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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Globular Cluster Candidates in the Sagittarius Dwarf Galaxy

Recently, new Sagittarius (Sgr) dwarf-galaxy globular clusters were discovered, which opens the question of the actual size of the Sgr globular cluster population, and therefore on our understanding of the Sgr galaxy formation and accretion history of the Milky Way. Based on Gaia EDR3 and SDSS IV DR16 (APOGEE-2) data sets, we performed an analysis of the color–magnitude diagrams (CMDs) of the eight new Sgr globular clusters found by Minniti et al. from a sound cleaning of the contamination of Milky Way and Sgr field stars, complemented by available kinematic and metal abundance information. The cleaned CMDs and spatial stellar distibutions reveal the presence of stars with a wide range of cluster membership probabilities. Minni 332 turned out to be a younger (<9 Gyr) and more metal-rich ([M/H] ≳ -1.0 dex) globular cluster than M54, the nuclear Sgr globular cluster; as could also be the case of Minni 342, 348, and 349, although their results are less convincing. Minni 341 could be an open cluster candidate (age < 1 Gyr, [M/H] ~ -0.3 dex), while the analyses of Minni 335, 343, and 344 did not allow us to confirm their physical reality. We also built the Sgr cluster frequency (CF) using available ages of the Sgr globular clusters and compared it with that obtained from the Sgr star formation history. Both CFs are in excellent agreement. However, the addition of eight new globular clusters with ages and metallicities distributed according to the Sgr age–metallicity relationship turns out in a remarkably different CF.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

CLASSY III. The Properties of Starburst-driven Warm Ionized Outflows

We report the results of analyses of galactic outflows in a sample of 45 low-redshift starburst galaxies in the COS Legacy Archive Spectroscopic SurveY (CLASSY), augmented by five additional similar starbursts with Cosmic Origins Spectrograph (COS) data. The outflows are traced by blueshifted absorption lines of metals spanning a wide range of ionization potential. The high quality and broad spectral coverage of CLASSY data enable us to disentangle the absorption due to the static interstellar medium (ISM) from that due to outflows. We further use different line multiplets and doublets to determine the covering fraction, column density, and ionization state as a function of velocity for each outflow. We measure the outflow's mean velocity and velocity width, and find that both correlate in a highly significant way with the star formation rate, galaxy mass, and circular velocity over ranges of four orders of magnitude for the first two properties. We also estimate outflow rates of metals, mass, momentum, and kinetic energy. We find that, at most, only about 20% of silicon created and ejected by supernovae in the starburst is carried out in the warm phase we observe. The outflows' mass-loading factor increases steeply and inversely with both circular and outflow velocity (log–log slope ~–1.6), and reaches ~10 for dwarf galaxies. We find that the outflows typically carry about 10%–100% of the momentum injected by massive stars and about 1%–20% of the kinetic energy. We show that these results place interesting constraints on, and new insights into, models and simulations of galactic winds.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

A Census of the 32 Ori Association with Gaia*

I have used high-precision photometry and astrometry from the third data release of Gaia (DR3) to identify candidate members of the 32 Ori association. Spectral types and radial velocities have been measured for subsets of the candidates using new and archival spectra. For the candidates that have radial velocity measurements, I have used UVW velocities to further constrain their membership, arriving at a final catalog of 169 candidates. I estimate that the completeness of the survey is ~90% for spectral types of ≲M7 (≳0.06 M ⊙ ). The histogram of spectral types for the 32 Ori candidates exhibits a maximum at M5 (~0.15 M ⊙ ), resembling the distributions measured for other young clusters and associations in the solar neighborhood. The available UVW velocities indicate that the association is expanding, but they do not produce a well-defined kinematic age. Based on their sequences of low-mass stars in color–magnitude diagrams, the 32 Ori association and Upper Centaurus-Lupus/Lower Centaurus-Crux (UCL/LCC) are coeval to within ±1.2 Myr, and they are younger than the β Pic moving group by ~3 Myr, which agrees with results from previous analysis based on the second data release of Gaia. Finally, I have used mid-IR photometry from the Wide-field Infrared Survey Explorer to check for excess emission from circumstellar disks among the 32 Ori candidates. Disks are detected for 18 candidates, half of which are reported for the first time in this work. The fraction of candidates at ≤M6 that have full, transitional, or evolved disks is 10/149 = 0.07$^{+0.03}_{-0.02'}$ which is consistent with the value for UCL/LCC.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Nearby stellar substructures in the Galactic halo from DESI Milky Way Survey Year 1 Data Release

We report five nearby ($d_{\mathrm{helio}} < 5$ kpc) stellar substructures in the Galactic halo from a subset of 138 661 stars in the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey Year 1 Data Release. With an unsupervised clustering algorithm, HDBSCAN*, these substructures are independently identified in Integrals of Motion ($E_{\rm tot}$, $L_{\rm z}$, $\log {J_r}$, $\log {J_z}$) space and Galactocentric cylindrical velocity space ($V_{R}$, $V_{\phi }$, $V_{z}$). We associate all identified clusters with known nearby substructures (Helmi streams, M18-Cand10/MMH-1, Sequoia, Antaeus, and ED-2) previously reported in various studies. With metallicities precisely measured by DESI, we confirm that the Helmi streams, M18-Cand10, and ED-2 are chemically distinct from local halo stars. We have characterized the chemodynamic properties of each dynamic group, including their metallicity dispersions, to associate them with their progenitor types (globular cluster or dwarf galaxy). Our approach for searching substructures with HDBSCAN* reliably detects real substructures in the Galactic halo, suggesting that applying the same method can lead to the discovery of new substructures in future DESI data. With more stars from future DESI data releases and improved astrometry from the upcoming Gaia Data Release 4, we will have a more detailed blueprint of the Galactic halo, offering a significant improvement in our understanding of the formation and evolutionary history of the Milky Way Galaxy.

dynamics↗
DOE OSTI2025

A Pride of Satellites in the Constellation Leo? Discovery of the Leo VI Milky Way Satellite Ultra-faint Dwarf Galaxy with DELVE Early Data Release 3

Abstract We report the discovery and spectroscopic confirmation of an ultra-faint Milky Way satellite in the constellation of Leo. This system was discovered as a spatial overdensity of resolved stars observed with Dark Energy Camera (DECam) data from an early version of the third data release of the DECam Local Volume Exploration (or DELVE) survey. The low luminosity ( M V = − 3.5 6 − 0.37 + 0.47 ; L V = 230 0 − 700 + 1200 L ⊙ ), large size ( R 1 / 2 = 9 0 − 30 + 30 pc), and large heliocentric distance ( D = 11 1 − 6 + 9 kpc) are all consistent with the population of ultra-faint dwarf galaxies (UFDs). Using Keck/DEIMOS observations of the system, we were able to spectroscopically confirm nine member stars, while measuring a tentative mass-to-light ratio of 70 0 − 500 + 1400 M ⊙ / L ⊙ and a nonzero metallicity dispersion of σ [ Fe / H ] = 0.1 9 − 0.11 + 0.14 , further confirming Leo VI’s identity as a UFD. While the system has a highly elliptical shape, ϵ = 0.5 4 − 0.29 + 0.19 , we do not find any conclusive evidence that it is tidally disrupting. Moreover, despite the apparent on-sky proximity of Leo VI to members of the proposed Crater-Leo infall group, its smaller heliocentric distance and inconsistent position in energy–angular momentum space make it unlikely that Leo VI is part of the proposed infall group.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Ultraviolet and X-ray properties of Coma’s ultra-diffuse galaxies

ABSTRACT Many ultra-diffuse galaxies (UDGs) have been discovered in the Coma cluster, and there is evidence that some, notably Dragonfly 44, have Milky Way-like dynamical masses despite dwarf-like stellar masses. We used X-ray, ultraviolet (UV), and optical data to investigate the star formation and nuclear activity in the Coma UDGs, and we obtained deep UV and X-ray data (Swift and XMM–Newton) for Dragonfly 44 to search for low-level star formation, hot circumgalactic gas, and the integrated emission from X-ray binaries. Among the Coma UDGs, we find UV luminosities consistent with quiescence but NUV − r colours indicating star formation in the past Gyr. This indicates that the UDGs were recently quenched. The r-band luminosity declines with projected distance from the Coma core. The Dragonfly 44 UV luminosity is also consistent with quiescence, with SFR$\lt 6\times 10^{-4} \, \mathrm{M}_{\odot }$ yr−1, and no X-rays are detected down to a sensitivity of 1038 erg s−1. This rules out a hot corona with a $M \gt 10^8 \, \mathrm{M}_{\odot }$ within the virial radius, which would normally be expected for a dynamically massive galaxy. The absence of bright, low-mass X-ray binaries is consistent with the expectation from the Galaxy total stellar mass, but it is unlikely if most low-mass X-ray binaries form in globular clusters, as Dragonfly 44 has a very large population. Based on the UV and X-ray analysis, the Coma UDGs are consistent with quenched dwarf galaxies, although we cannot rule out a dynamically massive population.

Lee, Chris H.↗
DOE OSTI2021

TOI-1231 b: A Temperate, Neptune-sized Planet Transiting the Nearby M3 Dwarf NLTT 24399

We report the discovery of a transiting, temperate, Neptune-sized exoplanet orbiting the nearby (d = 27.5 pc), M3V star TOI-1231 (NLTT 24399, L 248-27, 2MASS J10265947-5228099). The planet was detected using photometric data from the Transiting Exoplanet Survey Satellite and followed up with observations from the Las Cumbres Observatory and the Antarctica Search for Transiting ExoPlanets program. Combining the photometric data sets, we find that the newly discovered planet has a radius of 3.65{sub −0.15}{sup +0.16} R{sub ⊕} and an orbital period of 24.246 days. Radial velocity measurements obtained with the Planet Finder Spectrograph on the Magellan Clay telescope confirm the existence of the planet and lead to a mass measurement of 15.5 ± 3.3 M {sub ⊕}. With an equilibrium temperature of just 330 K, TOI-1231 b is one of the coolest small planets accessible for atmospheric studies thus far, and its host star’s bright near-infrared brightness (J = 8.88, K {sub s} = 8.07) makes it an exciting target for the Hubble Space Telescope and the James Webb Space Telescope. Future atmospheric observations would enable the first comparative planetology efforts in the 250–350 K temperature regime via comparisons with K2-18 b. Furthermore, TOI-1231's high systemic radial velocity (70.5 km s{sup −1}) may allow for the detection of low-velocity hydrogen atoms escaping the planet by Doppler, shifting the H i Lyα stellar emission away from the geocoronal and interstellar medium absorption features.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Discovery and characterization of five new eclipsing AM CVn systems

ABSTRACT AM CVn systems are ultra-compact, hydrogen-depleted, and helium-rich, accreting binaries with degenerate or semidegenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 min. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) light curves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra show double-peaked H e lines but also show metals, including K and Zn, elements that have never been detected in AM CVn systems before. By modelling the high-speed photometry, we measured the mass and radius of the donor star, potentially constraining the evolutionary channel that formed these AM CVn systems. We determined that the average mass of the accreting white dwarf is ≈0.8 M⊙, and that the white dwarfs in long-period systems are hotter than predicted by recently updated theoretical models. The donors have a high entropy and are a factor of ≈2 more massive compared to zero-entropy donors at the same orbital period. The large donor radius is most consistent with H e-star progenitors, although the observed spectral features seem to contradict this. The discovery of five new eclipsing AM CVn systems is consistent with the known observed AM CVn space density and estimated ZTF recovery efficiency.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

The Cocytos Stream: A Disrupted Globular Cluster from our Last Major Merger?

The census of stellar streams and dwarf galaxies in the Milky Way provides direct constraints on galaxy formation models and the nature of dark matter. The DESI Milky Way survey (with a footprint of 14,000$~deg{^2}$ and a depth of $r<19$ mag) delivers the largest sample of distant metal-poor stars compared to previous optical fiber-fed spectroscopic surveys. This makes DESI an ideal survey to search for previously undetected streams and dwarf galaxies. We present a detailed characterization of the Cocytos stream, which was re-discovered using a clustering analysis with a catalog of giants in the DESI year 3 data, supplemented with Magellan/MagE spectroscopy. Our analysis reveals a relatively metal-rich ([Fe/H]$=-1.3$) and thick stream (width$=1.5^\circ$) at a heliocentric distance of $\approx 25$ kpc, with an internal velocity dispersion of 6.5-9 km s$^{-1}$. The stream's metallicity, radial orbit, and proximity to the Virgo stellar overdensities suggest that it is most likely a disrupted globular cluster that came in with the Gaia-Enceladus merger. We also confirm its association with the Pyxis globular cluster. Our result showcases the ability of wide-field spectroscopic surveys to kinematically discover faint disrupted dwarfs and clusters, enabling constraints on the dark matter distribution in the Milky Way.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

A prescription for the asteroseismic surface correction

In asteroseismology, the surface effect refers to a disparity between the observed and the modelled frequencies in stars with solar-like oscillations. It originates from improper modelling of the surface layers. Correcting the surface effect usually requires using functions with free parameters, which are conventionally fitted to the observed frequencies. On the basis that the correction should vary smoothly across the H–R diagram, we parameterize it as a simple function of surface gravity, effective temperature, and metallicity. We determine this function by fitting a wide range of stars. The absolute amount of the surface correction decreases with luminosity, but the ratio between it and νmax increases, suggesting the surface effect is more important for red giants than dwarfs. Applying the prescription can eliminate unrealistic surface correction, which improves parameter estimations with stellar modelling. Using two open clusters, we found a reduction of scatter in the model-derived ages for each star in the same cluster. As an important application, we provide a new revision for the Δν scaling relation that, for the first time, accounts for the surface correction. The values of the correction factor, f Δν , are up to 2 percent smaller than those determined without the surface effect considered, suggesting decreases of up to 4 percent in radii and up to 8 percent in masses when using the asteroseismic scaling relations. This revision brings the asteroseismic properties into an agreement with those determined from eclipsing binaries. The new correction factor and the stellar models with the corrected frequencies are available at https://www.github.com/parallelpro/surface.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Probing for the host galaxies of the fast X-ray transients XRT 000519 and XRT 110103

Over the past few years, ~30 extragalactic fast X-ray transients (FXRTs) have been discovered, mainly in Chandra and XMM-Newton data. Their nature remains unclear, with proposed origins, including a double neutron star merger, a tidal disruption event involving an intermediate-mass black hole and a white dwarf, or a supernova shock breakout. A decisive differentiation between these three promising mechanisms for their origin requires an understanding of the FXRT energetics, environments, and/or host properties. We present optical observations obtained with the Very Large Telescope for the FXRTs XRT 000519 and XRT 110103 and Gran Telescopio Canarias observations for XRT 000519 designed to search for host galaxies of these FXRTs. In the g s , r s , and R-band images, we detect an extended source on the north-west side of the $\sim \, 1^{\prime \prime }$ (68 per cent confidence) error circle of the X-ray position of XRT 000519 with a Kron magnitude of g s = 26.29 ± 0.09 (AB magnitude). We discuss the XRT 000519 association with the probable host candidate for various possible distances, and we conclude that if XRT 000519 is associated with the host candidate a supernova shock breakout scenario is likely excluded. No host galaxy is found near XRT 110103 down to a limiting magnitude of R > 25.8.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Solar Neighborhood. XLVII. Comparing M-dwarf Models with Hubble Space Telescope Dynamical Masses and Spectroscopy

We use HST/STIS optical spectroscopy of 10 M dwarfs in five closely separated binary systems to test models of M-dwarf structure and evolution. Individual dynamical masses ranging from 0.083 to 0.405 M {sub ⊙} for all stars are known from previous work. We first derive temperature, radius, luminosity, surface gravity, and metallicity by fitting the BT-Settl atmospheric models. We verify that our methodology agrees with empirical results from long-baseline optical interferometry for stars of similar spectral types. We then test whether or not evolutionary models can predict those quantities given the stars’ known dynamical masses and the conditions of coevality and equal metallicity within each binary system. We apply this test to five different evolutionary model sets: the Dartmouth models, the MESA/MIST models, the models of Baraffe et al., the PARSEC models, and the YaPSI models. We find marginal agreement between evolutionary model predictions and observations, with few cases where the models respect the condition of coevality in a self-consistent manner. We discuss the pros and cons of each family of models and compare their predictive power.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Magellan/IMACS Spectroscopy of Grus I: A Low Metallicity Ultra-faint Dwarf Galaxy

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 $\langle$[Fe/ H]$\rangle$ = -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 -1 and a velocity dispersion of σ rv = $2.5_{-0.8}^{+1.3}$ km s -1 , which results in a dynamical mass of M 1/2 (r h ) = $8^{+12}_{-4}$ x 10 5 M ⊙ and a mass-to-light ratio of M/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 > 80 M ⊙ /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 (ρ 1/2 ~ $3.5^{+5.7}_{-2.7}$ x 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.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

The metallicity distribution in the core of the Sagittarius dwarf spheroidal: Minimising the metallicity biases

We present the metallicity and radial velocity for 450 bona fide members of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy, measured from high-resolution spectra (R ≃ 18 000) obtained with FLAMES at the VLT. The targets were carefully selected (a) to sample the core of the main body of Sgr dSph while avoiding contamination from the central stellar nucleus, and (b) to prevent any bias on the metallicity distribution by selecting targets based on their Gaia parallax and proper motions. All the targets selected in this way were confirmed as radial velocity members. We used this sample to derive the first metallicity distribution of the core of Sgr dSph, which is virtually unaffected by metallicity biases. The observed distribution ranges from [Fe/H] ≃ -2.3 to [Fe/H] ≃ 0.0, with a strong, symmetric, and relatively narrow peak around [Fe/H] ≃ -0.5 and a weak and extended metal-poor tail, in which only 13.8 ± 1.9% of the stars have [Fe/H] < -1.0. We confirm previous evidence of correlations between chemical and kinematical properties of stars in the core of Sgr. In our sample, stars with [Fe/H] ≥ -0.6 display a lower velocity dispersion and a higher rotation amplitude than those with [Fe/H] < - 0.6, confirming previous suggestions of disk/halo structure for the progenitor of the system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

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

ABSTRACT 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.

Almeida, Andres (ORCID:0009000007332479)↗
DOE OSTI2021

Spectroscopic Confirmation of the Sixth Globular Cluster in the Fornax Dwarf Spheroidal Galaxy*

The Fornax dwarf spheroidal galaxy has an anomalous number of globular clusters, five, for its stellar mass. There is a longstanding debate about a potential sixth globular cluster (Fornax 6) that has recently been "rediscovered" in DECam imaging. We present new Magellan/M2FS spectroscopy of the Fornax 6 cluster and Fornax dSph. Combined with literature data we identify ~15–17 members of the Fornax 6 cluster, showing that this overdensity is indeed a star cluster and associated with the Fornax dSph. The cluster is significantly more metal-rich (mean metallicity of $\overline{[\mathrm{Fe}/{\rm{H}}]}$ = –0.71 ± 0.05) than the other five Fornax globular clusters (–2.5 < [Fe/H] < –1.4) and more metal-rich than the bulk of Fornax. We measure a velocity dispersion of ${5.6}_{-1.6}^{+2.0}\mathrm{km}\,{{\rm{s}}}^{-1}$ corresponding to an anomalously high mass-to-light of 15 < M/L < 258 at 90% confidence when calculated assuming equilibrium. Two stars inflate this dispersion and may be either Fornax field stars or as yet unresolved binary stars. Alternatively, the Fornax 6 cluster may be undergoing tidal disruption. Based on its metal-rich nature, the Fornax 6 cluster is likely younger than the other Fornax clusters, with an estimated age of ~2 Gyr when compared to stellar isochrones. The chemodynamics and star formation history of Fornax shows imprints of major events such as infall into the Milky Way, multiple pericenter passages, star formation bursts, and/or potential mergers or interactions. Any of these events may have triggered the formation of the Fornax 6 cluster.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

MaNGA 8313-1901: Gas Accretion Observed in a Blue Compact Dwarf Galaxy?

Gas accretion is an important process in the evolution of galaxies, but it has limited direct observational evidences. In this paper, we report the detection of a possible ongoing gas accretion event in a blue compact dwarf (BCD) galaxy, MaNGA 8313-1901, observed by the Mapping Nearby Galaxies and Apache Point Observatory (MaNGA) program. This galaxy has a distinct off-centered blue clump to the northeast (the NE clump) that shows low metallicity and enhanced star formation. The kinematics of the gas in the NE clump also seems to be detached from the host BCD galaxy. Together with the metallicity drop of the NE clump, it suggests that the NE clump likely has an external origin, such as gas accretion or galaxy interaction, rather than an internal origin, such as an H II complex in the disk. After removing the underlying host component, we find that the spectrum of the "pure" clump can match very well with a modeled spectrum containing a stellar population of the young stars (≤7 Myr) only. This may imply that the galaxy is experiencing an accretion of cold gas, instead of a merger event involving galaxies with significant preexisting old stars. We also find signs of another clump (the SW clump) at the southwest corner of the host galaxy, and the two clumps may share the same origin of gas accretion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Diverse Chemo-Dynamical Properties of Nitrogen-Rich Stars Identified from Low-Resolution Spectra

The second generation of stars in the globular clusters (GCs) of the Milky Way (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 Sloan Digital Sky Survey and Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey, and present their dynamical properties. The N-rich population (NRP) and N-normal population (NNP) among our giant sample do not exhibit similarities in either in their metallicity distribution function (MDF) 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 Gaia Sausage/Enceladus, Sequoia, and Sagittarius dwarf galaxies, as well as with presently unrecognized progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗