Stars and stellar evolution

arXiv:2605.02639v2 Announce Type: replace Abstract: The candidate gravitational wave (GW) event S250818k was one of only three non-retracted LIGO-Virgo-KAGRA public alerts issued during the fourth observing run of the network (O4) with a binary neutron star (BNS) merger classification probability exceeding one percent. This triggered a prompt search for a potential electromagnetic (EM) counterpart in the large localisation error region (949 deg$^2$ projected in the sky at 90% credible level). The transient SN2025ulz, discovered by the Zwicky Transient Facility (ZTF) during the search, attracted a great deal of attention due to a potential spatial and temporal coincidence, and due to its initial fast decay and featureless spectrum. Here, we report on the follow up of this transient by the Electromagnetic counterparts of gravitational wave sources at the Very Large Telescope (ENGRAVE) Collaboration. We conducted an extensive multi-wavelength observational campaign, which led to the spectral classification of the transient as a type IIb supernova (SN), indicating that it is unrelated to the candidate GW event. In this article, we describe our observing strategies, data reduction, and interpretation. All of our results confirm and strengthen our classification of the source, and also show that shock cooling tails associated with type IIb SNe are one of the most prominent contaminants in kilonova searches.

arXiv:2502.17556v3 Announce Type: replace Abstract: We present optical-to-near-infrared (NIR) photometry and spectroscopy of the Type Ia supernova (SN Ia) 2024epr, including NIR spectra observed within two days of first light. The early-time optical spectra show strong, high-velocity Ca and Si features near rarely-observed velocities at $\sim$0.1$c$, and the NIR spectra show a C I "knee." Despite early-time, high-velocity features, SN 2024epr evolves into a normal SN Ia, albeit with stronger peak-light Ca absorption than other SNe Ia with the same light curve shape. Although we infer a normal decline rate, $\Delta m_{15}(B)=1.09\pm0.12$ mag, from the light-curve rise, SN 2024epr is a Branch "cool" object and has red early-time colors ($g-r\approx0.15$ mag at $-10$ days). The high velocities point to a density enhancement in the outer layers of the explosion, predicted by some models, but thick-shell He-detonation models do not match the smoothly rising light curve or apparent lack of He in our early-time NIR spectra. No current models (e.g., delayed detonation or thin He shell double detonation) appear to reproduce all observed properties, particularly the unusual early-time colors. Such constraints are only possible for SN 2024epr from the earliest optical and NIR observations, highlighting their importance for constraining SN Ia models. Finally, we identify several literature SNe Ia with intermediate mass elements at $\sim$30\,000 km s$^{-1}$ within days after the explosion that evolve into otherwise normal SNe Ia at peak light, suggesting the early-time spectra of SNe Ia may hide a broad diversity of observational characteristics.

arXiv:2607.27467v1 Announce Type: new Abstract: Emission from photoionised gas surrounding young stellar populations ($H\text{II}$ regions) provides critical diagnostics of the physical conditions in star forming galaxies. This emission constrains the gas properties, the nature of ionising sources, and generates essential features for determining galaxy redshifts. To leverage spectroscopic observations to test galaxy formation models, it is essential to incorporate these emissions into synthetic datasets. Here, we present the integration of photoionised gas emission into the SYNTHESIZER package (https://synthesizer-project.github.io) and demonstrate its application. We quantify the impact of key modelling assumptions - including stellar population synthesis models, initial mass functions, ionisation parameter, gas density, geometry, abundance pattern, elemental depletion, and dust - on spectral diagnostics. Furthermore, we demonstrate the versatility of SYNTHESIZER through its application in different scenarios ranging from exploring emission in toy parametric models to large-volume cosmological simulations with realistic star formation and metal enrichment histories. Taken together, SYNTHESIZER provides a flexible, physically motivated framework to model stellar and nebular emissions, serving as a vital link between theory and observations in the era of next-generation spectroscopic missions.

arXiv:2607.27332v1 Announce Type: new Abstract: iLocater is a high-resolution near-infrared extreme precision radial velocity (EPRV) spectrograph that was deployed to the Large Binocular Telescope (LBT) in June 2026. iLocater operates over $\lambda=966-1312$ nm with a median resolving power of $R=205,000$ as measured in the laboratory. We present the commissioning and initial on-sky verification program using solar and night-time observations at the LBT. First light was achieved on 27 June 2026, and nearly 150 on-sky spectra have now been recorded. Observations include single stars ranging in spectral type from B5 to M6. iLocater uses the LBT AO system, and we have demonstrated its ability to obtain spatially resolved spectra of close ($\theta < 1''$) binary stars.

arXiv:2607.26147v1 Announce Type: new Abstract: Stellar winds are a primary source of uncertainty in predicting the masses of black holes (BHs) from massive stars. At solar metallicity, theoretical models lead to widely divergent results due to differing wind prescriptions. A key obstacle remains the lack of systematic investigations across a common parameter space. To address this, we construct a ``Wind Atlas'' using detailed 1D MESA stellar evolution models and population synthesis techniques to estimate the Galactic population of solar metallicity BH progenitors. We systematically investigate 14 distinct wind models, ranging from the most traditional and widespread prescriptions to the most recent. By evaluating stellar evolution across this extensive grid, we show that the final BH mass is dictated by a fundamental bifurcation: whether a star collapses as a cool supergiant or is first stripped of its envelope to become a Wolf-Rayet (WR) star. If a star enters the WR stage, its strong thick winds dominate, making the final mass sensitive to the WR wind prescription while largely erasing the memory of its prior mass-loss history. Conversely, stars that face core collapse as supergiants form significantly more massive BHs, producing a mass peak around an initial mass of 40 $M_\odot$. Rather than simply reproducing these divergent outcomes, our comprehensive evaluation demonstrates that this bifurcation is universally controlled by the highly uncertain mass loss during the cool supergiant phase. This framework strongly constrains the problem of BH mass prediction by identifying two key bottlenecks for future studies: envelope stripping efficiency and WR mass-loss rates. Our atlas provides a clear baseline for interpreting current theoretical discrepancies and testing wind models against observational constraints, such as the Galactic WR/OB population ratio.

arXiv:2607.26133v1 Announce Type: new Abstract: Recently, several long-duration gamma-ray bursts (GRBs) associated with kilonovae have cast doubt on the traditional, dichotomous mapping between gamma-ray duration and progenitor system. Here, we investigate the rates and properties of bursts which appear to cross this dichotomy using a sample of GRBs for which progenitor constraints are possible. We first build a sample of known Swift-detected GRBs at z<0.3, finding 8 short- and 21 long-duration GRBs. Of these long GRBs, we find 9 bursts with deep limits on supernova emission, evidence for kilonova emission, or association with a quiescent galaxy (31% $\pm$ 9% of all GRBs at z<0.3), implying that a significant fraction of nearby long GRBs likely do not come from massive stars. At z<0.3, no short GRB has an observed supernova counterpart. We find comparable numbers when expanding to z<0.5 and other gamma-ray telescopes, though we obtain a decreased fraction of bursts with robust constraints on a progenitor. We further find that the long GRBs with no associated supernovae possess on-average fainter afterglows and lie in less star-forming host galaxies than those with supernovae, supporting that these events may originate in compact object mergers. We estimate approximate volumetric rates, finding similar (on-axis) rates for short GRBs and supernova-less long GRBs of $\sim 0.5-2.5$ Gpc$^{-3}$ yr$^{-1}$, although a search for possible low-redshift hosts of the complete Swift catalog suggests that our sample may be $\sim$50% complete. If supernova-less long GRBs arise from compact object mergers, this implies that $\sim$ 30-70% of all z<0.3 Swift long GRBs may arise from mergers and that the $z<0.3$ rates of mergers from long and short GRBs are comparable. These findings hold substantial implications for gravitational-wave coincidence and heavy element enrichment.

arXiv:2607.14251v1 Announce Type: new Abstract: Metal-enriched white dwarfs provide a unique insight into the composition of exoplanet interiors. These stars accrete the debris of disrupted planetary bodies, and hence, measuring the stellar parameters and photospheric abundances yields the bulk compositions of the parent bodies. At present, over 1750 debris-accreting white dwarfs are known, but just a few dozen are sufficiently enriched to allow a detailed abundance study. Here we report the analysis of 12 highly metal-enriched white dwarfs observed within the Data Release~1 of the Dark Energy Spectroscopic Instrument (DESI). We characterised their stellar parameters and photospheric metal abundances and we identified between three and ten different elements in their optical spectra, including most of the rock-forming species: O, Mg, Si, Ca and Fe. We conclude that the accreted bodies broadly resemble compositions found within the inner Solar System such as primitive meteorites, processed material or planetary cores. Six of the systems allowed a more thorough analysis: four of the parent bodies are composed of dry rock-forming elements; and two of them of something akin to a water-rich planetesimal. Thus, this study establishes DESI as a potent survey for identifying metal-rich targets, yielding reliable compositions of accreted exoplanetary material.

arXiv:2607.13020v1 Announce Type: new Abstract: Studies of the oldest and most metal-poor stars in the Milky Way have confirmed a common abundance signature of high carbon enrichment, coupled with a subsolar abundance pattern of neutron-capture elements. The so-called CEMP-no stars have been speculated to be bona fide population II stars, potentially tracing the nucleosynthesis of the very first stars formed in the universe. However, constraining the binary nature of CEMP-no stars is crucial for understanding their abundance patterns. In previous radial-velocity monitoring of CEMP-no stars, the binary fraction has tentatively been found to vary with carbon enhancement. Here we present the results of radial-velocity monitoring of 30 CEMP-no stars over five years. Combined with literature data, this yields a total sample of 90 CEMP-no stars with constrained binary statuses, providing a larger statistical sample to investigate the CEMP-no binary fraction as a function of carbon enrichment. We find an overall binary frequency of $50^{+13}_{-13}\%$ among high-carbon ($A(\mathrm{C}) \ge7.3$) CEMP-no stars, compared to $18^{+5}_{-4}\%$ for low-carbon ($A(\mathrm{C}) <7.3$) stars, establishing for the first time a statistically significant increase in the CEMP-no binary frequency as a function of carbon at the 2$\sigma$ confidence level. Of the confirmed binary systems, we derive orbital parameters for four new ones, which, combined with literature data, amount to a total of 12 CEMP-no binaries with constrained orbits. We discuss these results in the context of the progenitors of CEMP-no stars; in particular, we explore the nature of the companion in these binary systems and the possibility of mass transfer.

arXiv:2607.02660v1 Announce Type: new Abstract: Protoplanetary disks often host substructures such as rings and gaps, which trace key processes in planet formation and dust evolution. However, narrow rings may remain unresolved due to limited observational resolution, hiding critical information about early planetesimal formation and the amount of dust present. We apply the azimuthal brightness modulation method, based on the modulation produced by multiple unresolved optically thick rings embedded in an optically thin background Scardoni+2024, to multi-wavelength ALMA observations of CI Tau to identify unresolved rings and constrain their geometry and optical depth. We analysed CI Tau archival ALMA continuum observations in bands 3, 6, and 7, extracting azimuthal brightness profiles along narrow annuli and comparing them with forward modeled synthetic observations of inclined disks containing unresolved rings. We detect the azimuthal signature at $\sim22$ au in all three bands, consistent with unresolved, optically thick rings embedded in a lower optical depth background. Multi wavelength modelling constrains the rings' geometry and optical depth, consistent with conditions expected for streaming instability and early planetesimal formation. Our results demonstrate the applicability of this azimuthal signature technique to real disks, reveal fine scale dust substructures in CI Tau, and illustrate a new method to study the early stages of planet formation below the nominal resolution limit.

arXiv:2607.00192v1 Announce Type: new Abstract: The disruption and accretion of planetary material onto white dwarfs is expected to be inherently dynamic and stochastic, potentially driving variability in the accretion rate and therefore the shape and depth of the photospheric metal absorption lines. This paper presents an 18-year optical spectroscopic monitoring campaign of five warm (11,000-23,000K) polluted white dwarfs with sinking timescales of days-months, observed using Magellan/MIKE and SALT/HRS to directly test this prediction. At four of the five systems, no statistically significant variability is detected over baselines of 15-18 years corresponding to hundreds to thousands of diffusion timescales, with inferred accretion rates stable to within 15-30% (1$\sigma$) showing remarkably stable accretion on decadal timescales. This implies that either the processes maintaining the accretion of the disrupted planetary material are stable on the same timescales, or that currently uncharacterized photospheric processes act to smooth observable abundance variations on these timescales. The one exception, WD 0106$-$328, shows statistically significant variability in the 4481A Mg II doublet from the ground-based data. Yet no significant equivalent width or abundance changes are seen between two Hubble Space Telescope ultraviolet spectra taken in 2016 and 2025, despite probing a larger set of transitions. This may imply that the ground-based observations witnessed a stochastic excursion from a stable baseline accretion rate, rather than a sustained change in the bulk accretion rate.

arXiv:2607.00098v1 Announce Type: new Abstract: We present a Gaia DR3 source-level census of \emph{Stellar Snake} complexes within 3\,kpc of the Sun. We define a Stellar Snake as a mutually coherent association of two or more stellar overdensities, characterised by consistent positions, kinematics, orbital invariants, ages, and chemical properties, rather than as a single gravitationally bound object. Moving beyond catalogue-driven searches seeded by known open clusters, our framework operates directly on individual Gaia sources to recover extended, low-density substructures and interconnecting stellar bridges. The multi-stage pipeline extracts statistically significant, non-overlapping base nodes, infers homogeneous parameters using a PointNet point-cloud regressor, and links these nodes into large-scale macro-structures across a 9D space spanning positions, tangential velocities, radial velocity, age \(\log t\), and orbital integrals \((E,L_Z)\). After FoF-topology cross-validation and boundary resolution, the final catalogue contains 1,256 Stellar Snake candidates comprising 802,489 unique member-star entries in 5,491 final base nodes selected from a 9,909-node input pool. Derived parameters are validated against external open-cluster catalogues and spectroscopic benchmarks. To quantify structural coherence, we introduce a graph-relation Snake Reliability Index (SRI), coupled with a peripheral-branch diagnostic and Gold/Silver/Bronze quality flags. At the population level, the census shows a broad age--metallicity pattern, a declining upper envelope of member-star entries toward older ages, and a projected association between young Snake nodes, nearby spiral-arm loci, and the Radcliffe Wave. This homogeneous inventory provides an observational foundation for probing the formation, coherence, and dynamical evolution of hierarchical stellar complexes in the Milky Way.

arXiv:2604.15197v3 Announce Type: replace Abstract: Binary interactions during the AGB phase can lead to the formation of chemically peculiar stars with overabundances of s-process elements. Only a few hundreds of these stars have been subject to detailed chemical or dynamical studies. This work aims at compiling a systematic sample of s-process-polluted candidates using GALAH DR4. We also want to compare their properties with those of confirmed s-polluted stars to have stronger evidence of their nature. GALAH DR4 uses neural networks and automatic spectral analysis methods as well as data of a lower spectral resolution than normally used to characterise these objects. Because of this, we built a validation sample, for which we obtained UVES@VLT and HERMES@Mercator high-resolution spectra. We compare our stellar parameters and abundances with those of the survey and use this validation to define the thresholds that a star in GALAH DR4 must pass to be flagged as a good s-process-rich candidate. Based on our comparisons, we define thresholds on [s/Fe], [Y/Fe], [Zr/Fe], [Ba/Fe], and [La/Fe]. We identified 1059 stars in GALAH DR4 that are good candidates to be s-process polluted stars, covering a broad parameter space. They share many similarities with the samples of confirmed s-rich stars, especially their ratios of heavy over light s-elements ([hs/ls]), which strengthen our confidence in the purity of the sample. We find that only 7% of the candidates have measured orbital periods and eccentricities, limiting for now a full comparison with confirmed Ba and related stars. However, their binary fraction is, as expected, higher than the one we found for the full GALAH DR4 catalogue. Our sample of candidates is almost five times larger than the number of currently confirmed polluted stars. This and the fact that it has been homogeneously treated by GALAH open very interesting avenues to confront nucleosynthesis and binary evolution models.

arXiv:2606.28293v1 Announce Type: new Abstract: The origin of the most massive gravitational-wave sources remains elusive. We show that gas accretion can be understood as a transport process in mass space, causing compact objects to migrate through a population at rates determined by the underlying growth law. Using a continuity-equation framework, we demonstrate that population evolution is governed primarily by the mass dependence of the accretion rate, $\dot m \propto m^\beta$. Accretion laws with $\beta>1$ naturally produce divergent evolution and generate extended high-mass tails, whereas $\beta<1$ leads to convergent evolution and compresses the population toward a narrower range of masses. We apply this framework to physically motivated accretion regimes and explore their consequences using analytical calculations and Monte Carlo population models. We show that sustained gas accretion can substantially broaden compact-object mass distributions, populate the high-mass end of gravitational-wave catalogs, and alter the mass-ratio distribution of compact-object binaries. In particular, collective accretion within compact binaries drives their mass ratios toward unity. Our results suggest that gaseous environments act as transport media that continuously reshape compact-object populations, providing a natural pathway toward the formation of massive mergers such as GW231123 and the high-mass tails increasingly revealed by gravitational-wave observations.

arXiv:2606.19455v1 Announce Type: new Abstract: The evolution of protoplanetary discs is intertwined with the process of planet formation, growth and migration. Studies of nearby star forming regions of different ages and properties provide the necessary information needed to understand the processes dictating their evolution. This paper presents the results of a spectroscopic study of the stellar and accretion properties of a large sample of 127 stars with protoplanetary discs in the Upper Scorpius region with disc dust masses inferred from ALMA continuum measurements. The accretion luminosity is derived from the excess UV continuum emission with respect to the photospheric and chromospheric one self-consistently with the stellar spectral types, extinction and luminosity, using FRAPPE. We apply a new method to evaluate upper limits to the accretion luminosity. In ~50% of cases we evaluate upper limits on the accretion luminosity, either because the S/N of the data is insufficient or because the measured value of the accretion luminosity is below the estimate of the emission due to chromospheric activity. The results show that the mass accretion rate has a weak correlation with the stellar mass, while no correlation is observed with disc properties such as dust mass or gas disc radius. The dispersion is larger than what is found in younger star forming regions such as Lupus and Cham. I, and suggests a fading of the correlations with age. We find no evidence that membership to Upper Scorpius sub-groups, nor the properties of known binary or transition discs can explain the origin of this dispersion. The lack of correlation and large dispersion of accretion rates challenge the current expectations of evolutionary models. The observed properties point to a decoupling of the inner and outer disc by the age of Upper Scorpius and a fading of the relations observed in younger star forming regions.

arXiv:2606.19455v1 Announce Type: new Abstract: The evolution of protoplanetary discs is intertwined with the process of planet formation, growth and migration. Studies of nearby star forming regions of different ages and properties provide the necessary information needed to understand the processes dictating their evolution. This paper presents the results of a spectroscopic study of the stellar and accretion properties of a large sample of 127 stars with protoplanetary discs in the Upper Scorpius region with disc dust masses inferred from ALMA continuum measurements. The accretion luminosity is derived from the excess UV continuum emission with respect to the photospheric and chromospheric one self-consistently with the stellar spectral types, extinction and luminosity, using FRAPPE. We apply a new method to evaluate upper limits to the accretion luminosity. In ~50% of cases we evaluate upper limits on the accretion luminosity, either because the S/N of the data is insufficient or because the measured value of the accretion luminosity is below the estimate of the emission due to chromospheric activity. The results show that the mass accretion rate has a weak correlation with the stellar mass, while no correlation is observed with disc properties such as dust mass or gas disc radius. The dispersion is larger than what is found in younger star forming regions such as Lupus and Cham. I, and suggests a fading of the correlations with age. We find no evidence that membership to Upper Scorpius sub-groups, nor the properties of known binary or transition discs can explain the origin of this dispersion. The lack of correlation and large dispersion of accretion rates challenge the current expectations of evolutionary models. The observed properties point to a decoupling of the inner and outer disc by the age of Upper Scorpius and a fading of the relations observed in younger star forming regions.

arXiv:2606.19429v1 Announce Type: new Abstract: We present a new framework for BayeSN, the hierarchical Bayesian SED model for type Ia supernovae (SNe Ia), incorporating cross-calibration of samples observed across heterogeneous telescopes. This framework is the first to parametrise the filter wavelength and zero-point offsets commonly used in SN~Ia cosmology within SN SED model training, enabling additional constraint on cross-calibration from SNe beyond the standard stellar-based cross-calibration pipeline. We apply this framework to train a new G26 BayeSN model on the same SED model training sample used in recent cosmological analyses, an order-of-magnitude increase over previous BayeSN training samples, and include a novel training methodology to leverage high-redshift SNe Ia in BayeSN training. We present the G26 model and apply it to the DES-SN5YR sample to assess performance, finding a 12 per cent reduction in $\sigma_{\rm NMAD}$ scatter when compared with SALT3.Dovekie; 0.164 mag compared with 0.185 mag for a sample of likely SNe Ia at $z < 0.7$, without bias corrections. We additionally present constraints on cross-calibration wavelength and zero-point shifts from our framework when using the latest `Dovekie' calibration constraints as a prior. This work is a key step towards a full end-to-end cosmological analysis with BayeSN; the new G26 model is incorporated within the public BayeSN code.

arXiv:2606.19455v1 Announce Type: new Abstract: The evolution of protoplanetary discs is intertwined with the process of planet formation, growth and migration. Studies of nearby star forming regions of different ages and properties provide the necessary information needed to understand the processes dictating their evolution. This paper presents the results of a spectroscopic study of the stellar and accretion properties of a large sample of 127 stars with protoplanetary discs in the Upper Scorpius region with disc dust masses inferred from ALMA continuum measurements. The accretion luminosity is derived from the excess UV continuum emission with respect to the photospheric and chromospheric one self-consistently with the stellar spectral types, extinction and luminosity, using FRAPPE. We apply a new method to evaluate upper limits to the accretion luminosity. In ~50% of cases we evaluate upper limits on the accretion luminosity, either because the S/N of the data is insufficient or because the measured value of the accretion luminosity is below the estimate of the emission due to chromospheric activity. The results show that the mass accretion rate has a weak correlation with the stellar mass, while no correlation is observed with disc properties such as dust mass or gas disc radius. The dispersion is larger than what is found in younger star forming regions such as Lupus and Cham. I, and suggests a fading of the correlations with age. We find no evidence that membership to Upper Scorpius sub-groups, nor the properties of known binary or transition discs can explain the origin of this dispersion. The lack of correlation and large dispersion of accretion rates challenge the current expectations of evolutionary models. The observed properties point to a decoupling of the inner and outer disc by the age of Upper Scorpius and a fading of the relations observed in younger star forming regions.

arXiv:2606.19429v1 Announce Type: new Abstract: We present a new framework for BayeSN, the hierarchical Bayesian SED model for type Ia supernovae (SNe Ia), incorporating cross-calibration of samples observed across heterogeneous telescopes. This framework is the first to parametrise the filter wavelength and zero-point offsets commonly used in SN~Ia cosmology within SN SED model training, enabling additional constraint on cross-calibration from SNe beyond the standard stellar-based cross-calibration pipeline. We apply this framework to train a new G26 BayeSN model on the same SED model training sample used in recent cosmological analyses, an order-of-magnitude increase over previous BayeSN training samples, and include a novel training methodology to leverage high-redshift SNe Ia in BayeSN training. We present the G26 model and apply it to the DES-SN5YR sample to assess performance, finding a 12 per cent reduction in $\sigma_{\rm NMAD}$ scatter when compared with SALT3.Dovekie; 0.164 mag compared with 0.185 mag for a sample of likely SNe Ia at $z < 0.7$, without bias corrections. We additionally present constraints on cross-calibration wavelength and zero-point shifts from our framework when using the latest `Dovekie' calibration constraints as a prior. This work is a key step towards a full end-to-end cosmological analysis with BayeSN; the new G26 model is incorporated within the public BayeSN code.

arXiv:2601.15285v2 Announce Type: replace Abstract: The A1V star $\gamma$ Oph, at a distance of 29.7 pc, is known from Spitzer imaging to host a debris disk with a large radial extent and from its spectral energy distribution to host inner warm dust. We imaged $\gamma$ Oph with JWST/MIRI at 15 and 25.5 $\mu$m, revealing smooth and radially broad emission that extends to a radius of at least 250 au at 25.5 $\mu$m. In contrast to JWST findings of an inner small-grain component with distinct ringed structures in Fomalhaut and Vega, the mid-infrared radial profile combined with prior ALMA imaging suggests a radially broad steady-state collisional cascade with the same grain size distribution throughout the disk. This further suggests that the system is populated by a radially broad planetesimal belt from tens of au or less to well over 200 au, rather than a narrow planetesimal belt from which the observed dust is displaced to appear broad. The disk is also found to be asymmetric, which could be modelled by a stellocentric offset corresponding to a small eccentricity of $\sim$0.03. Such a disk eccentricity could be induced by a mildly eccentric $<10\,M_\mathrm{Jup}$ giant planet outside 10 au, or a more eccentric companion up to stellar mass at a few au, without producing a resolvable radial gap in the disk.

arXiv:2606.18849v1 Announce Type: new Abstract: We present a sample of 88,959 K-giants from DESI Milky Way Survey Year 3 data, which we use to characterize the chemo-dynamical properties of the stellar halo at Galactocentric distances of 12 to ~100 kpc. Using HDBSCAN, we identify five prominent stellar halo substructures: Aleph, the Sagittarius stream, Gaia Sausage-Enceladus (GSE), Cetus-Palca and the Orphan-Chenab stream. We present the properties of each of these structures as they appear in our catalog, and examine how uncertainties on distance affect the characterization of substructure with this approach. We also examine regions associated with previously reported overdensities (such as the Virgo Overdensities and the Sagittarius spur) that we do not recover with HDBSCAN. The size and distance range of our catalog allows us to explore in detail the residual stellar halo, comprising stars that we do not associate with any substructure. We find that samples of ~2000 outer halo stars with both highly prograde and highly retrograde angular momenta have similar metallicity distribution functions (MDFs), which do not resemble the MDFs of either GSE or Sagittarius. Both the prograde and retrograde residual halo MDFs are bimodal, with a metal-poor peak at [Fe/H] ~ -2 and a metal-rich peak at [Fe/H] ~ -1.3 (prograde) or -1.5 (retrograde). The MDF for lower angular momentum residual halo K-giants does not show clear evidence for a metal-poor peak, and broadly resembles the MDF of GSE, even at much lower binding energies than GSE itself. We discuss possible interpretations of these findings for GSE accretion scenarios.