arXiv subjects

Lu

Publications and source records attributed to Lu.

At least 19 recordsLinked to original sources

arXiv2026

Modeling Rotation in the Old, Cold Domain: Implications on Gyrochronology and the Stellar Magnetic Wind

Gyrochronology ties stellar rotation periods to ages. It is well studied in the young, open cluster age domain, but there are few constraints on gyrochronology in the old field star regime. In this work we use gyro-kinematic ages to explore the spin down of stars in this formerly inaccessible domain. Using forward modeling techniques which relax strict Rossby scaled assumptions, we find evidence for a departure from a standard spin down models. This departure can be explained with a mass-dependent term either in the global strength of the stellar wind or in the relationship between angular velocity and wind strength. Models with this additional mass- dependence help explain prior difficulty in fitting open cluster rotation distributions across the full mass range. Additionally, we use rotation models to identify an mass-dependent age domain over which wind driven stellar spin down can be isolated from other physical effects. For lower mass stars, this age domain is most affected by the core-envelope coupling timescale, where as higher mass stars are more subject to inertial effects late in their main sequence lifetimes. Using simple, analytic models of stellar spin down in this domain is unable to determine whether the stellar wind has a mass-dependence or if the stellar wind is strictly Skumanich-like in nature. In future studies of rotational and dynamo evolution we advocate for a forward modeling approach to gyrochronology over purely empirical approaches due to its superior ability to trace physical effects governing stellar spin down.

astro-ph.SR↗

arXiv2026

Host--Non-host Differences in Stellar Chemistry, Activity, and Birth Radius: Hints of Distinct Formation Environments for Earth-like Planets and Sub-Neptunes

Host-star properties provide important clues to planet formation and evolution, yet it remains unclear whether the observed differences between planet-hosting stars and stars without detected planets reflect genuine signatures of planet formation or underlying differences in stellar populations. Using a homogeneous sample of 28,383 Kepler-LAMOST-Gaia dwarf and subgiant stars, including 629 host stars with 865 planets, we compare host stars with age-mass matched non-host stars after correcting for distribution-induced matching biases. Most host-non-host differences disappear when the full planet sample is considered. However, separating planets by radius reveals distinct trends. At the high-abundance end of the [O/Fe], [Mg/Fe], and [Si/Fe] distributions, Earth-like hosts tend to be more O-rich but more Mg-poor and Si-poor than their age-mass matched non-host stars, whereas sub-Neptune hosts tend to show the opposite behavior. Sub-Neptune hosts also tend to exhibit lower chromospheric activity and smaller birth radii than comparable non-host stars. These results suggest that host-non-host differences become apparent primarily after separating planets by size and that Earth-like planets and sub-Neptunes may be associated with distinct formation environments and evolutionary pathways. We also find that hot-Jupiter hosts are tentatively more metal-rich, chromospherically active, and formed at smaller Galactic birth radii than hosts of longer-period Jupiters.

astro-ph.SR↗

arXiv2026

Milky Way Mapper decoded abundances -- II: From patterns to paths

The element abundances of Milky Way disc stars encode entangled imprints of multiple enrichment processes, making it difficult to uncover the underlying chemical evolution. Here we re-project 16 stellar abundances for 199,290 red giant stars ([Fe/H]$ > -1$) into a set of (4) shared enrichment patterns, providing a generative framework for learning the organising structure of the Milky Way disc. The relative contributions of these patterns vary systematically across the disc, revealing a low-dimensional enrichment basis that responds coherently to global drivers of disc evolution. By grouping stars according to their pattern contributions, we identify coherent enrichment pathways that exhibit strong chemo-spatial correlations and are stratified in both age and height above the plane, linking radial growth to vertical disc structure. Stars occupying similar positions along these enrichment pathways also show coherent vertical deviations across radius, indicating that the low-dimensional chemical structure captures the disc's response to dynamical perturbations. We identify a transition in enrichment behaviour at approximately 6 Gyr, marking the onset of a more chemically mixed regime with increasing contributions from delayed sources. Within this connected system, the observed $\alpha$-bimodality arises within a shared, low-dimensional abundance structure, with stars populating continuous sequences of changing enrichment fractions that are tightly coupled to spatial, temporal, and orbital coordinates across the Milky Way disc.

astro-ph.GA↗

arXiv2026

Milky Way Mapper decoded abundances -- I. Shared disc enrichment patterns

Elemental abundances in the Milky Way disc trace its star-formation and enrichment history, but predicting these abundances from theory is limited by uncertain nucleosynthetic yields and poorly constrained chemical evolution models. Large surveys provide many abundances that enable multi-dimensional insight. However, having so much data available complicates joint visualisation and physical interpretation. Here, we examine the element abundances of 70,057 red giant stars from the Milky Way Mapper survey ([Fe/H] $> -1$), using 16 elements (O,~Mg,~Al,~Si,~S,~K,~Ca,~Ti,~V, ~Cr, Mn,~Fe,~Co,~Ni,~Ce,~Nd). To tackle the challenges of joint-interpretation of these elements, we build a generative data-driven model, expressing each star's abundance vector as a linear combination of a few ($4$) latent nucleosynthetic patterns. These patterns are shared among the population but vary in fraction between stars. The model accurately generates the measured abundances, with $\chi^2 < 3$ (5) for $\sim$ 80\% (95\%) of stars. Model failures, where stars' abundances are not generated by the latent basis reveal accreted material and the role of multiple channels of metal-poor disk enrichment. We associate the recovered patterns, which represent high-precision ($\sigma_P \sim 3$\%) nucleosynthetic channels, with specific enrichment sources; (early and late) core-collapse supernovae, supernovae Type Ia, and asymptotic giant branch stars. We subsequently explore how the dominance of enrichment channels varies across age, metallicity and spatial extent of the disk, and show that enrichment patterns tightly couple to orbital properties. Mean pattern fractions vary smoothly with enrichment, and change rapidly across the valley between the high- and low-$\alpha$ sequences. Our results provide a framework for improving our understanding of Galactic evolution in the Milky Way.

astro-ph.GA↗

arXiv2026

BenchGuard: Who Guards the Benchmarks? Automated Auditing of LLM Agent Benchmarks

As benchmarks grow in complexity, many apparent agent failures are not failures of the agent at all - they are failures of the benchmark itself: broken specifications, implicit assumptions, and rigid evaluation scripts that penalize valid alternative approaches. We propose employing frontier LLMs as systematic auditors of evaluation infrastructure, and realize this vision through BenchGuard, the first automated auditing framework for task-oriented, execution-based agent benchmarks. BenchGuard cross-verifies all benchmark artifacts via structured LLM protocols, optionally incorporating agent solutions or execution traces as additional diagnostic evidence. Deployed on two prominent scientific benchmarks, BenchGuard identified 12 author-confirmed issues in ScienceAgentBench - including fatal errors rendering tasks unsolvable - and exactly matched 83.3% of expert-identified issues on the BIXBench Verified-50 subset, catching defects that prior human review missed entirely. A full audit of 50 complex bioinformatics tasks costs under USD 15, making automated benchmark auditing a practical and valuable complement to human review. These findings point toward AI-assisted benchmark development, where frontier models serve not only as subjects of evaluation but as active participants in validating the evaluation infrastructure itself.

cs.CL↗

arXiv2026

mmAnomaly: Leveraging Visual Context for Robust Anomaly Detection in the Non-Visual World with mmWave Radar

mmWave radar enables human sensing in non-visual scenarios-e.g., through clothing or certain types of walls-where traditional cameras fail due to occlusion or privacy limitations. However, robust anomaly detection with mmWave remains challenging, as signal reflections are influenced by material properties, clutter, and multipath interference, producing complex, non-Gaussian distortions. Existing methods lack contextual awareness and misclassify benign signal variations as anomalies. We present mmAnomaly, a multi-modal anomaly detection framework that combines mmWave radar with RGBD input to incorporate visual context. Our system extracts semantic cues-such as scene geometry and material properties-using a fast ResNet-based classifier, and uses a conditional latent diffusion model to synthesize the expected mmWave spectrum for the given visual context. A dual-input comparison module then identifies spatial deviations between real and generated spectra to localize anomalies. We evaluate mmAnomaly on two multi-modal datasets across three applications: concealed weapon localization, through-wall intruder localization, and through-wall fall localization. The system achieves up to 94% F1 score and sub-meter localization error, demonstrating robust generalization across clothing, occlusions, and cluttered environments. These results establish mmAnomaly as an accurate and interpretable framework for context-aware anomaly detection in mmWave sensing.

cs.CV↗

arXiv2026

Insights into the Exoplanet Radius Valley from Host-Star Ages, Activity, Chemistry, and Birth Radius

The radius valley, a bimodal feature in the size distribution of close-in small exoplanets, is widely interpreted as a signature of atmospheric loss and therefore provides a key constraint on the formation and atmospheric evolution of these planets. We investigate its dependence on host-star properties using 769 planets orbiting 558 stars, for which we derive stellar ages, chromospheric activity, and Galactic birth radius, together with elemental abundances. We find that the radius valley is not fully established at ages $\sim 3$ Gyr and evolves over gigayear timescales, with its prominence strongly affected by stellar population mixing. The dependence on magnetic activity is non-monotonic: a clear valley is present even among magnetically quiet stars, while highly active systems do not show a systematically stronger depletion. The valley morphology also varies with stellar composition: the valley is strongest in metal-poor stars, weakens near solar metallicity, and partially strengthens again at the highest metallicities. In addition, the valley shows sensitivity to refractory element ratios such as [Mg/Si], while correlations with [C/O] are weaker, indicating a dependence on planetary interior structure. Our results are more consistent with a dominant role for core-powered atmospheric mass loss than with purely irradiation-driven photoevaporation. Finally, the radius valley also depends on the Galactic birth environment, with systems near the estimated solar birth radius $\sim 4.5$ kpc showing a high fraction of Earth-like planets and a well-defined bimodal structure, suggesting that the Solar System formed in a region with a well-developed Earth-sized planet population.

astro-ph.SR↗

arXiv2026

Charge Migration and Residual Non-Linearity in NIRSpec BOTS Observations

We investigate the effect of charge migration and residual non-linearity on the JWST/NIRSpec G395H NRS1 and NRS2 detectors using Bright Object Time Series (BOTS) observations of the ultra-hot Jupiter WASP-121b. These full-orbit phase curve observations were taken over 37.8 hours (1.57 days), and provide an excellent testbed of the non-linearity behavior of NRS1 and NRS2 over long timescales. For both detectors, our analysis demonstrates charge losses at the center of the spectral trace and charge excesses at the trace edges. We find that the NRS1 detector displays ~3x larger deviations from linearity compared to NRS2. Given the large transit (~1.5%) and eclipse (~0.5%) signals for WASP-121b, we also investigate variations in the distribution of charge throughout the time-series observation. Our results show that charge distribution varies at different planetary orbital phases for NRS1, which manifests as a change in the morphology and shape of the spectral trace over the course of the time-series. The effect of charge distribution on the trace shape is not evident for NRS2.

astro-ph.IM↗

arXiv2025

mmCounter: Static People Counting in Dense Indoor Scenarios Using mmWave Radar

mmWave radars struggle to detect or count individuals in dense, static (non-moving) groups due to limitations in spatial resolution and reliance on movement for detection. We present mmCounter, which accurately counts static people in dense indoor spaces (up to three people per square meter). mmCounter achieves this by extracting ultra-low frequency (< 1 Hz) signals, primarily from breathing and micro-scale body movements such as slight torso shifts, and applying novel signal processing techniques to differentiate these subtle signals from background noise and nearby static objects. Our problem differs significantly from existing studies on breathing rate estimation, which assume the number of people is known a priori. In contrast, mmCounter utilizes a novel multi-stage signal processing pipeline to extract relevant low-frequency sources along with their spatial information and map these sources to individual people, enabling accurate counting. Extensive evaluations in various environments demonstrate that mmCounter delivers an 87% average F1 score and 0.6 mean absolute error in familiar environments, and a 60% average F1 score and 1.1 mean absolute error in previously untested environments. It can count up to seven individuals in a three square meter space, such that there is no side-by-side spacing and only a one-meter front-to-back distance.

cs.CV↗

arXiv2025

C3PO V: Comoving stellar pairs indicate rotational spin-down drives the main-sequence Li-Dip

The lithium-dip observed in mid-F dwarfs remains a long-standing challenge for stellar evolution models. We present high-precision stellar parameters and A(Li) for 22 new comoving pairs, primarily located on the hotter side of the Li-Dip. Combined with pairs from the C3PO catalog, our sample includes 124 stars with Teff between 6000 and 7300 K, encompassing and extending slightly beyond the Li-Dip. Among them, 49 comoving pairs (98 stars) have both components within the temperature range of interest. Using this expanded set of comoving pairs observed with high-resolution spectroscopy, we show that rotational spin-down is the dominant process responsible for Li depletion in the Li-Dip. First, within comoving pairs, the star with v sin i > 12 km/s shows higher A(Li) than its more slowly rotating companion within the Li-Dip, indicating that rotation-dependent mixing drives lithium depletion. Second, we observe a correlation between A(Li) and v sin i: fast rotators retain higher A(Li) with less scatter, while slow rotators show lower A(Li) and greater dispersion. Third, among slow rotators, A(Li) varies widely, suggesting that differences in initial rotation rates and spin-down histories influence how much Li is depleted. Some stars may have formed as fast rotators and spun down rapidly, leading to more Li depletion, while others may have started as slow rotators and retained more of their initial Li. These results demonstrate that rotational induced mixing plays an important role in shaping the Li-Dip beyond the effects of stellar age and mass.

astro-ph.SR↗

arXiv2025

Measuring Long Stellar Rotation Periods (>10 days) from TESS FFI Light Curves is Possible: An Investigation Using TESS and ZTF

The rotation period of a star is an important quantity that provides insight into its structure and state. For stars with surface features like starspots, their periods can be inferred from brightness variations as these features move across the stellar surface. TESS, with its all-sky coverage, is providing the largest sample of stars for obtaining rotation periods. However, most of the periods have been limited to shorter than the 13.7-day TESS orbital period due to strong background signals (e.g., scattered light) on those timescales. In this study, we investigated the viability of measuring longer periods (> 10 days) from TESS light curves for stars in the Northern Continuous Viewing Zone (NCVZ). We first created a reference set of 272 period measurements longer than 10 days for K & M dwarfs in the NCVZ using data from the Zwicky Transient Facility (ZTF) that we consider as the "ground truth" given ZTF's long temporal baseline of 6+ years. We then used the unpopular pipeline to de-trend TESS light curves and implemented a modified Lomb-Scargle (LS) periodogram that accounts for flux offsets between observing sectors. For 179 out of the 272 sources (66%), the TESS-derived periods match the ZTF-derived periods to within 10%. The match rate increases to 81% (137 out of 170) when restricting to sources with a TESS LS power that exceeds a threshold. Our results confirm the capability of measuring periods longer than 10 days from TESS data, highlighting the dataset's potential for studying slow rotators.

astro-ph.SR↗

arXiv2025

Trials and Tribulations of Chemodynamical Tagging: Investigating the Recovery of Open Clusters in the Gaia DR3-GALAH DR4 Catalog

Chemodynamical tagging has been suggested as a powerful tool to trace stars back to their birth clusters. However, the efficacy of chemodynamical tagging as a means to recover individual stellar clusters is still under debate. In this study, we present a detailed investigation of chemodynamical tagging of open clusters using both dynamical and chemical data from the \textit{Gaia} DR3 and GALAH DR4 surveys, respectively. Using a sample of open clusters and mock field stars, we conduct a bootstrap analysis to evaluate every unique combination of orbital components ($E, J_R, J_\phi, J_Z$) and chemical abundances ([X/Fe] for O, Na, Mg, Al, Si, K, Ca, Sc, Ti, Cr, Mn, Ni, Cu, Y, and Ba) on how well they recover open clusters when used as parameters in the clustering algorithm, HBDSCAN. We find that using primarily dynamical orbital parameters leads to the highest recovery rate of open cluster stars. Nevertheless, even employing the best performing parameter combinations leads to low open cluster recovery rates. We find that, in most cases, chemodynamical tagging of open clusters using blind clustering algorithms is not efficient, which is in line with previous theoretical and observational work. However, we show that the addition of cuts based on metallicity, age, and birth radii in order to reduce the size of the clustering catalog can marginally improve the recovery rate of open clusters.

astro-ph.GA↗

arXiv2025

StarFlow: Leveraging Normalizing Flows for Stellar Age Estimation in SDSS-V DR19

Understanding the ages of stars is crucial for unraveling the formation history and evolution of our Galaxy. Traditional methods for estimating stellar ages from spectroscopic data often struggle with providing appropriate uncertainty estimations and are severely constrained by the parameter space. In this work, we introduce a new approach using normalizing flows, a type of deep generative model, to estimate stellar ages for evolved stars with improved accuracy and robust uncertainty characterization. The model is trained on stellar masses for evolved stars derived from asteroseismology and predicts the relationship between the carbon and nitrogen abundances of a given star and its age. Unlike standard neural network techniques, normalizing flows enable the recovery of full likelihood distributions for individual stellar ages, offering a richer and more informative perspective on uncertainties. Our method yields age estimations for 378,720 evolved stars and achieves a typical absolute age uncertainty of approximately 2 Gyr. By intrinsically accounting for the coverage and density of the training data, our model ensures that the resulting uncertainties reflect both the inherent noise in the data and the completeness of the sampled parameter space. Applying this method to data from the SDSS-V Milky Way Mapper, we have produced the largest stellar age catalog for evolved stars to date.

astro-ph.SR↗

arXiv2025

A Data-Driven M Dwarf Model and Detailed Abundances for $\sim$17,000 M Dwarfs in SDSS-V

The cool temperatures of M dwarf atmospheres enable complex molecular chemistry, making robust characterization of M dwarf compositions a long-standing challenge. Recent modifications to spectral synthesis pipelines have enabled more accurate modeling of M dwarf atmospheres, but these methods are too slow for characterizing more than a handful of stars at a time. Data-driven methods such as The Cannon are viable alternatives, and can harness the information content of many M dwarfs from large spectroscopic surveys. Here, we train The Cannon on M dwarfs with FGK binary companions from the Sloan Digital Sky Survey-V/Milky Way Mapper (SDSS-V/MWM), with spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE). The FGK-M pairs are assumed to be chemically homogeneous and span $-$0.56 $<$ [Fe/H] $<$ 0.31 dex. The resulting model is capable of inferring M dwarf $T_{\textrm{eff}}$ and elemental abundances for Fe, Mg, Al, Si, C, N, O, Ca, Ti, Cr, and Ni with median uncertainties of 13 K and 0.018$-$0.029 dex, respectively. We test the model by verifying that it reproduces reported abundance values of M dwarfs in open clusters and benchmark M dwarf datasets, as well as expected metallicity trends from stellar evolution. We apply the model to 16,590 M dwarfs in SDSS-V/MWM and provide their detailed abundances in our accompanying catalog.

astro-ph.SR↗

arXiv2025

Exoplanet Occurrence Rate with Age for FGK Stars in Kepler

We measure exoplanet occurrence rate as a function of isochrone and gyrochronology ages using confirmed and candidate planets identified in Q1-17 DR25 Kepler data. We employ Kepler's pipeline detection efficiency to correct for the expected number of planets in each age bin. We examine the occurrence rates for planets with radii $0.2 \leq Rp \leq 20$ R$_\oplus$ and orbital periods $0.2 \leq P \leq 100$ days for FGK stars with ages between $1.5-8$ Gyr using the inverse detection efficiency method. We find no significant trend between occurrence rate and stellar ages; a slight, decreasing trend (within $1.5-2.5$ $\sigma$) only emerges for low-mass and metal-rich stars that dominate our sample. We isolate the effects of mass and metallicity on the occurrence rate trend with age, but find the results to be inconclusive due to weak trends and small sample size. Our results hint that the exoplanet occurrence rate may decrease over time due to dynamical instability from planet-planet scattering or planet ejection, but accurate ages and larger sample sizes are needed to resolve a clear relation between occurrence rate and age.

astro-ph.EP↗

arXiv2024

ChronoFlow: A Data-Driven Model for Gyrochronology

Gyrochronology is a technique for constraining stellar ages using rotation periods, which change over a star's main sequence lifetime due to magnetic braking. This technique shows promise for main sequence FGKM stars, where other methods are imprecise. However, the observed dispersion in rotation rates for similar coeval stars has historically been difficult to characterize. To properly understand this complexity, we have assembled the largest standardized data catalog of rotators in open clusters to date, consisting of $\approx$8,000 stars across 30 open clusters/associations spanning ages of 1.5 Myr to 4 Gyr. We have also developed ChronoFlow: a flexible data-driven model which accurately captures observed rotational dispersion. We show that ChronoFlow can be used to accurately forward model rotational evolution, and to infer both cluster and individual stellar ages. We recover cluster ages with a statistical uncertainty of 0.06 dex ($\approx$15%), and individual stellar ages with a statistical uncertainty of 0.7 dex. Additionally, we conducted robust systematic tests to analyze the impact of extinction models, cluster membership, and calibration ages. These contribute an additional 0.06 dex of uncertainty in cluster age estimates, resulting in a total error budget of 0.08 dex. We apply ChronoFlow to estimate ages for M34, NGC 2516, NGC 6709, and the Theia 456 stellar stream. Our results show that ChronoFlow can precisely estimate the ages of coeval stellar populations, and constrain ages for individual stars. Furthermore, its predictions may be used to inform physical spin down models. ChronoFlow is publicly available at https://github.com/philvanlane/chronoflow.

astro-ph.SR↗

arXiv2024

The temporal and spatial variations of lithium abundance in the Galactic disc

This study investigates the temporal and spatial variations in lithium abundance within the Milky Way using a sample of 22,034 main-sequence turn-off (MSTO) stars and subgiants, characterised by precise stellar ages, 3D NLTE (non-local thermodynamic equilibrium) lithium abundances, and birth radii. Our results reveal a complex variation in lithium abundance with stellar age: a gradual increase from 14 Gyr to 6 Gyr, followed by a decline between 6 Gyr and 4.5 Gyr, and a rapid increase thereafter. We find that young Li-rich stars (ages $<$ 4 Gyr, A(Li) $>$ 2.7 dex) predominantly originate from the outer disc. By binning the sample according to guiding center radius and z$_{\rm max}$, we observe that these young Li-rich stars migrate radially to the local and inner discs. In addition, the stars originating from the inner disc experienced a rapid Li enrichment process between 8 Gyr and 6 Gyr. Our analysis suggests that the age range of Li-dip stars is 4-5 Gyr, encompassing evolution stages from MSTO stars to subgiants. The Galactic radial profile of A(Li) (with respect to birth radius), as a function of age, reveals three distinct periods: 14-6 Gyr ago, 6-4 Gyr ago, and 4-1 Gyr ago. Initially, the lithium abundance gradient is positive, indicating increasing Li abundance with birth radius. During the second period, it transitions to a negative and broken gradient, mainly affected by Li-dip stars. In the final period, the gradient reverts to a positive trend.

astro-ph.GA↗

arXiv2024

When LAMOST meets Gaia DR3 Exploring the metallicity of open clusters

Context. Open clusters (OCs) are valuable probes of stellar population characteristics. Their age and metallicity provide insights into the chemical enrichment history of the Milky Way. By studying the metallicity of OCs, we can explore the spatial distribution of composition across the Galaxy and understand stellar birth radii through chemical tagging. However, inferring the original positions of OCs remains a challenge. Aims. This study investigates the distribution of metallicity in the solar neighborhood using data from Gaia DR3 and LAMOST spectra. By measuring accurate ages and metallicities, we aim to derive birth radii and understand stellar migration patterns. Methods. We selected 1131 OCs within 3 kpc of the Sun from Gaia DR3 and LAMOST DR8 low-resolution spectra (R=1800). To correct the LAMOST data, we incorporated high-resolution spectra from GALAH DR3 (R=28000) using an artificial neural network. The average metallicity of the OCs was derived from reliable [Fe/H] values of their members. We examined the metallicity distribution across the Galaxy and calculated birth radii based on age and metallicity. Results. The correction method reduces the systematic offset in LAMOST data. We found a metallicity gradient as a function of Galactocentric distance and guiding radii. Comparisons with chemo-dynamic simulations show that observed metallicity values are slightly lower than predicted when uncertainties are ignored, but the metallicity gradients align with previous studies. We also inferred that many OCs near the Sun likely originated from the outer Galactic disk.

astro-ph.GA↗