arXiv subjects

Chervin F. P. Laporte

Publications and source records attributed to Chervin F. P. Laporte.

At least 19 recordsLinked to original sources

arXiv2026

Stirred, not shaken: Dislodging $ω$ Centauri from the Sausage galaxy through bar resonances

We test the idea proposed by Laporte \& Orkney that $ω$ Centauri ($ω$Cen) is the central nuclear star cluster of the Gaia-Sausage-Enceladus galaxy (GSE) which got dislodged by the bar. We run a series of simulations in time-evolving bar potentials with present-day pattern speeds within Portail et al's $Ω_{b}=39\pm3.5\,\rm{km\,s^{-1}\,kpc^{-1}}$ and rate of slow-down $\dotΩ_{\rm b}= 4.5 \pm 1.4 \,\rm{km \,s^{-1}\,kpc^{-1}\, Gyr^{-1}}$ to study how bar resonances modulate the angular momentum and energies of both GSE-debris and $ω$Cen's location in $(E,L_{z})$ space. We find that the retrograde $3:2$ resonance is capable of inflicting changes of $ΔE\approx0.4\times10^{5}\,\rm{km^{2}\, s^{-2}}$, $ΔL_{z}\approx0.3-0.5\times10^{3}\, \rm{km \, s^{-1}\, kpc^{-1}}$ within timescales of $Δt\approx 0.5-1\,\rm{Gyr}$, well within the bar's lifespan $t_{\rm{bar}}\approx10-8\,\rm{Gyr}$. These results are independent of the choice of potential or modelling. We uncover a sample of N-rich stars with metallicities $-2.0<[\rm{Fe/H}]<-1.0$ and aluminium enrichments consistent with $ω$Cen. In ($E,L_{z}$) space, the map reveals a corridor of trailing N-rich stars clustered about the region connecting $ω$Cen to the GSE's debris centroid, matching $ω$Cen's past predicted track. This provides the best evidence yet that $ω$Cen is the nuclear star cluster of the GSE. Using M54 and $ω$Cen's chemistry we show that neither first nor second generations follow general galaxy-wide scale chemical enrichment trends. This is empirical evidence that GC chemical trends are a cluster-scale physics problem, not a galaxy-wide one.

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arXiv2026

The Role of Dark Matter in Driving Disk Perturbations: Application to LMC-SMC Like Galaxy Interactions

Dark-matter (DM) models predict that satellite galaxies distort their host's DM halo. As such, both the satellite and the distorted halo can perturb the host's stellar disk. The LMC-SMC system is a nearby $\sim1:10$~mass-ratio binary with a well-constrained orbit and well-characterized LMC disk perturbations. Hence, the system offers unique opportunities to test DM theory, provided the impact of halo torques on the LMC's disk can be characterized along with the SMC's torques. We use halo Basis Function Expansions (BFEs) of an N-body simulation of an isolated LMC-SMC-like interaction to quantify the time-dependent torques exerted on the LMC's disk by the SMC and the LMC's DM halo. We find that the halo torques arise from the quadrupole halo distortion, and the inner (R$<6$ kpc) LMC disk warps are the most promising probe of this quadrupole. For the first time, we construct a BFE to quantify the LMC's disk perturbations resulting from the SMC and halo torques. We find that the LMC's disk is significantly perturbed prior to the Clouds' Milky Way infall, possessing warps with a mean vertical extent of $\sim1$ kpc. Finally, we apply multi-channel singular spectral analysis (mSSA) to the time-series of the disk BFE coefficients, halo quadrupole, and the LMC-SMC orbit. mSSA successfully separates the temporal evolution of the LMC warps into components correlated with the halo versus the SMC. Our framework identifies correlated evolution between the LMC's halo distortions and the LMC's disk perturbations, enabling the Clouds to be a precision laboratory for DM physics.

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arXiv2026

GASTRO library II: Exploring Chemical Bimodalities in Disk Galaxies with GSE-like Mergers and Massive Star-forming Clumps

We use several smoothed particle hydrodynamics+N-body models as part of the GASTRO library to study the role of high-density star-forming clumpy regions and a single merger on the formation of the $α$-rich and $α$-poor populations in the disk galaxies. These experiments are tailored to mimic what is expected to be the Gaia-Sausage/Enceladus (GSE) accretion event, which occurred circa 10 Gyr ago in the Milky Way (MW). We find that either an early clumpy phase or a retrograde merger significantly reduces the star formation rate (SFR) of the disk, giving rise to a chemical bimodality qualitatively similar to the MW's. The decrease of the SFR as the cause of the chemical bimodality is consistent with previous idealized and cosmological simulations. On the other hand, a prograde radial merger does not significantly modify the SFR of the disk, resulting in no clear chemical bimodality. We further show that stars originating from the inner regions ($R_{form}<4$ kpc) do not create the disk's chemical bimodality, although they can enhance it. Finally, only the models with an early clumpy phase can produce a significant fraction of old, age $>11$ Gyr, $α-$poor stars with disk-like orbits, similar to what has been recently observed in the MW. Our results strengthen the case of clumpy disky galaxies observed at redshift $z\approx 1-2$ as likely progenitors of our Galaxy.

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arXiv2026

The Last Galactic Firework: Timing the last significant merger with stars, globular clusters and $ω$Centauri

We present a robust method to empirically infer the timing of the last significant merger in the Milky Way which is tested against fully cosmological models of galaxy formation. We apply it to Milky Way subgiant stars with spectro-photometric ages, finding that the last significant merger (Gaia-Sausage-Enceladus, GSE), occurred $\sim11\,$Gyrs ago. This coincides with the birth of a coeval in-situ group of globular clusters (GCs), which constrains the merger-induced starburst (hereafter {\it Tainá}) to have occurred at $11.2\pm 0.1\,\rm{Gyr}$, the most precise dating of this merger event. The GSE's most metal-rich GCs were also born around this time ($τ=10.9\pm0.1\,\rm{Gyr}$) and likely formed during the merger interaction prior to disruption of the GSE. We argue that $ω$ Centauri is the most likely candidate for the surviving remnant of the GSE, and show that its stellar populations have final ages and metallicities consistent with the GSE GCs together with observational evidence it may have been affected by bar resonances. Furthermore, we argue that the mean metallicity for which stellar orbits transition from halo-like to disc-like kinematics shows an upward inflexion point at $[\rm{Fe/H}]\sim-1.33$, and this sets an upper-limit for the age when the disc was forming. To corroborate this, we identify proto-MW GCs with highly disc-like orbits that formed before the last significant merger (with ages up to $τ=13.0\pm0.5\,\rm{Gyr}$). This places the disc formation time as far back as as $z_{\rm disc\, form}\gtrsim4$.

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arXiv2026

Build-up and survival of the disc: From numerical models of galaxy formation to the Milky Way

We study the build-up and survival of angular momentum in the stellar disc using a statistical suite of cosmological simulations of Milky Way-mass galaxies. Our results show that stellar kinematics at $z=0$ rarely recover the true times of disc spin-up, due to the disruptive impact of massive radial merger events. The proto-disc (i.e. Aurora) and kicked-up disc stars (the Splash) become indistinguishable at low metallicities, and the local fraction of kicked-up disc stars remains $<20$ per cent even after major mergers. In contrast, observations from Gaia and legacy surveys reveal that Galactic $α$-rich populations as old as $τ=13.5\,\rm{Gyr}$ show significant rotation, with median $η> 0.75$. This places strong constraints on the total merger ratio between the proto-Milky Way and its last significant merger (Gaia-Sausage Enceladus, GSE), favouring minor mergers with mass ratios $< 1:4$. We present the age-metallicity relation for the stellar halo and estimate the interaction epoch at $τ_{\rm{spin\text{-}up}}\simeqτ_{\rm{GSE}}\sim11\,\rm{Gyr}$. We note an abrupt dearth of halo and Splash stars after a lookback time of $10\,\rm{Gyr}$, marking the end of the merger interaction. Finally, we show that Globular Clusters in the metallicity range $-0.8<\rm{[Fe/H]}<-0.3$ share a formation time of $τ_{\rm{starburst}}\sim11\,\rm{Gyr}$, which we interpret as a signature of a starburst triggered by the first pericentric interaction of the GSE. This is remarkable corroboration between our GSE interaction and starburst times of $τ_{\rm GSE}=τ_{\rm starburst} \sim 11\,$Gyr.

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arXiv2026

Mapping the Distorted Dark Matter Distribution of the LMC-SMC System Prior to Milky Way Infall with Basis Function Expansions

The SMC orbits within the LMC's dark matter (DM) halo in a $\sim$1:10 mass-ratio encounter. The LMC:Milky Way (MW) interaction is also $\sim$1:10, and is expected to perturb the MW's DM distribution. However, no framework exists to quantify the severity of these perturbations over multiple pericenters and longer periods of time, such as the LMC-SMC interaction history. We construct basis function expansions of a high-resolution \textit{N}-body simulation of the Clouds interacting in isolation and analyze their DM distributions at an epoch approximating the time of their infall to the MW. Our goal is to quantify how the Clouds distort each other's DM distributions \textit{without} the MW. The LMC halo's response to the SMC includes a $\sim 20$ kpc long dynamical friction wake and the displacement of the LMC's density center during each SMC pericenter, which produces two overdensities in the LMC halo (at $\sim$60 and $\sim$100 kpc) at MW infall. The SMC's tidal radius at infall is just $\sim4$ kpc, at which point the SMC has lost two-thirds of its initial DM mass to the LMC. The distortions to the Clouds' halos produce a highly asymmetric acceleration field. Accurate orbit integration in the LMC-SMC system must account for the time-dependent shapes of both halos. The SMC-induced perturbations in the LMC DM halo resemble the MW-LMC system, and persist over multiple SMC pericenters. We conclude that 1:10 satellite-host encounters induce characteristic deformations in both DM halos across host-mass scales, with implications for merger rates and tests of DM models.

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arXiv2026

A nearly pristine star from the Large Magellanic Cloud

The first stars formed out of pristine gas, causing them to be so massive that none are expected to have survived until today. If their direct descendants were sufficiently low-mass stars, such stars could exist today and would be recognizable by having the lowest metallicities (abundance of elements heavier than helium). We present the independent identification and detailed chemical analysis of the star SDSS J0715-7334, finding ultra-low elemental abundances of both iron and carbon ([Fe/H] = -4.3, [C/Fe] < -0.2) and total metallicity Z < 7.8 x 10^{-7} (log Z/Zsun < -4.3). The star's orbit indicates that it originates from the halo of the Large Magellanic Cloud. Its heavy element abundance pattern can be explained by a primordial supernova with an initial mass of 30 solar masses. This star is over ten times more chemically pristine than the most extreme high-redshift galaxies currently found by the James Webb Space Telescope. It is sufficiently metal-poor that current models of low-mass star formation require dust cooling to explain its existence.

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arXiv2025

The Milky Way in context: The formation of galactic discs and chemical sequences from a cosmological perspective

We study the formation of chemical sequences in the stellar disc of Milky Way (MW)-mass galaxies in a full cosmological context with the auriga simulations. We focus on the conditions giving rise to bi-modal $α$-chemistry in the MW disc and the potential influence of mergers (e.g. Gaia-Enceladus, GSE). We find a wide diversity of chemical sequences, without correlation between the emergence of dichotomous $α$-chemistry and GSE-like mergers. The transition between multiple $α$-sequences is sequential, and is mediated by modulations in the star formation rate (SFR). In some cases, this can be caused by the starburst and subsequent quiescence induced by mergers. In others, by exhaustion or violent disruption of the gas disc. Realisations with singular sequences either lack significant modulations in their SFR, or form too late to have a significant high-$α$ sequence. The metallicity overlap between the high-$α$ and low-$α$ sequences (as seen in the Solar neighbourhood of the MW) arises from accretion of metal-poor gas from the circum-galactic medium. This depends on gas disc thickness, with thinner discs losing their metal-poor extremities. Gas donation from singular gas-rich merger events are incapable of driving long-lived metal dilution ($Δ\text{[Fe/H]} \gtrsim 0.3$), and we rule-out this scenario for the low-$α$ sequence in the MW. Finally, the shape of $α$-sequences in the [Fe/H] versus [Mg/Fe] plane is related to long-term SFR trends, with a downward slanted locus (as is observed in the low-$α$ sequence of the MW) owing to a sustained or declining SFR.

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arXiv2025

Shaping the Milky Way. II. The dark matter halo's response to the LMC's passage in a cosmological context

The distribution of dark matter in the Milky Way (MW) is expected to exhibit a large-scale dynamical response to the recent infall of the LMC. This event produces a dynamical friction wake and shifts the MW's halo density center. The structure of this response encodes information about the LMC- MW mass ratio, the LMC's orbit, the MW halo's pre-infall structure and could provide constraints on dark matter physics. To extract this information, a method to separate these effects and recover the initial shape of the MW's halo is required. Here, we use basis function expansions to analyze the halo response in eighteen simulations of MW-LMC-like interactions from the MWest cosmological, dark-matter-only zoom-in simulations. The results show that mergers similar to the LMC consistently generate a significant dipole and a secondary quadrupole response in the halo. The dipole arises from the host density center displacement and halo distortions, and its amplitude scales as the square of the MW-LMC mass ratio, peaking 0.2-0.7 Gyr after the LMC's pericenter. The quadrupole's strength depends primarily on the original axis ratios of the host halo, though contributions from the dynamical friction wake cause it to peak less than 0.3 Gyr before pericenter. Future measurements of both the dipole and quadrupole imprints of the LMC's passage in the density of the MW's stellar halo should be able to disentangle these effects and provide insight into the initial structure of the MW's halo, the MW's response, and the mass of the LMC.

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arXiv2025

Analogues of the Milky Way-Sagittarius interaction in the TNG50: effect on the Milky Way

The Sagittarius Dwarf Galaxy is undoubtedly being disrupted in the tidal field of the Milky Way. The Milky Way disc is also found to be in a state of disequilibrium. The role of the Sagittarius in driving or contributing to this disequilibrium has been extensively investigated. Most of these studies, however, assume an initially near-equilibrium disc. It was also hypothesized that the passage of Sagittarius could have increased the star-forming activity in the Solar Neighbourhood. We check whether galaxies that have undergone cosmological evolution are affected by interactions analogous to those between the Sagittarius and the Milky Way. We use the high-resolution simulation TNG50 to look for pairs similar to the Milky Way and Sagittarius. We search within redshift z=1-0 for discs from the MW/M31 sample that interacted with a satellite more massive than 10 billion Msun, had a pericenter smaller than 50 kpc, and was on an approximately polar orbit. We exclude cases where, within 1 Gyr of the pericenter, a similar interaction occurred. In 90 percent of cases, a passage of the Sagittarius analogue had no significant effect on either the vertical velocity field of the disc or the star formation history. A response in vertical stellar kinematics can be found mostly in cold discs and mildly correlates with the strength of interactions. For star formation, the studied interactions had an effect only when little to no star formation was ongoing prior to the interaction, often due to previously disturbed star-forming discs, e.g., from AGN activity. Our results indicate that stellar discs in TNG50 are frequently vertically perturbed preceding pericenter passages of Sagittarius analogues. Future studies using other simulations and extragalactic surveys will help establish whether vertical disequilibrium is a common feature of disc galaxies or an artifact of the specific setup studied.

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arXiv2025

Shaping the Milky Way: The interplay of mergers and cosmic filaments

The large-scale morphology of Milky Way (MW)-mass dark matter (DM) halos is shaped by two key processes: filamentary accretion from the cosmic web and interactions with massive satellites. Disentangling their contributions is essential for understanding galaxy evolution and constructing accurate mass models of the MW. We analyze the time-dependent structure of MW-mass halos from zoomed cosmological-hydrodynamical simulations by decomposing their mass distribution into spherical harmonic expansions. We find that the dipole ($\ell=1$) and quadrupole ($\ell=2$) moments dominate the gravitational power spectrum, encoding key information about the halo's shape and its interaction with the cosmic environment. While the dipole reflects transient perturbations from infalling satellites and damps on dynamical timescales, the quadrupole -- linked to the halo's triaxiality -- is a persistent feature. We show that the quadrupole's orientation aligns with the largest filaments, imprinting a long-lived memory on the halo's morphology even in its inner regions ($\sim30$ kpc). At the virial radius, the quadrupole distortion can reach 1-2 times the spherical density, highlighting the importance of environment in shaping MW-mass halos. Using multivariate Singular Spectrum Analysis, we successfully disentangle the effects of satellite mergers and filamentary accretion on quadrupole. We find that the quadrupolar response induced by LMC-mass satellites has an order of magnitude larger gravitational power than in spherically symmetric MW models. This highlights the need for models that incorporate the MW's asymmetry and time-evolution, with direct consequences for observable structures such as disk warps, the LMC-induced wake, and stellar tracers -- particularly in the era of precision astrometry.

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arXiv2025

Dark matter spiral arms in Milky Way-like halos

The coupling between the dark matter (DM) halo and the stellar disc is a key factor in galactic evolution. While the interaction between structures like the Galactic bar and DM halos has been explored (e.g. slowing down of the bar due to dynamical friction), the effect of spiral arms on the DM halo distribution has received limited attention. We analyze a suite of simulations featuring strong stellar spiral arms, ranging in complexity from test-particle models to fully cosmological hydrodynamical simulations. Using Fourier transforms, we characterize the phase and amplitude of the stellar spirals at different times and radii. We then apply the same methodology to DM particles near the stellar disc and compare trends in Fourier coefficients and phases between the two components. We detect a clear spiral arm signal in the DM distribution, correlated with the stellar spirals, confirming the reaction of the halo. The strength of the DM spirals consistently measures around 10\% of that of the stellar spiral arms. In the $N$-body simulation, the DM spiral persistently trails the stellar spiral arm by approximately $10^\circ$. A strong spiral signal of a few km\,s$^{-1}$ appears in the radial, azimuthal, and vertical velocities of halo particles, distinct from the stellar kinematic signature. In a test-particle simulation with an analytical spiral potential (omitting self-gravity), we reproduce a similar density and kinematic response, showing that the test-particle halo responds in the same way as the $N$-body halo. Finally, we also find the rest of the simulations, indicating that the dynamical signatures of the forced response in the DM halo are independent of the dynamical origin of the stellar spiral arms. We reveal the ubiquitous presence of DM spiral arms in Milky Way-like galaxies, driven by a forced response to the stellar spiral potential. (ABR)

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arXiv2025

Azimuthal metallicity variations, spiral structure, and the failure of radial actions based on assuming axisymmetry

We study azimuthal variations in the mean stellar metallicity, <[Fe/H]>, in a self-consistent, isolated simulation in which all stars form out of gas. We find <[Fe/H]> variations comparable to those observed in the Milky Way and which are coincident with the spiral density waves. The azimuthal variations are present in young and old stars and therefore are not a result of recently formed stars. Similar variations are present in the mean age and alpha-abundance. We measure the pattern speeds of the <[Fe/H]>-variations and find that they match those of the spirals, indicating that spirals are the cause of the metallicity patterns. Because younger stellar populations are not just more [Fe/H]-rich and alpha-poor but also dynamically cooler, we expect them to more strongly support spirals, which is indeed the case in the simulation. However, if we measure the radial action, J_R, using the Stackel axisymmetric approximation, we find that the spiral ridges are traced by regions of high J_R, contrary to expectations. Assuming that the passage of stars through the spirals leads to unphysical variations in the measured J_R, we obtain an improved estimate of J_R by averaging over a 1 Gyr time interval. This time-averaged J_R is a much better tracer of the spiral structure, with minima at the spiral ridges. We conclude that the errors incurred by the axisymmetric approximation introduce correlated deviations large enough to render the instantaneous radial actions inadequate for tracing spirals.

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arXiv2024

[X/Fe] Marks the Spot: Mapping Chemical Azimuthal Variations in the Galactic Disk with APOGEE

Chemical cartography of the Galactic disk provides insights to its structure and assembly history over cosmic time. In this work, we use chemical cartography to explore chemical gradients and azimuthal substructure in the Milky Way disk with giant stars from APOGEE DR17. We confirm the existence of a radial metallicity gradient in the disk of $Δ$[Fe/H]/$Δ$R $\sim -0.066 \pm 0.0004$ dex/kpc and a vertical metallicity gradient of $Δ$[Fe/H]/$Δ$Z $\sim -0.164 \pm 0.001$ dex/kpc. We find azimuthal variations ($\pm0.1$ dex) on top of the radial metallicity gradient that have been previously established with other surveys. The APOGEE giants show strong correlations with stellar age and the intensity of azimuthal variations in iron; older stellar populations show the largest deviations from the radial metallicity gradient. Beyond iron, we show that other elements (e.g., Mg, O) display azimuthal variations at the $\pm0.05$ dex-level across the Galactic disk. We illustrate that moving into the orbit-space could help constrain the mechanisms producing these azimuthal metallicity variations. These results suggest that the spiral arms of the Galaxy are not solely responsible for azimuthal metallicity variations and other Galactic processes are at play.

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arXiv2024

On the Galactic rotation curve inferred from the Jeans equations Assessing its robustness using Gaia DR3 and cosmological simulations

Several works have recently applied Jeans modelling to Gaia-based datasets to infer the circular velocity curve for the Milky Way. Such works have consistently found evidence for a continuous decline in the rotation curve beyond $\sim$15kpc possibly indicative of a light dark matter halo. We used Gaia DR3 RVS data, supplemented with Bayesian distances to determine the radial variation of the second moments of the velocity distribution for stars close to the Galactic plane. We have used these profiles to determine the rotation curve using the Jeans equations under the assumption of axisymmetry and explored how they vary with azimuth and above and below the Galactic disk plane. We have applied the same methodology to an N-body simulation of a Milky Way-like galaxy impacted by a satellite akin the Sagittarius dwarf and to the Auriga suite of cosmological simulations. We reveal evidence of disequilibrium and deviations from axisymmetry closer in. We find that the second moment of $V_R$ flattens out at $R \gtrsim 12.5$kpc, and that the second moment of $V_ϕ$ is different above and below the plane for $R \gtrsim 11$kpc. The simulations indicate that these features are typical of galaxies that have been perturbed by external satellites. They also suggest that the difference between the true circular velocity curve and that inferred from Jeans equations can be as high as 15$\%$, but is likely of order 10$\%$ for the Milky Way. This is of larger amplitude than the systematics inherent to Jeans equations. However, if the density of the tracer population were truncated at large radii, the erroneous conclusion of a steeply declining rotation curve can be reached. We find that steady-state axisymmetric Jeans modelling becomes less robust at large radii, indicating that particular caution is needed when interpreting the rotation curve inferred in those regions.

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arXiv2024

Mapping the anisotropic Galactic stellar halo with Blue Horizontal Branch stars

We use Legacy Survey photometric data to probe the stellar halo in multiple directions of the sky using a probabilistic methodology to identify Blue Horizontal Branch (BHB) stars. The measured average radial density profile follows a double power law in the range $ 5 < r_{gc}/{\rm kpc} < 120$, with a density break at $r_{gc}\approx20$ kpc. This description, however, falls short, depending on the chosen line-of-sight, with some regions showing no signature of a break in the profile and a wide range of density slopes, e.g. outer slope $-5.5 \lesssim α_{out} \lesssim -4$, pointing towards a highly anisotropic stellar halo. This explains in part the wide range of density profiles reported in the literature owing to different tracers and sky coverage. Using our detailed 3-D stellar halo density map, we quantify the shape of the Pisces overdensity associated with the transient wake response of the Galaxy's (dark) halo to the Large Magellanic Cloud (LMC). Measured in the LMC's coordinate system, Pisces stands above the background, is 60 degrees long and 25 degrees wide and aligned with the LMC's orbit. This would correspond to a wake width of $\sim 32$ kpc at $\sim 70$ kpc. We do not find a statistically significant signature of the collective response in density as previously reported in the literature measured with K giant stars, despite our larger numbers. We release the catalogue constructed in this study with 95,446 possible BHB stars and their BHB probability.

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arXiv2024

Spectacular nucleosynthesis from early massive stars

Stars formed with initial mass over 50 Msun are very rare today, but they are thought to be more common in the early universe. The fates of those early, metal-poor, massive stars are highly uncertain. Most are expected to directly collapse to black holes, while some may explode as a result of rotationally powered engines or the pair-creation instability. We present the chemical abundances of J0931+0038, a nearby low-mass star identified in early followup of SDSS-V Milky Way Mapper, which preserves the signature of unusual nucleosynthesis from a massive star in the early universe. J0931+0038 has relatively high metallicity ([Fe/H] = -1.76 +/- 0.13) but an extreme odd-even abundance pattern, with some of the lowest known abundance ratios of [N/Fe], [Na/Fe], [K/Fe], [Sc/Fe], and [Ba/Fe]. The implication is that a majority of its metals originated in a single extremely metal-poor nucleosynthetic source. An extensive search through nucleosynthesis predictions finds a clear preference for progenitors with initial mass > 50 Msun, making J0931+0038 one of the first observational constraints on nucleosynthesis in this mass range. However the full abundance pattern is not matched by any models in the literature. J0931+0038 thus presents a challenge for the next generation of nucleosynthesis models and motivates study of high-mass progenitor stars impacted by convection, rotation, jets, and/or binary companions. Though rare, more examples of unusual early nucleosynthesis in metal-poor stars should be found in upcoming large spectroscopic surveys.

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arXiv2023

The Prince and the Pauper: Evidence for the early high-redshift formation of the Galactic $α$-poor disc population

Context. The presence of [$α$/Fe]-[Fe/H] bi-modality in the Milky Way disc has animated the Galactic archaeology community since more than two decades. Aims. Our goal is to investigate the chemical, temporal, and kinematical structure of the Galactic discs using abundances, kinematics, and ages derived self-consistently with the new Bayesian framework SAPP. Methods. We employ the public Gaia-ESO spectra, as well as Gaia EDR3 astrometry and photometry. Stellar parameters and chemical abundances are determined for 13 426 stars using NLTE models of synthetic spectra. Ages are derived for a sub-sample of 2 898 stars, including subgiants and main-sequence stars. The sample probes a large range of Galactocentric radii, $\sim$ 3 to 12 kpc, and extends out of the disc plane to $\pm$ 2 kpc. Results. Our new data confirm the known bi-modality in the [Fe/H] - [$α$/Fe] space, which is often viewed as the manifestation of the chemical thin and thick discs. The over-densities significantly overlap in metallicity, age, and kinematics, and none of these is a sufficient criterion for distinguishing between the two disc populations. Different from previous studies, we find that the $α$-poor disc population has a very extended [Fe/H] distribution and contains $\sim$ 20$\%$ old stars with ages of up to $\sim$ 11 Gyr. Conclusions. Our results suggest that the Galactic thin disc was in place early, at look-back times corresponding to redshifts z $\sim$ 2 or more. At ages $\sim$ 9 to 11 Gyr, the two disc structures shared a period of co-evolution. Our data can be understood within the clumpy disc formation scenario that does not require a pre-existing thick disc to initiate a formation of the thin disc. We anticipate that a similar evolution can be realised in cosmological simulations of galaxy formation.

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