Birth Radii Research Articles

When LAMOST meets Gaia DR3. Exploring the metallicity of open clusters

Open clusters (OCs) are excellent probes as their age and abundance can be tightly constrained, allowing us to explore the distribution of metallicity and composition across the disk of the Milky Way. By conducting a comprehensive analysis of the metallicity of OCs, we can obtain valuable information about the history of their chemical enrichment. Moreover, by observing stars in different regions of the Milky Way, we can identify significant spatial structures in their chemical composition and abundance. This enables us to understand stellar birth radii through chemical tagging. Nevertheless, it remains challenging to infer the original positions of OCs using current data alone. The aim of this study is to investigate the distribution of metallicity in the solar neighborhood using a large dataset from Gaia DR3 combined with LAMOST spectra. With accurate ages and metallicity measurements, we can determine birth radii for the stars and attempt to understand their migration pattern. We chose a total of 1131 OCs within 3 Kpc of the Sun from the Gaia DR3 and LAMOST DR8 low-resolution spectral database (R=1800). We used an artificial neural network to correct the LAMOST data by incorporating high-resolution spectral data from GALAH DR3 (R=28000). The average metallicity of the OCs was determined based on the reliable Fe/H values for their members. We then examined the distribution of metallicity across different regions within the Galaxy and inferred birth radii of the OCs from their age and metallicity. The correction method presented here can partially eliminate the systematic offset for LAMOST data. We discuss the metallicity trend as a function of Galactocentric distance and the guiding radii. We also compare these observational results with those from chemo-dynamic simulations. Values derived from observational metallicity data are slightly lower than predicted values when the uncertainties are not considered. However, the metallicity gradients are consistent with previous calculations. Finally, we investigated the birthplace of OCs and find hints that the majority of OCs near the Sun have migrated from the outer Galactic disk.

The chemical evolution of the Milky Way thin disk using solar twins

In this study we address whether the age--metallicity relation (AMR) deviates from the expected trend of metallicity increasing smoothly with age. We also show the presence (or absence) of two populations, as recently claimed using a relatively small dataset. Moreover, we studied the Milky Way thin disk's chemical evolution using solar twins, including the effect of radial migration and accretion events. In particular, we exploited high-resolution spectroscopy of a large sample of solar twins in tandem with an accurate age determination to investigate the Milky Way thin disk age--metallicity relationship. Additionally, we derived the stars' birth radius and studied the chemical evolution of the thin disk. We discovered that statistical and selection biases can lead to a misinterpretation of the observational data. An accurate accounting of all the uncertainties led us to detect no separation in the AMR into different populations for solar twins around the Sun ($-0.3 < Fe/H < 0.3$ dex). This lead us to the conclusion that the thin disk was formed relatively smoothly. For the main scenario of the Milky Way thin disk formation, we suggest that the main mechanism for reaching today's chemical composition around the Sun is radial migration with the possible contribution of well-known accretion events such as Gaia -Enceladus/Sausage (GES) and Sagittarius (Sgr).

LMC stars and where to find them: inferring birth radii for external galaxies

ABSTRACT It is well known that stars are subject to radial migration, i.e. over time, they move away from their birth location. This dynamical process tends to mix different stellar populations and hence hinders the determination of the true chemical evolution of a galaxy (e.g. metallicity gradients). 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 few 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 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 per cent median uncertainty for individual stars in galaxies with i) orderliness of the orbits, $\langle v_\phi \rangle /\sigma _{v} > 2 $, ii) a dark matter halo mass greater or equal to approximately the LMC mass (∼2 × 1011 ${\rm M}_\odot$), and iii) after the average azimuthal velocity of the stellar disc reaches ∼70 per cent 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.

A path towards constraining the evolution of the interstellar medium and outflows in the Milky Way using APOGEE

ABSTRACT In recent years, the study of the Milky Way has significantly advanced due to extensive spectroscopic surveys of its stars, complemented by astroseismic and astrometric data. However, it remains disjoint from recent advancements in understanding the physics of the Galactic interstellar medium (ISM). This paper introduces a new model for the chemical evolution of the Milky Way that can be constrained on stellar data, because it combines a state-of-the-art ISM model with a Milky Way stellar disc model. Utilizing a data set of red clump stars from APOGEE, known for their precise ages and metallicities, we concentrate on the last 6 billion years – a period marked by Milky Way’s secular evolution. We examine the oxygen abundance in the low-$\alpha$ disc stars relative to their ages and birth radii, validating or constraining critical ISM parameters that remain largely unexplored in extragalactic observations. The models that successfully reproduce the radius–metallicity distribution and the age–metallicity distribution of stars without violating existing ISM observations indicate a need for modest differential oxygen enrichment in Galactic outflows, meaning that the oxygen abundance of outflows is higher than the local ISM abundance, irrespective of outflow mass loading. The models also suggest somewhat elevated ISM gas velocity dispersion levels over the past 6 billion years compared to galaxies of similar mass. The extra turbulence necessary could result from energy from gas accretion onto the Galaxy, supernovae clustering in the ISM, or increased star formation efficiency per freefall time. This work provides a novel approach to constraining the Galactic ISM and outflows, leveraging the detailed insights available from contemporary Milky Way surveys.

Chemical clocks and their time zones: understanding the [s/Mg]–age relation with birth radii

ABSTRACT The relative enrichment of s-process to α-elements ([s/α]) has been linked with age, providing a potentially useful avenue in exploring the Milky Way’s chemical evolution. However, the age–[s/α] relationship is non-universal, with dependencies on metallicity and current location in the Galaxy. In this work, we examine these chemical clock tracers across birth radii (${R}_\text{birth}$), recovering the inherent trends between the variables. We derive ${R}_\text{birth}$ and explore the [s/α]–age–${R}_\text{birth}$ relationship for 36 652 APOGEE DR17 red giant and 24 467 GALAH DR3 main-sequence turn-off and subgiant branch disc stars using [Ce/Mg], [Ba/Mg], and [Y/Mg]. We discover that the age–$\rm [{\it s}/Mg]$ relation is strongly dependent on birth location in the Milky Way, with stars born in the inner disc having the weakest correlation. This is congruent with the Galaxy’s initially weak, negative $\rm [{\it s}/Mg]$ radial gradient, which becomes positive and steep with time. We show that the non-universal relations of chemical clocks is caused by their fundamental trends with ${R}_\text{birth}$ over time, and suggest that the tight age–$\rm [{\it s}/Mg]$ relation obtained with solar-like stars is due to similar ${R}_\text{birth}$ for a given age. Our results are put into context with a Galactic chemical evolution model, where we demonstrate the need for data-driven nucleosynthetic yields.

Exploring the Sun’s birth radius and the distribution of planet building blocks in the Milky Way galaxy: a multizone Galactic chemical evolution approach

ABSTRACT We explore the influence of the Milky Way galaxy’s chemical evolution on the formation, structure, and habitability of the Solar system. Using a multizone Galactic chemical evolution (GCE) model, we successfully reproduce key observational constraints, including the age–metallicity ([Fe/H]) relation, metallicity distribution functions, abundance gradients, and [X/Fe] ratio trends for critical elements involved in planetary mineralogy, including C, O, Mg, and Si. Our GCE model suggests that the Sun formed in the inner Galactic disc, Rbirth,⊙ ≈ 5 kpc. We also combined a stoichiometric model with the GCE model to examine the temporal evolution and spatial distribution of planet building blocks within the Milky Way galaxy, revealing trends in the condensed mass fraction (fcond), iron-to-silicon mass fraction (firon), and water mass fraction (fwater) over time and towards the inner Galactic disc regions. Specifically, our model predicts a higher fcond in the protoplanetary disc within the inner regions of the Milky Way galaxy, as well as an increased firon and a decreased fwater in the inner regions. Based on these findings, we discuss the potential impact of the Sun’s birth location on the overall structure and habitability of the Solar system.

The individual abundance distributions of disc stars across birth radii in GALAH

ABSTRACT Individual abundances in the Milky Way disc record stellar birth properties [e.g. age, birth radius (Rbirth)], and capture the diversity of the star-forming environments over time. Assuming an analytical relationship between ([Fe/H], [α/Fe]), and Rbirth, we examine the distributions of individual abundances [X/Fe] of elements C, O, Mg, Si, Ca (α), Al (odd-z), Mn (iron-peak), Y, and Ba (neutron-capture) for stars in the Milky Way. We want to understand how these elements might differentiate environments across the disc. We assign tracks of Rbirth in the [α/Fe] versus [Fe/H] plane as informed by expectations from simulations for ∼59 000 GALAH stars in the solar neighborhood (R ∼ 7−9 kpc) which also have inferred ages. Our formalism for Rbirth shows that older stars (∼10 Gyrs) have an Rbirth distribution with smaller mean values (i.e. $\bar{R}_{\mbox{birth}} \sim 5\pm 0.8$ kpc) compared to younger stars (∼6 Gyrs; $\bar{R}_{\mbox{birth}} \sim 10\pm 1.5$ kpc), for a given [Fe/H], consistent with inside–out growth. The α-, odd-z, and iron-peak element abundances decrease as a function of Rbirth, whereas the neutron-capture abundances increase. The Rbirth–[Fe/H] gradient we measure is steeper compared to the present-day gradient (−0.066 dex kpc−1 versus −0.058 dex kpc−1), which we also find true for Rbirth–[X/Fe] gradients. These results (i) showcase the feasibility of relating the birth radius of stars to their element abundances, (ii) demonstrate that the Milky Way abundance gradients across Rbirth have evolved to be shallower over time, and (iii) offer an observational comparison to element abundance distributions in hydrodynamical simulations.

Unveiling the time evolution of chemical abundances across the Milky Way disc with APOGEE

ABSTRACT Chemical abundances are an essential tool in untangling the Milky Way’s enrichment history. However, the evolution of the interstellar medium abundance gradient with cosmic time is lost as a result of radial mixing processes. For the first time, we quantify the evolution of many observational abundances across the Galactic disc as a function of lookback time and birth radius, $\rm \text{R}_\text{birth}$. Using an empirical approach, we derive $\rm \text{R}_\text{birth}$ estimates for 145 447 APOGEE DR17 red giant disc stars, based solely on their ages and $\rm [Fe/H]$. We explore the detailed evolution of six abundances [Mg, Ca (α), Mn (iron-peak), Al, C (light), Ce (s-process)] across the Milky Way disc using 87 426 APOGEE DR17 red giant stars. We discover that the interstellar medium had three fluctuations in the metallicity gradient ∼9, ∼6, and ∼4 Gyr ago. The first coincides with the end of high-α sequence formation around the time of the Gaia–Sausage–Enceladus disruption, while the others are likely related to passages of the Sagittarius dwarf galaxy. A clear distinction is found between present-day observed radial gradients with age and the evolution with lookback time for both [X/Fe] and [X/H], resulting from the significant flattening and inversion in old populations due to radial migration. We find the $\rm [Fe/H]$–$\rm [\alpha /Fe]$ bimodality is also seen as a separation in the $\rm \text{R}_\text{birth}$–$\rm [X/Fe]$ plane for the light and α-elements. Our results recover the chemical enrichment of the Galactic disc over the past 12 Gyr, providing tight constraints on Galactic disc chemical evolution models.

Chemodynamical ages of small-scale kinematic structures of the galactic disc in the solar neighbourhood from ∼250 000 K and M dwarfs

ABSTRACT We combine photometric metallicities with astrometry from Gaia DR3 to examine the chemodynamic structure of ∼250 000 K dwarfs in the solar neighbourhood (SN). In kinematics, we observe ridges/clumps of ‘kinematic groups’, like studies of more massive main-sequence stars. Here, we note clear differences in both metallicity and vertical velocity as compared with the surrounding regions in velocity space and hypothesize this is due to differences in mean age. To test this, we develop a method to estimate the age distribution of subpopulations of stars. In this method, we use GALAH data to define probability distributions of W versus [M/H] in age bins of 2 Gyr and determine optimal age distributions as the best-fitting weighted sum of these distributions. This process is then validated using the GALAH subset. We estimate the probable age distribution for regions in the kinematic plane, where we find significant substructure that is correlated with the kinematic groups. Most notably, we find an age gradient across the Hercules streams that is correlated with birth radius. Finally, we examine the bending and breathing modes as a function of age. From this, we observe potential hints of an increase in the bending amplitude with age, which will require further analysis in order to confirm it. This is one of the first studies to examine these chemodynamics in the SN using primarily low-mass stars and we hope these findings can better constrain dynamical models of the Milky Way due to the increase in resolution the sample size provides.

The role of radial migration on tracing lithium evolution in the Galactic disc

ABSTRACTWith the calculated guiding centre radius Rguiding and birth radius Rbirth, we investigate the role of radial migration on the description of lithium evolution in the Galactic disc based on the upper envelope of the A(Li) versus [Fe/H] diagram. Using migration distances, we find that stars in the solar neighbourhood are born at different locations in the Galactic disc, and cannot all be explained by models of chemical evolution in the solar neighbourhood. It is found that the upper envelope of the A(Li) versus [Fe/H] diagram varies significantly with Rbirth, which explains the decrease of Li for super-metal-rich (SMR) stars because they are non-young stars born in the inner disc. The upper envelope of Li-Rbirth plane fits very well with chemical evolution models by Grisoni et al. for Rbirth = 7–12 kpc, outside which young stars generally lack sufficient time to migrate to the solar neighbourhood. For stars born in the solar neighbourhood, the young open clusters and the upper envelope of field stars with age <3 Gyr fit well with theoretical prediction. We find that calculations using stars with ages less than 3 Gyr are necessary to obtain an undepleted Li upper envelope, and that stars with solar age (around 4.5 Gyr) have depleted around 0.3 dex from the original value based on the chemical evolution model of Grisoni et al.

The GALAH survey: chemical clocks

ABSTRACT We present the first large-scale study that demonstrates how ages can be determined for large samples of stars through Galactic chemical evolution. Previous studies found that the elemental abundances of a star correlate directly with its age and metallicity. Using this knowledge, we derive ages for 214 577 stars in GALAH DR3 using only overall metallicities and chemical abundances. Stellar ages are estimated via the machine learning algorithm XGBoost for stars belonging to the Milky Way disc with metallicities in the range −1 < [Fe/H] < 0.5, using main-sequence turn-off stars as our training set. We find that stellar ages for the bulk of GALAH DR3 are precise to 1–2 Gyr using this method. With these ages, we replicate many recent results on the age-kinematic trends of the nearby disc, including the solar neighbourhood’s age–velocity dispersion relationship and the larger global velocity dispersion relations of the disc found using Gaia and GALAH. These results show that chemical abundance variations at a given birth radius are small, and that strong chemical tagging of stars directly to birth clusters may prove difficult with our current elemental abundance precision. Our results highlight the need to measure abundances for as many nucleosynthetic production sites as possible in order to estimate reliable ages from chemistry. Our methods open a new door into studies of the kinematic structure and evolution of the disc, as ages may potentially be estimated to a precision of 1–2 Gyr for a large fraction of stars in existing spectroscopic surveys.

Reliability and limitations of inferring birth radii in the Milky Way disc

ABSTRACT Recovering the birth radii of observed stars in the Milky Way is one of the ultimate goals of Galactic Archaeology. One method to infer the birth radius and the evolution of the interstellar medium (ISM) metallicity assumes a linear relation between the ISM metallicity with radius at any given look-back time. Here, we test the reliability of this assumption by using four zoom-in cosmological hydrodynamic simulations from the NIHAO-UHD project. We find that one can infer precise birth radii only when the stellar disc starts to form, which for our modelled galaxies happens ∼10 Gyr ago, in agreement with recent estimates for the Milky Way. With a current day measurement of ISM metallicity gradient of −0.05 dex and a dispersion of 0.03 dex, the intrinsic uncertainty in inferring Rbirth is ∼0.6 kpc. At later times, the linear correlation between the ISM metallicity and radius increases, as stellar motions become more ordered and the azimuthal variations of the ISM metallicity start to drop. The formation of a central bar and perturbations from mergers can increase this uncertainty in the inner and outer disc, respectively.

Quantifying radial migration in the Milky Way: inefficient over short time-scales but essential to the very outer disc beyond ∼15 kpc

ABSTRACT Stellar radial migration plays an important role in reshaping a galaxy’s structure and the radial distribution of stellar population properties. In this work, we revisit reported observational evidence for radial migration and quantify its strength using the age–[Fe/H] distribution of stars across the Milky Way with APOGEE data. We find a broken age–[Fe/H] relation in the Galactic disc at r > 6 kpc, with a more pronounced break at larger radii. To quantify the strength of radial migration, we assume stars born at each radius have a unique age and metallicity, and then decompose the metallicity distribution function (MDF) of mono-age young populations into different Gaussian components that originated from various birth radii at rbirth < 13 kpc. We find that, at ages of 2 and 3 Gyr, roughly half the stars were formed within 1 kpc of their present radius, and very few stars (<5 per cent) were formed more than 4 kpc away from their present radius. These results suggest limited short-distance radial migration and inefficient long-distance migration in the Milky Way during the last 3 Gyr. In the very outer disc beyond 15 kpc, the observed age–[Fe/H] distribution is consistent with the prediction of pure radial migration from smaller radii, suggesting a migration origin of the very outer disc. We also estimate intrinsic metallicity gradients at ages of 2 and 3 Gyr of −0.061 and −0.063 dex kpc−1, respectively.

The Homogeneity of the Star-forming Environment of the Milky Way Disk over Time

Abstract Stellar abundances and ages afford the means to link chemical enrichment to galactic formation. In the Milky Way, individual element abundances show tight correlations with age, which vary in slope across ([Fe/H]–[α/Fe]). Here, we step from characterizing abundances as measures of age, to understanding how abundances trace properties of stellar birth environment in the disk over time. Using measurements from ∼27,000 APOGEE stars (R = 22,500, signal-to-noise ratio > 200), we build simple local linear models to predict a sample of elements (X = Si, O, Ca, Ti, Ni, Al, Mn, Cr) using (Fe, Mg) abundances alone, as fiducial tracers of supernovae production channels. Given [Fe/H] and [Mg/H], we predict these elements, [X/H], to about double the uncertainty of their measurements. The intrinsic dispersion, after subtracting measurement errors in quadrature is ≈0.015–0.04 dex. The residuals of the prediction (measurement − model) for each element demonstrate that each element has an individual link to birth properties at fixed (Fe, Mg). Residuals from primarily massive-star supernovae (i.e., Si, O, Al) partially correlate with guiding radius. Residuals from primarily supernovae Ia (i.e., Mn, Ni) partially correlate with age. A fraction of the intrinsic scatter that persists at fixed (Fe, Mg), however, after accounting for correlations, does not appear to further discriminate between birth properties that can be traced with present-day measurements. Presumably, this is because the residuals are also, in part, a measure of the typical (in)-homogeneity of the disk’s stellar birth environments, previously inferred only using open cluster systems. Our study implies at fixed birth radius and time that there is a median scatter of ≈0.01–0.015 dex in elements generated in supernovae sources.

The GALAH Survey: dependence of elemental abundances on age and metallicity for stars in the Galactic disc

ABSTRACT Using data from the GALAH survey, we explore the dependence of elemental abundances on stellar age and metallicity among Galactic disc stars. We find that the abundance of most elements can be predicted from age and [Fe/H] with an intrinsic scatter of about 0.03 dex. We discuss the possible causes for the existence of the abundance–age–metallicity relations. Using a stochastic chemical enrichment scheme that takes the volume of supernovae remnants into account, we show the intrinsic scatter is expected to be small, about 0.05 dex or even smaller if there is additional mixing in the ISM. Elemental abundances show trends with both age and metallicity and the relationship is well described by a simple model in which the dependence of abundance ([X/Fe]) on age and [Fe/H] are additively separable. Elements can be grouped based on the direction of their abundance gradient in the (age,[Fe/H]) plane and different groups can be roughly associated with three distinct nucleosynthetic production sites, the exploding massive stars, the exploding white dwarfs, and the AGB stars. However, the abundances of some elements, like Co, La, and Li, show large scatter for a given age and metallicity, suggesting processes other than simple Galactic chemical evolution are at play. We also compare the abundance trends of main-sequence turn-off (MSTO) stars against that of giants, whose ages were estimated using asteroseismic information from the K2 mission. For most elements, the trends of MSTO stars are similar to that of giants. The existence of abundance relations implies that we can estimate the age and birth radius of disc stars, which is important for studying the dynamic and chemical evolution of the Galaxy.

Fundamental relations for the velocity dispersion of stars in the Milky Way

ABSTRACT We explore the fundamental relations governing the radial and vertical velocity dispersions of stars in the Milky Way, from combined studies of complementary surveys including GALAH, LAMOST, APOGEE, the NASA Kepler and K2 missions, and Gaia DR2. We find that different stellar samples, even though they target different tracer populations and employ a variety of age estimation techniques, follow the same set of fundamental relations. We provide the clearest evidence to date that, in addition to the well-known dependence on stellar age, the velocity dispersions of stars depend on orbital angular momentum Lz, metallicity, and height above the plane |z|, and are well described by a multiplicatively separable functional form. The dispersions have a power-law dependence on age with exponents of 0.441 ± 0.007 and 0.251 ± 0.006 for σz and σR, respectively, and the power law is valid even for the oldest stars. For the solar neighbourhood stars, the apparent break in the power law for older stars, as seen in previous studies, is due to the anticorrelation of Lz with age. The dispersions decrease with increasing Lz until we reach the Sun’s orbital angular momentum, after which σz increases (implying flaring in the outer disc) while σR flattens. For a given age, the dispersions increase with decreasing metallicity, suggesting that the dispersions increase with birth radius. The dispersions also increase linearly with |z|. The same set of relations that work in the solar neighbourhood also work for stars between 3 < R/kpc < 20. Finally, the high-[α/Fe] stars follow the same relations as the low-[α/Fe] stars.

Keeping It Cool: Much Orbit Migration, yet Little Heating, in the Galactic Disk

Abstract A star in the Milky Way’s disk can now be at a Galactocentric radius quite distant from its birth radius for two reasons: either its orbit has become eccentric through radial heating, which increases its radial action J R (“blurring”), or merely its angular momentum L z has changed and thereby its guiding radius (“churning”). We know that radial orbit migration is strong in the Galactic low-α disk and set out to quantify the relative importance of these two effects, by devising and applying a parameterized model ( ) for the distribution in the stellar disk. This model describes the orbit evolution for stars of age τ and metallicity , presuming that coeval stars were initially born on (near-)circular orbits, and with a unique at a given birth angular momentum and age. We fit this model to APOGEE red clump stars, accounting for the complex selection function of the survey. The best-fit model implies changes of angular momentum of and changes of radial action as at 8 kpc. This suggests that the secular orbit evolution of the disk is dominated by diffusion in angular momentum, with radial heating being an order of magnitude lower.

Super Metal-rich Stars in the LAMOST Survey: A Test on Radial Migration

Abstract Super metal-rich stars with [Fe/H] > 0.4 are selected from LAMOST DR6, and two groups, the blue and the red, are found in the T eff versus logg diagram with a temperature gap between them. In combination with Gaia DR2, stellar positions, velocities, and orbits are calculated, and spatial distributions, kinematical properties, and orbital parameters are compared between the two groups. The blue group shows mainly thin-disk kinematics and spans a wide R range of 6–12 kpc, while the red group has both the thin-disk and the thick-disk kinematics with a narrower range of R = 6–10 kpc. The kinematical and orbital parameters of stars in the blue group indicate that they could belong to the young population with age less than 1 Gyr, rather than blue stragglers of the old population. The orbital parameters, R p , R a , and R g , of the red group with the thick-disk kinematics are smaller than those with the thin-disk kinematics. The distributions of birth radius and migration distance indicate that radial migration is a favorable origin for the red group, especially those with the thick-disk kinematics, but not for the blue group. The relative magnitude of radial migration is of 51% for the whole sample and of 64% for the red group only. The corotation radial migration caused by the bars and spiral arms at resonances is the most likely mechanism for explaining these properties of SMR stars in this work.

Yule-Simpson’s paradox in Galactic Archaeology

Simpson's paradox, or Yule-Simpson effect, arises when a trend appears in different subsets of data but disappears or reverses when these subsets are combined. We describe here seven cases of this phenomenon for chemo-kinematical relations believed to constrain the Milky Way disk formation and evolution. We show that interpreting trends in relations, such as the radial and vertical chemical abundance gradients, the age-metallicity relation, and the metallicity-rotational velocity relation (MVR), can lead to conflicting conclusions about the Galaxy past if analyses marginalize over stellar age and/or birth radius. It is demonstrated that the MVR in RAVE giants is consistent with being always strongly negative, when narrow bins of [Mg/Fe] are considered. This is directly related to the negative radial metallicity gradients of stars grouped by common age (mono-age populations) due to the inside out disk formation. The effect of the asymmetric drift can then give rise to a positive MVR trend in high-[alpha/Fe] stars, with a slope dependent on a given survey's selection function and observational uncertainties. We also study the variation of lithium abundance, A(Li), with [Fe/H] of AMBRE:HARPS dwarfs. A strong reversal in the positive A(Li)-[Fe/H] trend of the total sample is found for mono-age populations, flattening for younger groups of stars. Dissecting by birth radius shows strengthening in the positive A(Li)-[Fe/H] trend, shifting to higher [Fe/H] with decreasing birth radius; these observational results suggest new constraints on chemical evolution models. This work highlights the necessity for precise age estimates for large stellar samples covering wide spatial regions.

APOGEE [C/N] Abundances across the Galaxy: Migration and Infall from Red Giant Ages

Abstract We present [C/N]–[Fe/H] abundance trends from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment survey, Data Release 14 (DR14), for red giant branch stars across the Milky Way (3 kpc < R < 15 kpc). The carbon-to-nitrogen ratio (often expressed as [C/N]) can indicate the mass of a red giant star, from which an age can be inferred. Using masses and ages derived by Martig et al., we demonstrate that we are able to interpret the DR14 [C/N]–[Fe/H] abundance distributions as trends in age–[Fe/H] space. Our results show that an anticorrelation between age and metallicity, which is predicted by simple chemical evolution models, is not present at any Galactic zone. Stars far from the plane ( kpc) exhibit a radial gradient in [C/N] (∼−0.04 dex kpc−1). The [C/N] dispersion increases toward the plane (σ [C/N] = 0.13 at kpc to σ [C/N] = 0.18 dex at ∣Z∣ < 0.5 kpc). We measure a disk metallicity gradient for the youngest stars (age < 2.5 Gyr) of −0.060 dex kpc−1 from 6 to 12 kpc, which is in agreement with the gradient found using young CoRoGEE stars by Anders et al. Older stars exhibit a flatter gradient (−0.016 dex kpc−1), which is predicted by simulations in which stars migrate from their birth radii. We also find that radial migration is a plausible explanation for the observed upturn of the [C/N]–[Fe/H] abundance trends in the outer Galaxy, where the metal-rich stars are relatively enhanced in [C/N].