arXiv subjects

Lu

Publications and source records attributed to Lu.

At least 37 records · Page 2Linked to original sources

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↗

arXiv2024

On Convective Turnover Times and Dynamos In Low-Mass Stars

The relationship between magnetic activity and Rossby number is one way through which stellar dynamos can be understood. Using measured rotation rates and X-ray to bolometric luminosity ratios of an ensemble of stars, we derive empirical convective turnover times based on recent observations and re-evaluate the X-ray activity-Rossby number relationship. In doing so, we find a sharp rise in the convective turnover time for stars in the mass range of $0.35-0.4\ \rm M_{\odot}$, associated with the onset of a fully convective internal stellar structure. Using $\texttt{MESA}$ stellar evolution models, we infer the location of dynamo action implied by the empirical convective turnover time. The empirical convective turnover time is found to be indicative of dynamo action deep within the convective envelope in stars with masses $0.1-1.2\ \rm M_{\odot}$, crossing the fully convective boundary. Our results corroborate past works suggesting that partially and fully convective stars follow the same activity-Rossby relation, possibly owing to similar dynamo mechanisms. Our stellar models also give insight into the dynamo mechanism. We find that empirically determined convective turnover times correlate with properties of the deep stellar interior. These findings are in agreement with global dynamo models that see a reservoir of magnetic flux accumulate deep in the convection zone before buoyantly rising to the surface.

astro-ph.SR↗

arXiv2024

The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars are Metal-Rich

Metallicities of both gas and stars decline toward large radii in spiral galaxies, a trend known as the radial metallicity gradient. We quantify the evolution of the metallicity gradient in the Milky Way as traced by APOGEE red giants with age estimates from machine learning algorithms. Stars up to ages of $\sim$9 Gyr follow a similar relation between metallicity and Galactocentric radius. This constancy challenges current models of Galactic chemical evolution, which typically predict lower metallicities for older stellar populations. Our results favor an equilibrium scenario, in which the gas-phase gradient reaches a nearly constant normalization early in the disk lifetime. Using a fiducial choice of parameters, we demonstrate that one possible origin of this behavior is an outflow that more readily ejects gas from the interstellar medium with increasing Galactocentric radius. A direct effect of the outflow is that baryons do not remain in the interstellar medium for long, which causes the ratio of star formation to accretion, $\dotΣ_\star / \dotΣ_\text{in}$, to quickly become constant. This ratio is closely related to the local equilibrium metallicity, since its numerator and denominator set the rates of metal production by stars and hydrogen gained through accretion, respectively. Building in a merger event results in a perturbation that evolves back toward the equilibrium state on $\sim$Gyr timescales. Under the equilibrium scenario, the radial metallicity gradient is not a consequence of the inside-out growth of the disk but instead reflects a trend of declining $\dotΣ_\star / \dotΣ_\text{in}$ with increasing Galactocentric radius.

astro-ph.GA↗

arXiv2024

Stellar Mergers or Truly Young? Intermediate-Age Stars on Highly-Radial Orbits in the Milky Way's Stellar Halo

Reconstructing the mass assembly history of the Milky Way relies on obtaining detailed measurements of the properties of many stars in the Galaxy, especially in the stellar halo. One of the most constraining quantities is stellar age, as it can shed light on the accretion time and quenching of star formation in merging satellites. However, obtaining reliable age estimates for large samples of halo stars is difficult. We report published ages of 120 subgiant halo stars with highly-radial orbits that likely belong to the debris of the \textsl{Gaia-Enceladus/Sausage}~(GES) galaxy. The majority of these halo stars are old, with an age distribution characterized by a median of 11.6~Gyr and 16$^{\rm th}$(84$^{\rm th}$) percentile of 10.5~(12.7)~Gyr. However, the distribution is skewed, with a tail of younger stars that span ages down to $\sim6$--$9$ Gyr. All highly-radial halo stars have chemical and kinematic/orbital quantities that associate them with the GES debris. Initial results suggest that these intermediate-age stars are not a product of mass transfer and/or stellar mergers, which can bias their age determination low. If this conclusion is upheld by upcoming spectro-photometric studies, then the presence of these stars will pose an important challenge for constraining the properties of the GES merger and the accretion history of the Galaxy.

astro-ph.GA↗

arXiv2024

The impact of stellar metallicity on rotation and activity evolution in the Kepler field using gyro-kinematic ages

In recent years, there has been a push to understand how chemical composition affects the magnetic activity levels of main sequence low-mass stars. Results indicate that more metal-rich stars are more magnetically active for a given stellar mass and rotation period. This metallicity dependence has implications for how the rotation periods and activity levels of low-mass stars evolve over their lifetimes. Numerical modelling suggests that at late ages more metal-rich stars should be rotating more slowly and be more magnetically active. In this work, we study the rotation and activity evolution of low-mass stars using a sample of Kepler field stars. We use the gyro-kinematic age dating technique to estimate ages for our sample and use the photometric activity index as our proxy for magnetic activity. We find clear evidence that, at late ages, more metal-rich stars have spun down to slower rotation in agreement with the theoretical modeling. However, further investigation is required to definitively determine whether the magnetic activity evolution occurs in a metallicity dependent way.

astro-ph.SR↗

arXiv2023

LMC Stars and Where to Find Them: Inferring Birth Radii for External Galaxies

It is well known that stars move away from their birth location over time via radial migration. This dynamical process makes computing the correct chemical evolution, e.g., metallicity gradients, of galaxies very difficult. This dynamical process makes inferring the chemical evolution of observed galaxies from their measured abundance gradients very difficult. One way to account for radial migration is to infer stellar birth radii for individual stars. Many attempts to do so have been performed over the last years, but are limited to the Milky Way as computing the birth position of stars requires precise measurements of stellar metallicity and age for individual stars that cover large Galactic radii. Fortunately, recent and future surveys will provide numerous opportunities for inferring birth radii for external galaxies such as the Large Magellanic Cloud (LMC). In this paper, we investigate the possibility of doing so using the NIHAO cosmological zoom-in simulations. We find that it is theoretically possible to infer birth radii with a ~ 25% median uncertainty for individual stars in galaxies with i) orderliness of the orbits, $\langle v_ϕ\rangle/σ_{v} >$ 2, ii) a dark matter halo mass greater or equal to approximately the LMC mass (~ 2 x 10$^{11} M_\odot$), and iii) after the average azimuthal velocity of the stellar disk reaches ~70% of its maximum. From our analysis, we conclude that it is possible and useful to infer birth radii for the LMC and other external galaxies that satisfy the above criteria.

astro-ph.GA↗

arXiv2023

Considerations for Master Protocols Using External Controls

There has been an increasing use of master protocols in oncology clinical trials because of its efficiency and flexibility to accelerate cancer drug development. Depending on the study objective and design, a master protocol trial can be a basket trial, an umbrella trial, a platform trial, or any other form of trials in which multiple investigational products and/or subpopulations are studied under a single protocol. Master protocols can use external data and evidence (e.g., external controls) for treatment effect estimation, which can further improve efficiency of master protocol trials. This paper provides an overview of different types of external controls and their unique features when used in master protocols. Some key considerations in master protocols with external controls are discussed including construction of estimands, assessment of fit-for-use real-world data, and considerations for different types of master protocols. Similarities and differences between regular randomized controlled trials and master protocols when using external controls are discussed. A targeted learning-based causal roadmap is presented which constitutes three key steps: (1) define a target statistical estimand that aligns with the causal estimand for the study objective, (2) use an efficient estimator to estimate the target statistical estimand and its uncertainty, and (3) evaluate the impact of causal assumptions on the study conclusion by performing sensitivity analyses. Two illustrative examples for master protocols using external controls are discussed for their merits and possible improvement in causal effect estimation.

stat.AP↗

arXiv2023

Revealing the Chemical Structure of the Magellanic Clouds with APOGEE. III. Abundance Gradients of the Small Magellanic Cloud

We determine radial- and age-abundance gradients of the Small Magellanic Cloud (SMC) using spectra of 2,062 red giant branch (RGB) field stars observed by SDSS-IV / APOGEE-2S. With coverage out to $\sim$9 kpc in the SMC, these data taken with the high resolution ($R \sim 22,500$) APOGEE $H$-band spectrograph afford the opportunity to measure extensive radial gradients for as many as 24 abundance ratios. The SMC is found to have an overall metallicity gradient of $-$0.0546 $\pm$ 0.0043 dex/kpc. Ages are calculated for every star to explore the evolution of the different abundance gradients. As a function of age, many of the gradients show a feature 3.66--5.58 Gyr ago, which is especially prominent in the [X/H] gradients. Initially many gradients flatten until about $\sim$5.58 Gyr ago, but then steepen in more recent times. We previously detected similar evolutionary patterns in the Large Magellanic Cloud (LMC) which are attributed to a recent interaction between the LMC and SMC. It is inferred that the feature in the SMC gradients was caused by the same interaction. The age-[X/Fe] trends, which track average [X/Fe] over time, are flat, demonstrating a slow enrichment history for the SMC. When comparing the SMC gradients to the LMC and MW, normalized to disk scale length ($R_\text{d}$), the [X/Fe] and [X/Mg] gradients are similar, but there is a dichotomy between the dwarfs and the Milky Way (MW) for the [X/H] gradients. The median MW [X/H] gradient around $-$0.125 dex/$R_\text{d}$ whilst the Clouds have gradients of about $-$0.075 dex/$R_\text{d}$.

astro-ph.GA↗

arXiv2023

Revealing the Chemical Structure of the Magellanic Clouds with APOGEE. II. Abundance Gradients of the Large Magellanic Cloud

We present the abundance gradients of the Large Magellanic Cloud (LMC) for 25 elemental abundance ratios and their respective temporal evolution as well as age-[X/Fe] trends using 6130 LMC field red giant branch (RGB) stars observed by SDSS-IV / APOGEE-2S. APOGEE is a high resolution ($R$ $\sim$22,500) $H$-band spectroscopic survey that gathered data on the LMC with broad radial and azimuthal coverage out to $\sim$10\degr. The calculated overall metallicity gradient of the LMC with no age binning is $-$0.0380 $\pm$ 0.0022 dex/kpc. We also find that many of the abundance gradients show a U-shaped trend as functions of age. This trend is marked by a flattening of the gradient but then a general steepening at more recent times. The extreme point at which all these gradients (with the U-shaped trend) begin to steepen is $\gtrsim$2 Gyr ago. In addition, some of the age-[X/Fe] trends show an increase starting a few Gyr before the extreme point in the gradient evolutions. A subset of the age-[X/Fe] trends also show maxima concurrent with the gradients' extreme points, further pinpointing a major event in the history of the LMC $\sim$2 Gyr ago. This time frame is consistent with a previously proposed interaction between the Magellanic Clouds suggesting that this is most likely the cause of the distinct trend in the gradients and age-[X/Fe] trends.

astro-ph.GA↗

arXiv2023

The impact of early massive mergers on the chemical evolution of Milky Way-like galaxies: insights from NIHAO-UHD simulations

Recent observations of the Milky Way (MW) found an unexpected steepening of the star-forming gas metallicity gradient around the time of the Gaia-Sausage-Enceladus (GSE) merger event. Here we investigate the influence of early ($t_{\mathrm{merger}}\lesssim5$ Gyr) massive ($M_{\mathrm{gas}}^{\mathrm{merger}}/M_{\mathrm{gas}}^{\mathrm{main}}(t_{\mathrm{merger}})\gtrsim10\%$) merger events such as the Gaia-Sausage Enceladus merger in the MW on the evolution of the cold gas metallicity gradient. We use the NIHAO-UHD suite of cosmological hydrodynamical simulations of MW-mass galaxies to study the frequency of massive early mergers and their detailed impact on the morphology and chemistry of the gaseous disks. We find a strong steepening of the metallicity gradient at early times for all four galaxies in our sample which is caused by a sudden increase in the cold gas disk size (up to a factor of 2) in combination with the supply of un-enriched gas ($\sim0.75$ dex lower compared to the main galaxy) by the merging dwarf galaxies. The mergers mostly affect the galaxy outskirts and lead to an increase in cold gas surface density of up to 200% outside of $\sim8$ kpc. The addition of un-enriched gas breaks the self-similar enrichment of the inter-stellar-medium and causes a dilution of the cold gas in the outskirts of the galaxies. The accreted stars and the ones formed later out of the accreted gas inhabit distinct tracks offset to lower [$α$/Fe] and [Fe/H] values compared to the main galaxy's stars. We find that such mergers can contribute significantly to the formation of a second, low-$α$ sequence as is observed in the MW.

astro-ph.GA↗

arXiv2022

The 3D Galactocentric velocities of Kepler stars: marginalizing over missing RVs

Precise Gaia measurements of positions, parallaxes, and proper motions provide an opportunity to calculate 3D positions and 2D velocities (i.e. 5D phase-space) of Milky Way stars. Where available, spectroscopic radial velocity (RV) measurements provide full 6D phase-space information, however there are now and will remain many stars without RV measurements. Without an RV it is not possible to directly calculate 3D stellar velocities, however one can infer 3D stellar velocities by marginalizing over the missing RV dimension. In this paper, we infer the 3D velocities of stars in the Kepler field in Cartesian Galactocentric coordinates (vx, vy, vz). We directly calculate velocities for around a quarter of all Kepler targets, using RV measurements available from the Gaia, LAMOST and APOGEE spectroscopic surveys. Using the velocity distributions of these stars as our prior, we infer velocities for the remaining three-quarters of the sample by marginalizing over the RV dimension. The median uncertainties on our inferred vx, vy, and vz velocities are around 4, 18, and 4 km/s, respectively. We provide 3D velocities for a total of 148,590 stars in the Kepler field. These 3D velocities could enable kinematic age-dating, Milky Way stellar population studies, and other scientific studies using the benchmark sample of well-studied Kepler stars. Although the methodology used here is broadly applicable to targets across the sky, our prior is specifically constructed from and for the Kepler field. Care should be taken to use a suitable prior when extending this method to other parts of the Galaxy.

astro-ph.GA↗

arXiv2021

Evolution of the Exoplanet Size Distribution: Forming Large Super-Earths Over Billions of Years

The radius valley, a bifurcation in the size distribution of small, close-in exoplanets, is hypothesized to be a signature of planetary atmospheric loss. Such an evolutionary phenomenon should depend on the age of the star-planet system. In this work, we study the temporal evolution of the radius valley using two independent determinations of host star ages among the California-Kepler Survey (CKS) sample. We find evidence for a wide and nearly empty void of planets in the period-radius diagram at the youngest system ages ($\lesssim$2-3 Gyr) represented in the CKS sample. We show that the orbital period dependence of the radius valley among the younger CKS planets is consistent with that found among those planets with asteroseismically determined host star radii. Relative to previous studies of preferentially older planets, the radius valley determined among the younger planetary sample is shifted to smaller radii. This result is compatible with an atmospheric loss timescale on the order of gigayears for progenitors of the largest observed super-Earths. In support of this interpretation, we show that the planet sizes which appear to be unrepresented at ages $\lesssim$2-3 Gyr are likely to correspond to planets with rocky compositions. Our results suggest the size distribution of close-in exoplanets, and the precise location of the radius valley, evolves over gigayears.

astro-ph.EP↗

arXiv2021

When is it permissible for artificial intelligence to lie? A trust-based approach

Conversational Artificial Intelligence (AI) used in industry settings can be trained to closely mimic human behaviors, including lying and deception. However, lying is often a necessary part of negotiation. To address this, we develop a normative framework for when it is ethical or unethical for a conversational AI to lie to humans, based on whether there is what we call "invitation of trust" in a particular scenario. Importantly, cultural norms play an important role in determining whether there is invitation of trust across negotiation settings, and thus an AI trained in one culture may not be generalizable to others. Moreover, individuals may have different expectations regarding the invitation of trust and propensity to lie for human vs. AI negotiators, and these expectations may vary across cultures as well. Finally, we outline how a conversational chatbot can be trained to negotiate ethically by applying autoregressive models to large dialog and negotiations datasets.

cs.CY↗

arXiv2021

Universal properties of the high- and low-α disk: small intrinsic abundance scatter and migrating stars

The detailed age-chemical abundance relations of stars measures time-dependent chemical evolution.These trends offer strong empirical constraints on nucleosynthetic processes, as well as the homogeneityof star-forming gas. Characterizing chemical abundances of stars across the Milky Way over time has been made possible very recently, thanks to surveys like Gaia, APOGEE and Kepler. Studies of the low-$α$ disk have shown that individual elements have unique age-abundance trends and the intrinsic dispersion around these relations is small. In this study, we examine and compare the age distribution of stars across both the high and low-$α$ disk and quantify the intrinsic dispersion of 16 elements around their age-abundance relations at [Fe/H] = 0 using APOGEE DR16. We find the high-$α$ disk has shallower age-abundance relations compared to the low-$α$ disk, but similar median intrinsic dispersions of ~ 0.04 dex, suggesting universal element production mechanisms for the high and low-$α$ disks, despite differences in formation history. We visualize the temporal and spatial distribution of disk stars in small chemical cells, revealing signatures of upside-down and inside-out formation. Further,the metallicity skew and the [Fe/H]-age relations - across radius indicates different initial metallicity gradients and evidence for radial migration. Our study is accompanied by an age catalogue for 64,317 stars in APOGEE derived usingThe Cannon with ~ 1.9 Gyr uncertainty across all ages (APO-CAN stars) as well as a red clump catalogue of 22,031 stars with a contamination rate of 2.7%.

astro-ph.GA↗

arXiv2021

Gyro-Kinematic Ages for around 30,000 Kepler Stars

Estimating stellar ages is important for advancing our understanding of stellar and exoplanet evolution and investigating the history of the Milky Way. However, ages for low-mass stars are hard to infer as they evolve slowly on the main sequence. In addition, empirical dating methods are difficult to calibrate for low-mass stars as they are faint. In this work, we calculate ages for Kepler F, G, and crucially K and M dwarfs, using their rotation and kinematic properties. We apply the simple assumption that the velocity dispersion of stars increases over time and adopt an age--velocity--dispersion relation (AVR) to estimate average stellar ages for groupings of coeval stars. We calculate the vertical velocity dispersion of stars in bins of absolute magnitude, temperature, rotation period, and Rossby number and then convert velocity dispersion to kinematic age via an AVR. Using this method, we estimate gyro-kinematic ages for 29,949 Kepler stars with measured rotation periods. We are able to estimate ages for clusters and asteroseismic stars with an RMS of 1.22 Gyr and 0.26 Gyr respectively. With our Astraea machine learning algorithm, which predicts rotation periods, we suggest a new selection criterion (a weight of 0.15) to increase the size of the McQuillian et al. (2014) catalog of Kepler rotation periods by up to 25%. Using predicted rotation periods, we estimated gyro-kinematic ages for stars without measured rotation periods and found promising results by comparing 12 detailed age--element abundance trends with literature values.

astro-ph.SR↗

arXiv2020

Spoiled for Choice? Personalized Recommendation for Healthcare Decisions: A Multi-Armed Bandit Approach

Online healthcare communities provide users with various healthcare interventions to promote healthy behavior and improve adherence. When faced with too many intervention choices, however, individuals may find it difficult to decide which option to take, especially when they lack the experience or knowledge to evaluate different options. The choice overload issue may negatively affect users' engagement in health management. In this study, we take a design-science perspective to propose a recommendation framework that helps users to select healthcare interventions. Taking into account that users' health behaviors can be highly dynamic and diverse, we propose a multi-armed bandit (MAB)-driven recommendation framework, which enables us to adaptively learn users' preference variations while promoting recommendation diversity in the meantime. To better adapt an MAB to the healthcare context, we synthesize two innovative model components based on prominent health theories. The first component is a deep-learning-based feature engineering procedure, which is designed to learn crucial recommendation contexts in regard to users' sequential health histories, health-management experiences, preferences, and intrinsic attributes of healthcare interventions. The second component is a diversity constraint, which structurally diversifies recommendations in different dimensions to provide users with well-rounded support. We apply our approach to an online weight management context and evaluate it rigorously through a series of experiments. Our results demonstrate that each of the design components is effective and that our recommendation design outperforms a wide range of state-of-the-art recommendation systems. Our study contributes to the research on the application of business intelligence and has implications for multiple stakeholders, including online healthcare platforms, policymakers, and users.

cs.LG↗

arXiv2020

Astraea: Predicting Long Rotation Periods with 27-Day Light Curves

The rotation periods of planet-hosting stars can be used for modeling and mitigating the impact of magnetic activity in radial velocity measurements and can help constrain the high-energy flux environment and space weather of planetary systems. Millions of stars and thousands of planet hosts are observed with the Transiting Exoplanet Survey Satellite (TESS). However, most will only be observed for 27 contiguous days in a year, making it difficult to measure rotation periods with traditional methods. This is especially problematic for field M dwarfs, which are ideal candidates for exoplanet searches, but which tend to have periods in excess of the 27-day observing baseline. We present a new tool, Astraea, for predicting long rotation periods from short-duration light curves combined with stellar parameters from Gaia DR2. Using Astraea, we can predict the rotation periods from Kepler 4-year light curves with 13% uncertainty overall (and a 9% uncertainty for periods > 30 days). By training on 27-day Kepler light curve segments, Astraea can predict rotation periods up to 150 days with 9% uncertainty (5% for periods > 30 days). After training this tool on these 27-day Kepler light curve segments, we applied \texttt{Astraea} to real TESS data. For the 195 stars that were observed by both Kepler and TESS, we were able to predict the rotation periods with 55% uncertainty despite the wild differences in systematics.

astro-ph.SR↗