremely Na-poor star ([Na/Fe] = –1.02) with high K- and Ni-abundance ratios may have undergone a distinct nucleosynthesis episode, associated with core-collapse supernovae (SNe) having a high explosion energy. We have also found a Mg-rich star ([Mg/Fe] = +0.73) with slightly enhanced Na and extremely low [Ba/Fe], indicating that its origin is not associated with neutron-capture events. On the other hand, the origin of the lowest Mg abundance ([Mg/Fe] = –0.61) star could be explained by accretion from a dwarf galaxy, or formation in a gas cloud largely polluted by SNe Ia. We have also explored the progenitor masses of our EMP stars by comparing their chemical-abundance patterns with those predicted by Population III SNe models, and find a mass range of 10–26 M ⊙, suggesting that such stars were primarily responsible for the chemical enrichment of the early Milky Way.

We present precise photometric estimates of stellar parameters, including effective temperature, metallicity, luminosity classification, distance, and stellar age, for nearly 26 million stars using the methodology developed in the first paper of this series, based on the stellar colors from the Stellar Abundances and Galactic Evolution Survey (SAGES) Data Release 1 and Gaia Early Data Release 3. The optimal design of stellar-parameter sensitive uv filters by SAGES has enabled us to determine photometric-metallicity estimates down to −3.5, similar to our previous results with the SkyMapper Southern Survey (SMSS), yielding a large sample of over five million metal-poor ([Fe/H] ≤ −1.0) stars and nearly one million very metal-poor ([Fe/H] ≤ −2.0) stars. The typical precision is around 0.1 dex for both dwarf and giant stars with [Fe/H] > −1.0, and 0.15–0.25/0.3–0.4 dex for dwarf/giant stars with [Fe/H] < −1.0. Using the precise parallax measurements and stellar colors from Gaia, effective temperature, luminosity classification, distance, and stellar age are further derived for our sample stars. This huge data set in the Northern sky from SAGES, together with similar data in the Southern sky from SMSS, will greatly advance our understanding of the Milky Way, in particular its formation and evolution.

최근 OpenAI사의 ChatGPT가 뜨거운 이슈로 부상하고 있다. ChatGPT는 자연언어를 사용하는 대화전문 인공지능 챗봇으로, OpenAI사의 언어기술인 GPT-3.5 모델에 기반하여 인간과 자연스러운 대화를 주고 받을 수 있는 챗봇이다. 2023년 2월 현재 ChatGPT는 세계에서 가장 진보한 챗봇으로 알려져 있다. 한편, 코로나가 한창이던 2020년 11월 정부는 “인공지능시대 교육정책방향과 핵심과제”를 제시하였다(관계부처 합동, 2020). 이 보고서는 인공지능 기술의 발전이 사회에 미치는 영향과 변화 속에서 대한민국 교육이 나아가야 할 방향과 방법을 제시한다. 본 논문에서는 정부가 제시한 AI 시대 교육의 비전에 대하여 ChatGPT 시대의 관점에서 비판적으로 고찰하고 대안을 제시하였다. 본 논문은 포지션 페이퍼로, 논쟁의 여지가 있는 이슈에 대하여 연구자의 의견을 제시하고 설득하기 위한 목적으로 쓰여졌다. 본 논문은 다음과 같은 순서로 논의가 이루어졌다. 우선, 정부가 출판한 보고서를 바탕으로 정부가 제시한 AI교육의 비전을 분석하였다. 이어서 정부가 제시한 AI교육의 비전의 문제점을 구성주의적 관점에서 비판하고, 이에 대한 대안으로서의 AI교육을 제시하였다. 본 연구를 통해 주장하는 바는 다음과 같이 요약된다. 1) AI를 활용한 맞춤형 교육은 입시위주, 주입식 교육을 강화하는 방법이다. 2) AI 교육으로 성취하고자 하는 인재상은 AI를 인지적 도구로 활용할 때에 가능하다.

The second generation of stars in the GCs of the MW exhibit unusually high N, Na, or Al, compared to typical Galactic halo stars at similar metallicities. The halo field stars enhanced with such elements are believed to have originated in disrupted GCs or escaped from existing GCs. We identify such stars in the metallicity range -3.0 < [Fe/H] < 0.0 from a sample of ~ 36,800 giant stars observed in the SDSS and LAMOST survey, and present their dynamical properties. The N-rich population and N-normal population among our giant sample do not exhibit similarities in either in their metallicity distribution function or dynamical properties. We find that, even though the MDF of the NRP looks similar to that of the MW's GCs in the range of [Fe/H] < -1.0, our analysis of the dynamical properties does not indicate similarities between them in the same metallicity range, implying that the escaped members from existing GCs may account for a small fraction of our N-rich stars, or the orbits of the present GCs have been altered by the dynamical friction of the MW. We also find a significant increase in the fraction of N-rich stars in the halo field in the very metal-poor (VMP; [Fe/H] < -2.0) regime, comprising up to ~ 20% of the fraction of the N-rich stars below [Fe/H] = -2.5, hinting that partially or fully destroyed VMP GCs may have in some degree contributed to the Galactic halo. A more detailed dynamical analysis of the NRP reveals that our sample of N-rich stars do not share a single common origin. Although a substantial fraction of the N-rich stars seem to originate from the GCs formed in situ, more than 60% of them are not associated with those of typical Galactic populations, but probably have extragalactic origins associated with GSE, Sequoia, and Sagittarius dwarf galaxies, as well as with presently unrecognized progenitors.

In this work, we study the phase-space and chemical properties of the Sagittarius (Sgr) stream, the tidal tails produced by the ongoing destruction of the Sgr dwarf spheroidal (dSph) galaxy, focusing on its very metal-poor (VMP; [Fe/H] < −2) content. We combine spectroscopic and astrometric information from SEGUE and Gaia EDR3, respectively, with data products from a new large-scale run of the StarHorse spectrophotometric code. Our selection criteria yield ∼1600 stream members, including >200 VMP stars. We find the leading arm ( b > 0°) of the Sgr stream to be more metal-poor, by ∼0.2 dex, than the trailing one ( b < 0°). With a subsample of turnoff and subgiant stars, we estimate this substructure’s stellar population to be ∼1 Gyr older than the thick disk’s. With the aid of an N -body model of the Sgr system, we verify that simulated particles stripped earlier (>2 Gyr ago) have present-day phase-space properties similar to lower metallicity stream stars. Conversely, those stripped more recently (<2 Gyr) are preferentially akin to metal-rich ([Fe/H] > −1) members of the stream. Such correlation between kinematics and chemistry can be explained by the existence of a dynamically hotter, less centrally concentrated, and more metal-poor population in Sgr dSph prior to its disruption, implying that this galaxy was able to develop a metallicity gradient before its accretion. Finally, we identified several carbon-enhanced metal-poor ([C/Fe] > +0.7 and [Fe/H] ≤ −1.5) stars in the Sgr stream, which might be in tension with current observations of its remaining core where such objects are not found.

We present precise photometric estimates of stellar parameters, including effective temperature, metallicity, luminosity classification, distance, and stellar age, for nearly 26 million stars using the methodology developed in the first paper of this series, based on the stellar colors from the Stellar Abundances and Galactic Evolution Survey (SAGES) DR1 and Gaia EDR3. The optimal design of stellar-parameter sensitive uv filters by SAGES has enabled us to determine photometric-metallicity estimates down to -3.5, similar to our previous results with the SkyMapper Southern Survey (SMSS), yielding a large sample of over five million metal-poor (MP; [Fe/H]≤ -1.0) stars and nearly one million very metal-poor (VMP; [Fe/H]≤ -2.0) stars. The typical precision is around 0.1 dex for both dwarf and giant stars with [Fe/H]>-1.0, and 0.15-0.25/0.3-0.4 dex for dwarf/giant stars with [Fe/H]<-1.0. Using the precise parallax measurements and stellar colors from Gaia, effective temperature, luminosity classification, distance and stellar age are further derived for our sample stars. This huge data set in the Northern sky from SAGES, together with similar data in the Southern sky from SMSS, will greatly advance our understanding of the Milky Way, in particular its formation and evolution.

We present a chemical and dynamical analysis of the leading arm (LA) and trailing arm (TA) of the Sagittarius (Sgr) stream, as well as for the Sgr dwarf galaxy core (SC), using red giant branch, main-sequence, and RR Lyrae stars from large spectroscopic survey data. The different chemical properties among the LA, TA, and SC generally agree with recent studies and can be understood by a radial metallicity gradient established in the progenitor of the Sgr dwarf, followed by preferential stellar stripping from the outer part of the Sgr progenitor. One striking finding is a relatively larger fraction of low-eccentricity stars ( e < 0.4) in the LA than in the TA and SC. The TA and SC exhibit very similar distributions. Considering that a tidal tail stripped off from a dwarf galaxy maintains the orbital properties of its progenitor, we expect that the e -distribution of the LA should be similar to that of the TA and SC. Thus, the disparate behavior of the e -distribution of the LA is of particular interest. Following the analysis of Vasiliev et al., we attempt to explain the different e -distribution by introducing a time-dependent perturbation of the Milky Way by the Large Magellanic Cloud's (LMC) gravitational pull, resulting in substantial evolution of the angular momentum of the LA stars to produce the low- e stars. In addition, we confirm from RR Lyrae stars with high eccentricity ( e > 0.6) that the TA stars farther away from the SC are also affected by disturbances from the LMC.

We report the result of a high-resolution spectroscopic study on seven magnesium (Mg) enhanced stars. The high Mg abundances in these stars imply that they were born in an environment heavily affected by the nucleosynthesis products of massive stars. We measure abundances of 16 elements including Mg and they show various abundance patterns implying their diverse origin. Three of our program stars show a very high Mg to Si ratio ([Mg/Si] asymptotic to 0.18-0.25), which might be well explained by fall-back supernovae or by supernovae with rapid rotating progenitors having an initial mass higher than about 20 M-?. Another three of our program stars have high light to heavy s-process element ratios ([Y/Ba] asymptotic to 0.30-0.44), which are consistent with the theoretical prediction of the nucleosynthesis in rapidly rotating massive stars with an initial mass of about M = 40 Mo. We also report a star having both high Y ([Y/Fe] = 0.2) and Ba ([Ba/Fe] = 0.28) abundance ratios, and it also shows the highest Zn abundance ratio ([Zn/Fe] = 0.27) among our sample, implying the nucleosynthesis by asymmetric supernova explosion induced by very rapid rotation of a massive progenitor having an initial mass between 20 M-? <= M <= 40 M-?. A relative deficiency of odd-number elements, which would be a signature of the pair-instability nucleosynthesis, is not found in our sample.

We continue our series of papers on phase-space distributions of stars in the Milky Way based on photometrically derived metallicities and Gaia astrometry, with a focus on the halo−disk interface in the local volume. To exploit various photometric databases, we develop a method of empirically calibrating synthetic stellar spectra based on a comparison with observations of stellar sequences and individual stars in the Sloan Digital Sky Survey, the SkyMapper Sky Survey, and the Pan-STARRS1 surveys, overcoming band-specific corrections employed in our previous work. In addition, photometric zero-point corrections are derived to provide an internally consistent photometric system with a spatially uniform metallicity zero-point. Using our phase-space diagrams, we find a remarkably narrow sequence in the rotational velocity (v ϕ ) versus metallicity ([Fe/H]) space for a sample of high proper-motion stars (>25 mas yr−1), which runs along Gaia Sausage/Enceladus (GSE) and the Splash substructures and is linked to the disk, spanning nearly 2 dex in [Fe/H]. Notably, a rapid increase of v ϕ from a nearly zero net rotation to ∼180 km s−1 in a narrow metallicity interval (−0.6 ≲ [Fe/H] ≲ −0.4) suggests that some of these stars emerged quickly on a short gas-depletion timescale. Through measurements of a scale height and length, we argue that these stars are distinct from those heated dynamically by mergers. This chain of high proper-motion stars provides additional support for recent discoveries suggesting that a starburst took place when the young Milky Way encountered the gas-rich GSE progenitor, which eventually led to the settling of metal-enriched gas onto the disk.