Thank you to everyone who attended the 2021 GALAH Survey Science Meeting, which was held from 22 to 24 June 2021. It was a hybrid format with people both online, and meeting in-person in Sydney, Australia. This page is an archive of the conference material, including links to recordings of the talks.

Many of the attendees for the 2021 GALAH Science Meeting. Some of them appear twice because we took photos for the Americas and Europe.

### Meeting Rationale

The GALAH Survey is an ongoing, ambitious stellar spectroscopic survey of the local Galactic volume, acquiring high-resolution optical spectra for over one million stars with the HERMES spectrograph at the Anglo-Australian Telescope. GALAH had its Third Data Release in November 2020, which provided reduced spectra, stellar parameters, elemental abundances, and radial velocities for about 600000 stars. Subsequent to this release was the Third Early Data Release from the Gaia astrometric mission. The combination of these two data releases has provided an unprecedented view of our Galaxy. This conference aimed to bring together researchers to present the latest research taking place using the GALAH survey, Gaia, and other stellar surveys.

This conference used a hybrid format with over 200 registrations from around the world. About 20 people in Sydney were able to meet in-person on the campus of the University of New South Wales in Sydney on 22 June and first half of 23 June 2021. Unfortunately a small COVID-19 cluster meant we moved the conference to fully online for the rest of 23 June and all of 24 June. The conference ran from 9 am to 6 pm Australian Eastern Time each day, so people from around the world joined when they were able to. We had a session in the Australian morning for the Americas to join, and a session in the Australian afternoon for the Europeans to join.

### Acknowledgement of Country

The in-person component of the 2021 GALAH Survey Science Meeting took on place on the unceded territory of the Bedegal peoples. We pay our respect to their elders past and present and extend that respect to all Aboriginal and Torres Strait Islander peoples who joined us.

### Invited speakers

• Tobias Buck (AIP)
• Sven Buder (ANU)
• Emily Cunningham (Flatiron)
• Diane Feuillet (Lund Observatory)
• Keith Hawkins (University of Texas at Austin)
• Paula Jofré (Universidad Diego Portales)
• Tadafumi Matsuno (Kapteyn Astronomical Institute)
• Melissa Ness (Columbia University)
• Sanjib Sharma (University of Sydney)
• Yuan-Sen Ting (ANU)

### Talk videos

All talks were broadcast and recorded via Zoom. When available each talk title is linked to recording to the talk on YouTube. There is also a playlist for each of the days: Day 1, Day 2, Day 3.

#### Tuesday 22 June

Sven Buder
ANU Canberra
Chemical Tagging and Chrono-Chemodynamics of accreted stars in GALAH
How similar is the recently identified structures of Gaia-Sausage-Enceladus (GSE) with the low-alpha halo? How (dis-)similar are selections of accreted stars when performed in dynamical and chemical space? I will present which elements and nucleosynthesis channels show the highest separation for accreted stars within GALAH DR3 and how the chrono-chemodynamic properties of these chemically tagged stars compares to common dynamical selection (like the energy-action or action-action space). The overlap of these selection, but especially the stars that do not overlap are most interesting as they could hold key information for the beginning, duration, and end of the merger of the GSE.
Emily Cunningham
Center for Computational Astrophysics, Flatiron Institute
The Imprint of Accretion History on the Milky Way Halo Chemical Plane
Kaley Brauer
Massachusetts Institute of Technology
Modeling Galactic Chemical Evolution in Dwarf Galaxies with Individual Stars
The dwarf galaxies that merged to form the Milky Way stellar halo all experienced complex galaxy formation processes including metal mixing, hierarchical galaxy merging, and bursty star formation. Currently, there is a severe lack of theoretical models that can capture the detailed physics that drives spreads in stellar abundance patterns. In particular, no models are capable of a direct, star-by-star comparison to observed abundances with a detailed treatment of stellar yields. We are working to change that. We present an initial overview of the Aeos project, the first hydrodynamic cosmological simulation of early galaxy formation that models feedback from individual stars and traces detailed elemental abundances. This project is ongoing but will provide insight into early galactic chemical evolution in dwarf galaxies and help us interpret observations of our own stellar halo.
Guilherme Limberg
Fragments of an Ancient Dwarf Galaxy
Within the hierarchical assembly paradigm, the Galactic stellar halo is expected to retain the chemodynamical signatures of merging events between the Milky Way and dwarf galaxies of various masses in the past. A widely utilized strategy to find the remnants of these primordial building blocks is to search for their clumping in phase space. The first discovery of a kinematically-cohesive group of stars with this method was reported in a seminal work by Helmi et al. (1999). Throughout the years, this substructure has been known as the Helmi stream (hereafter "HStr''). Intending to find additional fragments of small systems accreted by the Galaxy, we recently conducted a detailed orbital analysis of ~1500 very metal-poor (VMP; [Fe/H] < -2.0) halo stars. In the process, we confirmed that many of these objects are compatible with HStr, populating its VMP tail with significant numbers. Most important, we found that several stars enhanced in rapid-neutron-capture (r-process; [Eu/Fe] > +0.3 and [Ba/Eu] < 0.0) elements are apparently associated with the stream. Following this clue, we utilized the combination between Gaia EDR3 and GALAH+ DR3 surveys to identify additional members of HStr spanning a considerably wider (by ~0.5 dex) metallicity interval (-2.5 < [Fe/H] < -1.0) than previously reported. We demonstrate that stars of HStr compose a clear declining sequence in [alpha/Fe] for increasing metallicity up to [Fe/H] ~ -1.0, in conformity with a shared chemical-evolution history. Moreover, every vetted star with [Fe/H] < -1.2 is r-process enhanced, providing remarkable evidence that, at such low-metallicity regime, the progenitor of HStr experienced enrichment in neutron-capture elements predominantly through this nucleosynthesis channel (perhaps via neutron-star mergers). Finally, the extended metallicity range further suggests an increase in [Ba/Eu] for higher [Fe/H], also indicative of a dwarf-galaxy origin. Given the available data, we can assert that this stellar stream constitutes the fragments of an ancient dwarf galaxy that was almost completely disrupted by its gravitational interaction with the Milky Way.
Ivanna Escala
Carnegie Observatories
Chemical Abundances in the Inner Halo and Giant Stellar Stream of M31
The Milky Way (MW) is the template for our understanding of halo formation. However, there is growing evidence that the MW’s formation history sets it apart from its sister galaxy, Andromeda (M31), calling its status as a template into question. A direct way to probe galaxy formation history is through chemical abundance measurements of individual stars. Until recently, such measurements were not available in M31, limiting the extent to which M31 could serve as a true complement to the MW. In this talk, I will present the first alpha-element abundance measurements of individual red giant branch stars in the inner halo of M31, including its Giant Stellar Stream (GSS). With measurements for over 200 individual stars, the Elemental Abundances in M31 survey has produced the largest homogeneous set of chemical abundance measurements in M31 to date. In particular, I will discuss how the abundance properties of the GSS, such as its negative metallicity gradient and high average alpha-enhancement, relate to the properties of the GSS progenitor and provide evidence in favor of a recent major merger scenario for the GSS's formation. I will also discuss the implications of these results for differences between the accretion history of M31 and the MW, focusing on comparisons between the GSS and Gaia-Enceladus-Sausage merger events.
Rosemary Wyse
Johns Hopkins University
Local Group Science with the Subaru Prime Focus Spectrograph
The Subaru Prime Focus Spectrograph (PFS) will provide the unique capability of a wide-field (1.3 degree) massively multiplexed (2394 reconfigurable fibers) spectrograph on an 8-m telescope. The PFS team is developing a five-year, 360-night survey as a Subaru Strategic Program, to address fundamental questions of cosmic evolution and the dark sector. We will investigate the formation and evolution of structure, from cosmological scales to the Local Group of galaxies. I will describe the astrophysical motivations for the planned observations of faint stars in the Milky Way, in its satellite galaxies and in M31, and the insight we hope to gain into how galaxies form and evolve, and into the nature of dark matter.
Miho N. Ishigaki
NAOJ
Chemical enrichment in the early Universe inferred from old Milky Way halo stars based on GALAH and Gaia
Metals produced by stars and supernova explosions in the first few billion years of the cosmic history have played a crucial role in setting the scene for the first galaxy formation in the Universe.The nature and metal yields of the earliest generations of stars and supernovae, however, remain largely unknown. To obtain insights into the relative contributions of different metal-enrichment sources in the early Universe, we select candidates of old nearby halo stars based on GALAH and Gaia EDR3. Using elemental abundance data from GALAH for the candidate old halo stars, we investigate stellar and supernova yield models of the first (metal-free) stars' supernovae, normal core-collapse supernovae, and Type Ia supernovae that explain observed abundances. We find that for the stars with [Fe/H] >-1.5, the models with contributions from both normal core-collapse supernovae and Type Ia supernovae best explain observed elemental abundance ratios ([X/Fe]) and [Fe/H]. In this talk, I present highlight results from our analysis and discuss possible implications on the metal-enrichment sources at the birthplaces of the old stellar populations in the Galactic halo.
Simon Campbell
Monash University
Exploring the evolution of Lithium in low-mass stars using GALAH: A new criterion for defining Li-rich giants & a ubiquitous Li production event?
The vast majority of low-mass stars are expected to destroy lithium gradually over the course of their lives, via low-temperature nuclear burning. About 1% of red giants buck this trend, showing much more Li in their atmospheres than expected. These are the infamous 'Li-rich giants' which still evade explanation after about 4 decades. The traditional definition of Li-rich giants, that they have A(Li) > 1.5 dex, comes from stellar models which robustly show that Li is depleted to this amount through dilution, as the stars' convective envelopes deepen when they first become red giants. Recent work using asteroseismology has now shown that most of the Li-rich giants are in fact core helium-burning red clump (RC) stars, rather than RGB stars. Low-mass RGB stars are known to deplete Li further as they ascend the giant branch, reaching very low abundances (A(Li) ~ -1 dex). We use GALAH Li abundances combined with Gaia data and asteroseismic parameters to investigate Li evolution in low-mass stars. We find the usual low rates of traditionally-defined Li-rich giants. However we also find that the average A(Li) on the RC is substantially higher than at the RGB tip. One explanation for this is that Li is produced in most, if not all, low-mass stars, somewhere between the RGB tip and the RC. A(Li) appears to increase from the RGB tip to the RC by a factor of 40.
Sarah Martell
University of New South Wales
What do we learn from lithium-rich giants in GALAH DR3?
The fact that evolved stars with high lithium abundances are only a small fraction of the overall population disguises a wealth of diversity in mass, metallicity and evolutionary state. I will present results from a recent study of 1262 lithium-rich giants in GALAH DR3. With a large data set we are able to expand on some previous literature results, finding significant differences in the occurrence rate of lithium richness as a function of metallicity for red giant branch versus red clump stars. We are also able to demonstrate that some stellar properties that have been claimed to be associated with lithium richness, including rotational velocity and infrared flux excess, are not effective predictors. While we do not arrive at a firm conclusion as to the physical mechanism(s) responsible for lithium enrichment in these stars, our analysis of the behaviour of the overall population can be used to inform models for that enrichment.
Jeffrey Simpson
UNSW
The Milky Way is not special: accreted stars also inhabit the Spite Plateau
The ESA Gaia astrometric mission has enabled the remarkable discovery that a large fraction of the stars near the Solar neighbourhood appear to be debris from a single in-falling system, the so-called Gaia-Enceladus. One exciting feature of this result is that it gives astronomers for the first time a large sample of easily observable, unevolved stars that formed in an extra-Galactic environment, which can be compared to stars that formed within our Milky Way. In this talk I will discuss using these stars to investigate the "Spite Plateau" – the near-constant lithium abundance observed in metal-poor dwarf stars across a wide range of metallicities (-3<[Fe/H]<-1). In particular our aim was to test whether the stars that formed in Gaia-Enceladus show a different Spite Plateau to Milky Way stars that inhabit the disk and halo. Individual galaxies could have different Spite Plateaus – e.g., the interstellar medium could be more depleted in lithium in a lower galactic mass system due to it having a smaller reservoir of gas. We find that the Gaia-Enceladus stars show the same lithium abundance as other likely accreted stars and in situ Milky Way stars, strongly suggesting that the "lithium problem" is not a consequence of the formation environment. This result fits within the growing consensus that the Spite Plateau, and more generally the "cosmological lithium problem" – the observed discrepancy between the amount of lithium in warm, metal-poor dwarf stars in our Galaxy, and the amount of lithium predicted to have been produced by Big Bang Nucleosynthesis – is the result of lithium depletion processes within stars.
Melissa Ness
Columbia University
Large spectroscopic surveys and data-driven insights into galactic formation from chemodynamics of stars
Sanjib Sharma
The University Of Sydney
Fundamental relations for the velocity dispersion of stars in the Milky Way
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 $L_z$, 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$\pm 0.007$ and 0.251$\pm 0.006$ for $\sigma_z$ and $\sigma_R$ respectively, and the power law is valid even for the oldest stars. For the solar neighborhood stars, the apparent break in the power law for older stars, as seen in previous studies, is due to the anti-correlation of $L_z$ with age. The dispersions decrease with increasing $L_z$ until we reach the Sun's orbital angular momentum, after which $\sigma_z$ increases (implying flaring in the outer disc) while $\sigma_R$ 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 neighborhood also work for stars between $3<R/{\rm kpc}<20$. Finally, the high-[$\alpha$/Fe] stars follow the same relations as the low-[$\alpha$/Fe] stars.
Christian Sahlholdt
Lund Observatory, Sweden
Age-metallicity distribution of GALAH turnoff stars
The combination of GALAH DR3 and Gaia EDR3 gives a sample of about 200,000 turnoff stars in the Galactic disc for which ages can be estimated by isochrone fitting. Instead of estimate individual ages for the stars, we have applied a new method for estimating the sample age-metallicity distribution by combining the age-metallicity probability densities of individual stars. In this talk I will present the results of applying this method to the full GALAH turnoff sample as well as subsamples divided by their kinematics and Galactic positions. The age-metallicity distribution of the full sample shows more structure than previous studies of the Galactic disc, including a minimum in the age distribution at about 5 Gyr and a bimodality at high metallicities. These results are related to recent models of formation and chemical evolution of the Galactic disc.
Josefina Montalban
University of Birmingham
Asteroseismic ages for Galactic archeology: from global seismic parameters to individual-mode frequencies
In the era of large spectroscopic, high-precision photometric, and astrometric surveys, the combination of these complementary data is the key to achieve a more accurate picture of our Galaxy. For instance, in the last decade, the symbiosis between spectroscopy and asteroseismology has provided important benefits for both disciplines. On the one hand, asteroseismic constraints on the surface gravity of stars were instrumental in improving the accuracy of spectroscopic analysis. On the other hand, detailed information on the photospheric chemical composition is an essential ingredient for the inference of stellar population properties based on asteroseismology. Since the first studies based on simple scaling relations between the global properties of the oscillation spectra and the parameters of stars exhibiting stochastic-type oscillations, significant improvements have been made thanks to a better understanding of the limitations of these scaling relations, and of the detailed structure of the oscillation spectra. In this contribution, we will present the work that our group has carried out in the last year, which has allowed a more precise characterisation of the stellar component of the thin and thick discs, and also a small fraction of the Milky Way halo. To this end, we have exploited different levels of information from the oscillation spectra (global oscillation properties with corrections based on theoretical models and individual oscillation frequencies) of Kepler red giants, in combination with chemical composition and radial velocity from APOGEE spectra. We will also show the potential of oscillation spectra of red giants observed by the TESS space telescope. Although these spectra are less accurate than those obtained by Kepler, they can still provide precise estimates of the stellar parameters, if combined with spectroscopic data and Gaia-eDR3 parallaxes.
Damien de Mijolla
University College London
Spectroscopic chemical similarity as metric learning
To understand the Milky Way disk's formation, we are interested in identifying chemically similar stars, which have presumably formed at similar locations. Current methods for estimating chemical abundances from stellar spectra make use of synthetic stellar spectra. This reliance on synthetic spectra, because of a mismatch with observations, limits the precision with which abundances can be estimated. Therefore, we are motivated to work with the spectra directly to capture and compare the chemical information in stars. In this talk I will present results on recovering chemically identical stars from stellar spectra alone. We employ metric learning to project spectra directly into a lower dimensional subspace which we use to encode a chemical distance between stars. Our approach rests on the assumption that, after suitable pre-processing to isolate chemical variance, stars within open-clusters can be identified directly from their spectra, given their near-identical chemical compositions. Therefore, a model trained to recognize open-cluster members provides a mapping to recover chemically identical stars. We demonstrate our approach on the APOGEE dataset. We identify held out open cluster stars with higher efficiency with our data-driven spectra approach, compared to when using stellar abundance measurements. We quantify our performance using the reference of field stars that are found to be as chemically similar as open cluster stars: with our method, only 0.6\% of field stars appear as chemically similar as birth siblings compared to 1.7\% when using stellar abundance labels.
Anke Arentsen
Observatoire astronomique de Strasbourg, France
Carbon-enhanced metal-poor stars in the inner Galaxy with PIGS
Old, metal-poor stars are typically searched for in the Galactic halo and the dwarf galaxies surrounding the Milky Way. However, a prediction of simulations is that the fraction of metal-poor stars that are very old is highest towards the centres of galaxies: in their bulges. Searching for metal-poor stars in the inner Milky Way faces many challenges and much is unknown about the properties of the metal-poor inner Galaxy. In this presentation, I will describe the Pristine Inner Galaxy Survey (PIGS) which has reached unprecedented efficiency in finding very metal-poor stars in the bulge region, employing metallicity-sensitive photometry to select candidates for spectroscopic follow-up. I will focus on recent PIGS results regarding the occurrence of carbon-enhanced metal-poor (CEMP) stars in the inner Galaxy. CEMP stars are very abundant in the halo at low metallicities and contain clues about the First Stars and/or about binary interactions. Previously only a handful of CEMP stars were known in the inner Galaxy, but we have discovered many more in PIGS. I will discuss what we can learn from them about the properties and origin of the metal-poor inner Galaxy.

#### Wednesday 23 June

Keith Hawkins
University of Texas at Austin
Galactic Archaeology and Stellar Chemistry Era of Large Surveys
Recently, we have entered an era of large spectroscopic and astrometric surveys which has begun to pave the way for the exciting advancements in Galactic and Stellar Astronomy. Combining data from the many multi-object spectroscopic surveys already underway (e.g., GALAH) and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will focus on the measurement and application (e.g., chemical tagging) of stellar chemistry in the context of large surveys.
Paula Jofre
Nucleo Astronomia, Universidad Diego Portales, Chile
Stellar phylogenies
We like to believe that chemical abundances of low-mass stars are a proxy of the stellar DNA, which can be used to find the building blocks of the Galaxy, e.g. do chemical tagging. We can go one step further and use this information to study heritability within stars and so build and analyse phylogenetic trees. I would like to discuss these ideas and see how could we apply phylogenetic tools into the GALAH survey.
Derek Shank
University of Notre Dame
Chemo-Dynamically Tagged Groups of Metal-Poor Stars from the GALAH DR3 Survey
Data from the GALactic Archaeology with HERMES (GALAH) Data Release 3 (DR3) spectroscopic survey are used to estimate dynamical parameters for stars selected from ¬¬an initial sample of ~1000 (primarily) moderate to highly r-process-enhanced metal-poor stars. With most stars in this sample for which sufficient kinematic data are available through GAIA Early Data Release 3 measurements of proper motions, Bayesian Bailer-Jones distances based on Gaia parallaxes and radial velocities from the GALAH DR3 survey, we derive their dynamical parameters with the code Action-based Galaxy Modelling Architecture (AGAMA). The resulting energies and orbital actions (E, Jr, Jφ, Jz) are clustered in dynamical space using the Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN) algorithm to identify Chemo-Dynamically Tagged Groups (CDTGs). We consider the dispersions in the abundances of [Fe/H], [C/Fe], [Y/Fe], [Ba/Fe] and [Eu/Fe], along with the CEMP class, in order to test the significance of their association with parent dwarf galaxies and possibly globular clusters disrupted into the Galactic halo.
Daniel Zucker
Macquarie University
Nix to Nyx: No chemical evidence of an extragalactic origin for the Nyx stream
The results from ESA's Gaia astrometric mission and deep photometric surveys have revolutionised our knowledge of the Milky Way. There are many ongoing efforts to search these data for stellar substructures to find evidence of individual accretion events that built up the Milky Way and its halo. One of these newly identified features, called Nyx, was announced as an accreted stellar stream traveling in the plane of the disk. Using a combination of elemental abundances and stellar parameters from the GALAH and APOGEE surveys, we find that the abundances of the highest likelihood Nyx members are entirely consistent with membership of the thick disk, and inconsistent with a dwarf galaxy origin. We conclude that the postulated Nyx stream is most probably a high-velocity component of the Milky Way's thick disk. With the growing availability of large data sets including kinematics, stellar parameters, and detailed abundances, the probability of detecting chance associations increases, and hence new searches for substructure require confirmation across as many data dimensions as possible.
Farnik Nikakhtar
University of Pennsylvania
New families in our Solar neighborhood: applying Gaussian Mixture models for objective classification of structures in the Milky Way and in simulations
The standard picture of galaxy formation motivates the decomposition of the Milky Way into 3-4 stellar populations with distinct kinematic and elemental abundance distributions: the thin disk, thick disk, bulge, and stellar halo. To test this idea, we construct a Gaussian mixture model (GMM) for both simulated and observed stars in the Solar neighborhood, using measured velocities and iron abundances (i.e., an augmented Toomre diagram) as the distributions to be decomposed. We compare results for the Gaia-APOGEE DR16 crossmatch catalog of the Solar neighborhood with those from a suite of synthetic Gaia-APOGEE crossmatches constructed from FIRE-2 cosmological simulations of Milky Way-mass galaxies. We find that in both the synthetic and real data, the best-fit GMM uses five independent components, some of whose properties resemble the standard populations predicted by galaxy formation theory. Two components can be identified unambiguously as the thin disk and another as the halo. However, instead of a single counterpart to the thick disk, there are three intermediate components with different age and alpha abundance distributions (although these data are not used to construct the model). We use decompositions of the synthetic data to show that the classified components indeed correspond to stars with different origins. By analogy with the simulated data, we show that our mixture model of the real Gaia-APOGEE crossmatch distinguishes the following components: (1) a classic thin disk of young stars on circular orbits (46%), (2) thin disk stars heated by interactions with satellites (22%), (3, 4) two components representing the velocity asymmetry of the alpha-enhanced thick disk (27%), and (5) a stellar halo consistent with early, massive accretion (4%).
Catherine Manea
University of Texas at Austin
A GALAH View of the Chemical Homogeneity, Li Abundances, and Ages of Stellar Strings Identified in Gaia
The advent of Gaia has spurred a surge in discoveries of new stellar populations in the Milky Way, including the discovery of nearly 300 elongated stellar clusters (called ’strings’) spanning hundreds of parsecs in length and mere tens of parsecs in width within 1 kpc of the Sun. In this work, we investigate the chemical distributions and chemical ages of 12 of these newfound stellar strings and associations using GALAH DR3. In this talk, I will discuss the results of this study, where we have found: (1) Li and other chemical clocks support the young ages of many of these structures, (2) all but 3 of these structures are generally more chemically homogeneous in [X/H] than their local fields, and (3) several structures are as homogenous as open clusters. The relatively homogenous nature of these extended structures challenges theories of star formation and mixing efficiency in the interstellar medium and informs the potential for chemically tagging stars born in strings.
Francisca Espinoza Rojas
The consistency of chemical clocks among coeval stars: prospects for chemical tagging
The abundance ratios of some chemical species have been found to correlate with stellar age, leading to the possibility of using stellar atmospheric abundances as stellar age indicators. These chemical clocks have been calibrated with solar-twins, open clusters and red giants, but it remains to be seen whether they can be effective at identifying coeval stars in a field population that spans a broad parameter space (i.e., the promise of chemical tagging). Since the components of wide binaries are known to be stars of common origins, they constitute ideal laboratories for testing the usefulness of chemical clocks for the age dating of field stars. We determined the abundances of a new sample of 5 binaries and collected data for other 31 systems from the literature in order to test the applicability of chemical clocks. We recover the well known result that the components of wide binaries have more consistent chemistry than that of random pairs. However, we also show for the first time that abundance ratios designed as chemical clocks are even more consistent among the components of wide binaries than their [X/Fe] ratios. Not only that, but the special case of the pair HIP 34426/HIP 34407 may indicate that chemical clocks are consistent for coeval stars even when the individual abundances are not. If the assumption that chemical clocks are reliable age indicators is correct, this would constitute first quantitative, statistically significant evidence that the components of wide binaries in the Galactic field are indeed coeval, validating a large body of published work that relies on that to be the case. Moreover, our results provide strong evidence that chemical clocks indeed carry important information about stellar birthplaces and chemical evolution, and thus we propose that including them in chemical tagging efforts may facilitate the identification of nowadays dissolved stellar groups.
Piyush Sharda
Australian National University
Gas phase metallicity history of Milky Way-like galaxies across cosmic time
This talk was not recorded.
The knowledge of the ISM metal history is crucial to understand GALAH observations that constrain the galactic chemical evolution of the Milky Way. To this effect, we develop and present a new model for the evolution of spatially resolved ISM metallicities in Milky Way-like galaxies from first principles. We show that ISM metallicities depend on four ratios that collectively describe the metal equilibration time-scale, production, transport, consumption, and loss. When normalized by metal diffusion, metallicity gradients in Milky Way-like galaxies are governed by the competition between radial advection and accretion of metal-poor gas from the cosmic web, both of which mix metals to flatten the gradients, as opposed to metal production which promotes the development of steep gradients. We use the model to study the cosmic evolution of spatially resolved metallicities across redshift, showing that the metallicity gradient in Milky Way-like galaxies has steepened over time, in good agreement with both observations and simulations. Our model also predicts that Milky Way-like galaxies had more metal enriched outflows in the past. Reproducing the observed mass-metallicity and mass-metallicity gradient relations in the local Universe from the model reveals that Milky Way-like galaxies transition from the metal advection-dominated to the gas accretion-dominated regime as they increase in mass. This transition mirrors the transition observed in the dominant source of turbulence in the disc, from gravity-driven at high redshifts to star formation feedback-driven at low redshifts. The model can be self-consistently incorporated into existing Galactic archaeology models that study radial migration, as well as the formation of the thin and the thick discs, and thus has the potential to further contribute to our understanding of Galactic chemical evolution.
Dennis Stello
UNSW
Galah-related science with K2 and TESS
I'll present recent results from the overlap sample of K2 GAP and K2-HERMES stars, demonstrating that previous mismatches between Kepler red giants and galactic population synthesis can largely be explained by the assumed thick disk metalicity in the model. I will further show hot-of-the-press results from full sky TESS asteroseismology giving us the largest seismic sample to date with over 100,000 stars.
Rob Wittenmyer
University of Southern Queensland
Know the star, know the planet: Insights on exoplanet radii from K2-HERMES
Accurate and precise radius estimates of transiting exoplanets are critical for understanding their compositions and formation mechanisms. To know the planet, we must know the host star in as much detail as possible. I describe the application of the K2-HERMES survey to exoplanetary characterisation. To date, we have derived host-star parameters and planet-candidate radii for 224 K2 candidate planets from Campaigns 1-13. Our results cast severe doubt on 30 K2 candidates, as we derive unphysically large radii, larger than 2 Jupiter radii. This work highlights the importance of obtaining accurate, precise, and self-consistent stellar parameters for ongoing large planet search programs - something that will only become more important in the coming years, as TESS and PLATO deliver their own harvests of exoplanets.
Jake Clark
University of Southern Queensland
What Can GALAH Teach Us About Exoplanetary Science?
The time of large-scale astronomical surveys is upon us. These huge spectral, astrometric and photometric surveys are providing us astronomers with the richest datasets to date, to better characterize the stars contained within the Milky Way like never before. With almost every star being orbited by at least one exoplanet across our galaxy, these large scale surveys can help better inform us in characterising confirmed and potential planet-hosting stars. In this talk, I will discuss how we've been able to utilise GALAH's last two data releases (DR2 and DR3), along with GAIA DR2 and EDR3 to better characterise over 125 exoplanet hosts and 250 candidate hosting stars, as well as 45,000 stars currently being observed by NASA's new planet-finding mission, TESS. I'll also be discussing how we can use GALAH's chemical abundances to help inform exoplanetary scientists on what types of planets TESS will likely uncover.
University of Groningen
It is still unclear if neutron star mergers are the dominant source for r-process elements in the Universe since the expected delay time distribution seems incompatible with the observed chemical abundance trend among stars in the Milky Way. Here we compare r-process element abundances of stars from an accreted dwarf galaxy, Gaia-Enceladus, with those formed in the Milky Way (in-situ stars) by using the data from the Gaia mission and the Galactic Archaeology with HERMES (GALAH) survey. The Gaia-Enceladus stars clearly show higher abundance of an r-process element with respect to an element formed by core-collapse supernovae. Three massive dwarf galaxies around the Milky Way are also known to show similar r-process enhancements. We show that a simple chemical evolution model that considers r-process enrichments by neutron star mergers can naturally explain the high r-process element abundance of these four galaxies. The high r-process element abundances are a consequence of lower star formation in these galaxies than the Milky Way and the delay time of neutron star mergers.
Diane Feuillet
Lund Observatory
Characterizing Gaia-Sausage-Enceladus and Sequoia with SkyMapper and APOGEE
Gaia DR2 has shown the Milky Way to have several previously unknown stellar populations; most notably the Gaia-Sausage-Enceladus (GSE) population. These could only be detected thanks to Gaia’s exquisite astrometry. However, the nature of these kinematic structures and their origin needs a deeper characterization of their stars than possible with Gaia DR2 data alone. We examine the kinematic properties, metallicity distribution functions, and population ages of stars possibly belonging to the GSE and Sequoia populations using SkyMapper and APOGEE elemental abundance information. GSE has a mean [Fe/H] of -1.2 and is most apparent at high JR. We show that the selection of GSE stars is quickly contaminated by disk stars below JR of 900 km/s. Using a population age analysis, we estimate both GSE and Sequoia to be old, peaking between 10 and 14 Gyr. We also find both GSE and Sequoia have characteristically low [Al/Fe].
Iryna Kushniruk
Stockholm University
Disentangling the stellar halo with the GALAH survey
Recently found stellar populations such as Gaia Enceladus/Sausage (GES), Sequoia, Splash, and Thamnos confirm that the Milky Way formed through multiple merger events. A combination of high-precision astrometric and spectroscopic data from the Gaia and GALAH surveys provides a golden opportunity to deeply investigate chemo-dynamical properties of accreted populations in order to better understand the formation history of the Milky Way. We present the analysis of chemical and kinematic properties of ~36000 carefully selected stars from the GALAH DR3 catalogue. We run the wavelet transform and t-SNE algorithms to search for characterise the substructure of the stellar halo. We detect GES, the “splashed” disk, Thamnos2, and one potentially new substructure in the radial (Jr) and azimuthal (Lz) actions space. In this talk we are going to discuss if GES is even bigger than previously thought, whether there is no in-situ halo but rather just a splashed disk, and whether Thamnos2 is a separate, metal-poor accretion event.
Akshara Viswanathan
Kapteyn Astronomical Institute, University of Groningen
A unique view of the metallicity structure of the Milky Way halo
The Milky Way stellar halo is diffuse and full of chemical and dynamical substructures that relate to the complex history of our galaxy in the distant past. Starting from a list of halo main sequence stars in Gaia eDR3 selected using the reduced proper motion method, we infer the metallicity information from the very-metallicity sensitive Pristine survey. This results in a set of ~0.76M stars with reliable photometric metallicities as well as distance estimates – the typical distance uncertainty for these stars is 7% and the heliocentric distance ranges between 0 and 21 kpc with a mean of 7 kpc, thereby probing much further out than would be possible using reliable Gaia parallaxes. From the distribution of binned velocity moments in the sky, several stream-like features stand out at higher distances in this sample. We study the metallicity structure of the halo in different distance slices and map out the metal-poor Milky Way halo across various lines of sight. Our results confirm an overall metallicity distribution function (MDF) of the local halo population with a peak at [Fe/H]=-1.6, consistent with literature results. We trace a probable extension of Phlegethon stream and another sub-structure that crosses Phlegethon from the [Fe/H]<-2 metallicity space in the sky. We use our results to better constrain the contributions of (kicked up) thick disk stars at different scale heights in this kinematically selected halo sample. By combining these two powerful surveys, we thus get a better view of some of the most intricate parts of our Galaxy.
Kris Youakim
Stockholm University
Chemical and kinematic tagging of star clusters with GALAH and Gaia
Stellar substructures provide clues to the accretion history of the Milky Way. In particular, the distributions of member stars from bound clusters reveal a lot of information about their interactions with our Galaxy, as well as the nature of the Galactic potential. However, it is difficult to identify member stars residing outside of the central core of these clusters because of contamination from foreground and background stars. Building on the work of Kos et al. 2018, we implement dimensionality reduction algorithms to search the high dimensional data set of GALAH and identify new cluster members through chemical and kinematic tagging. We find that the larger data set and improved abundances from GALAH DR3 (Buder et al. 2020), as well as the inclusion of more parameters, in particular kinematic parameters computed from the Gaia data, improve the capability to differentiate cluster members from background stars. We also demonstrate that this is an effective method to identify members of accreted halo substructures such as the Gaia-Enceladus-Sausage.

#### Thursday 24 June

Yuan-Sen Ting
ANU
How Many Elements Matter?
Emily Griffith
Ohio State
Abundance Ratio Trends in the Milky Way Disk and Bulge:
Ambitious Galactic spectroscopic surveys such as GALAH have obtained high-resolution, high signal-to-noise ratio spectra of hundreds of thousands of stars, spanning large swaths of the Milky Way. The high-resolution surveys provide detailed chemical fingerprints for each program star, typically measuring 15-30 elements per star. A key question to these surveys is how many of these elements actually contain independent information. It has long been recognized that the ratio of alpha-elements to iron peak elements is an important dimension of stellar abundance variation in addition to overall metallicity. However, the evidence on variations beyond the metallicity and alpha-enhancement is mixed. Some studies of stars' multi-element abundance distributions suggest at least 5-7 significant dimensions, but others show that many elemental abundances can be predicted to high accuracy from [Fe/H] and [Mg/Fe] (or [Fe/H] and age) alone. In this talk, I will reconcile these seemingly contradictory results. I will show that both propositions can be, and are, simultaneously true. In particular, I will discuss, although one could infer elemental abundances to high accuracy with only [Fe/H] and [Alpha/Fe] elements, residual abundances can display clear correlations between other elements, which signal cannot be explained by only two elements. I will demonstrate that cross-element correlations are a much more sensitive probe of hidden structure than dispersion, and they can be measured precisely in a large sample even if star-by-star measurement noise is comparable to the intrinsic scatter. In short, many elements have an independent story to tell, even for the "mundane'' disk stars and elements produced by core-collapse and Type Ia supernovae. The only way to learn these lessons is to measure the abundances directly, and not merely infer them.
Aarya Patil
University of Toronto
Functional Data Analysis for extracting the Intrinsic Dimensionality of Spectra
By dividing stellar samples into low-Ia (high-[Mg/Fe]) and high-Ia (low-[Mg/Fe]) populations, we examine the median trends of [X/Mg] versus [Mg/H] for elements. These median abundance trends inform us on aspects of our Galactic enrichment history. By fitting median abundance sequences with the two-process model, we empirically derive an element's fractional CCSN contribution and predict the stellar abundances from only the [Mg/Fe] and [Fe/H] values for a star. In this talk, I will present the similarities and differences between the abundance ratio trends in the Milky Way's disk and bulge, the two-process model fits to the median trends, and empirical constraints on CCSN and SNIa contribution to elements in APOGEE and GALAH. I will discuss the two-process model's predicted abundances and explore the interesting differences between the predicted and observed values.
James W. Johnson
The Ohio State University
A Hybrid Model for Stellar Migration and Chemical Evolution in the Milky Way
Numerical simulations of galaxy evolution suggest stars can migrate significant distances from their birth radius, but the impact of this effect on galactic chemical evolution is poorly understood. In this talk, I will discuss the models introduced by Johnson et al. (2021, arxiv:2103.09838), which describe the Milky Way disc as a series of concentric rings combined with a prescription for stellar migration informed by a hydrodynamic simulation. The model successfully reproduces many observed features of the Galaxy's alpha and iron-peak abundance structure, such as the considerable intrinsic scatter in the stellar age-metallicity relation and the presence of young and intermediate-age alpha-enhanced stars in the solar neighbourhood. However, the models fail to reproduce the infamous [alpha/Fe] dichotomy, and no one variation simultaneously explains all observables. Time permitting, I will discuss follow-up investigations to these models, such as their predicted nitrogen abundances as well as alternate star formation histories.
Borja Anguiano
University of Virginia
Anisotropies in the Local Velocity Field of the Galactic disk and the Local Dark Matter density
The local dark matter density encodes valuable information about the local shape of the Milky Way's dark matter halo near the plane, providing constraints on galaxy formation models, the merger history of the Galaxy and even alternative gravity theories. I present an ongoing exploration of the vertical and radial kinematics of stars in the solar vicinity as tracers of the relative contributions of baryons and dark matter to the overall surface mass density of the disk. This work exploits the latest astrometric solution from the Gaia mission together with stellar abundances from the APOGEE survey, which is key to labeling the different population structures in the Milky Way.
Michael Hayden
University of Sydney
Chemical Clocks with GALAH DR3
Using data from the GALAH survey, we explore the dependence of elemental abundances on stellar age andmetallicity among Galactic disc stars. We find that the abundance of most elements can be predicted fromage and [Fe/H] with an intrinsic scatter of about 0.03 dex. Inverting these relations, we are able to use chemistry alone to predict the age of a star. This is the first large scale study that demonstrates ages are able to be determined for large samples of stars directly through Galactic chemical evolution. Stellar ages are estimated via the machine learning algorithm XGBoost, using main sequence turn off stars as our input training set. We find that the stellar ages for the bulk of the GALAH DR3 sample are accurate to 1-2 Gyr using this method.
Emily Wisnioski
ANU
Chemical evolution: connecting high redshift scaling relations to GALAH
This talk was not recorded.
I will discuss the formation and evolutionary paths of star-forming galaxies. Kinematics and structural properties have revealed that the majority of 'normal' star-forming galaxies at early times host thick disk-like structure and a turbulent gas-rich interstellar medium. I will present results from high redshift surveys of molecular and ionised gas to trace the evolution of spatially-resolved dynamics and star formation. These results when combined with local galactic archeology studies provide new insights into how the structures of local galaxies including the Milky Way were formed. I will discuss these observational results within the framework of theoretical models for the formation of the local stellar age-velocity dispersion relation and the scale height - scale length plane. Future work will focus on using global scaling relations (e.g. MZR, M*-R) to model the formation of the age-metallicity relation as measured by GALAH.
Tilman Hartwig
University of Tokyo
Multiplicity of the first stars confirmed by supervised classification of extremely metal-poor stars
I will present how the multiplicity of the first stars can be derived from the abundance patterns of extremely metal-poor (EMP) stars in the Milky Way. Based on theoretical models of the chemical yields of the first supernovae, we train Support Vector Machines to classify EMP stars. This AI-based approach predicts if a specific abundance pattern is consistent with supernova enrichment by one or by several progenitor stars (mono- or multi-enriched). By applying the trained classifier to actual observations, we find that most EMP stars are multi-enriched, which is the first observational confirmation for the multiplicity of the first stars. The predictive power of such data-driven approaches scales with the number of EMP stars, for which large surveys will be beneficial.
Kirsten Banks
UNSW
Mapping Stellar Interiors with Spectroscopy
Helium core burning red clump stars are very effective standard candles and can be used to accurately map much of the Galaxy. However, they have very similar spectral features to inert helium core red giant branch (RGB) stars, which are not standard candles. These two types of stars can be effectively distinguished using asteroseismology, however, this can be very time-intensive. In this research, we explore the spectro-seismic connection of these stars and their internal structures and investigate which spectral features are most sensitive to asteroseismic classification.
Christian Lehmann
Swinburne University of Technology
Discovering Distant Solar Twins to Test Fundamental Physics
Solar twins stars are important for probing stellar and Galactic chemical evolution, and we propose to expand their usage to testing fundamental physics. We aim to test for variations in the fine structure constant "α" - the strength of electromagnetism - with changing dark matter density, which requires finding solar twins much closer to the Galactic Centre compared to the Sun. We aim to probe as close as 4kpc to the Galactic Centre. The goal of the project is to detect hundreds of distant new solar twins and this requires a highly multiplexed spectrograph, i.e. HERMES. However, the uncertainties of the main stellar parameters [Teff, [Fe/H], and log(g)] from The Cannon technique applied in the GALAH survey are too large to confidently select solar twins (e.g. Teff to within 200K). We present a novel approach that leverages GALAH survey spectra to enable a fully differential measurement of the stellar parameters, relative to a high-precision solar reference spectrum, using low-SNR HERMES spectra (SNR ~ 25). I will demonstrate that the precision obtained is sufficient to confidently select solar twins up to 4kpc away with HERMES. I will also report on our recent HERMES observing campaign to discover such distant solar twins, and outline our approach to measuring the fine-structure constant with the best ~40 of these with VLT/ESPRESSO.
Arvind Hughes
Macquarie University/MPIA
Searching for Extremely Metal-Poor Stars with GALAH: A How-To Guide
Astronomy is being transformed by massive stellar spectroscopic surveys, such as RAVE, APOGEE and GALAH. The next generation of surveys -- including WEAVE, 4MOST and SDSS V's MWM -- will expand the available spectroscopic data well into the millions of stars. How are we to efficiently sift through these huge datasets to reliably identify rare and interesting stars? As a case study, I present a hybrid methodology to target Extremely Metal-Poor (EMP) stars within the GALAH survey, with exciting results: the identification of 26 candidates with Fe/H < 3.0. Such techniques can be adapted to find other classes of objects in large spectroscopic databases.
Chiaki Kobayashi
University of Hertfordshire
Chemodynamical Simulations of Milky Way: the bimodality in other elements
I have been running chemodynamical hydrodynamical simulations from cosmological initial conditions, including the latest nucleosynthesis yields for single stars (Kobayashi, Karakas, Lugaro 2020), Type Ia supernovae (Kobayashi, Leung, Nomoto 2020), and neutron-capture processes in various astronomical sites: AGB stars, electron-capture supernovae, neutron star mergers, and magneto-rotational supernovae. In the simulation the disk grow inside-out, following cosmological gas inflow and regulated by supernova feedback, with small effects of radial gas flow and stellar migration. [Alpha/Fe] ratios show a bimodality depending on the location within the Milky Way, similar to observations (Vincenzo & Kobayashi 2020). In this talk I also predict the bimodality found in other elements, explain the origins, and discuss how it will be useful to understand the formation history of the Milky Way.
Tobias Buck
AIP
The challenge of simultaneously matching the observed diversity of chemical abundance patterns in cosmological hydrodynamical simulations
With the advent of large spectroscopic surveys the amount of high quality chemo-dynamical data in the Milky Way (MW) increased tremendously. Accurately and correctly capturing and explaining the detailed features in the high-quality observational data is notoriously difficult for state-of-the-art numerical models. In order to keep up with the quantity and quality of observational datasets, improved prescriptions for galactic chemical evolution need to be incorporated into the simulations. Here we present a new, flexible, time resolved chemical enrichment model for cosmological simulations. Our model allows to easily change a number of stellar physics parameters such as the shape of the initial mass function (IMF), stellar lifetimes, chemical yields or SN Ia delay times. We implement our model into the Gasoline2 code and perform a series of cosmological simulations varying a number of key parameters, foremost evaluating different stellar yield sets for massive stars from the literature. We find that total metallicity, total iron abundance and gas phase oxygen abundance are robust predictions from different yield sets and in agreement with observational relations. On the other hand, individual element abundances, especially alpha-elements show significant differences across different yield sets and none of our models can simultaneously match constraints on the dwarf and MW mass scale. This offers a unique way of observationally constraining model parameters. For MW mass galaxies we find for most yield tables tested in this work a bimodality in the [\alpha/Fe] vs. [Fe/H] plane of rather low intrinsic scatter potentially in tension with the observed abundance scatter.
Laura Magrini
INAF Osservatorio di Arcetri
Post-main sequence lithium evolution: insights from the database of open clusters in the Gaia-ESO survey
We have constrained the mixing processes in low-mass stars by investigating the behaviour of the Li surface abundance after the main sequence, taking advantage of the data from the Gaia-ESO iDR6 and from Gaia eDR3. The comparison with stellar evolution models confirms that classical models cannot reproduce the lithium abundances observed in the metallicity and mass regimes covered by the data. The models that include the effects of both rotation-induced mixing and thermohaline instability account for the Li abundance trends observed in our sample, in all metallicity and mass ranges. The differences between the results of the classical models and of the rotation models largely differ (up to ~2 dex), making lithium the best element to constrain stellar mixing processes in low-mass stars. We discuss the Li abundances in the red clump stars, confirming that a large fraction of them have A(Li) higher than expected by models, and than some mixing episodes might be necessary during the He-flash phase.
Valeria Grisoni
SISSA (Italy)
Galactic chemical evolution models: from lithium to europium
In this talk, I will discuss chemical evolution models of the Milky Way in the light of the most recent observational data from Galactic surveys and missions. Indeed, we are in a golden era for this field of research thanks to the advent of large spectroscopic surveys, such as GALAH. In this way, detailed stellar abundances of stars in the Milky Way can be measured. Then, by means of detailed chemical evolution models, it is possible to predict the chemical abundances expected in the stars of each Galactic component: halo, thick disc, thin disc and bulge. From the comparison between data and model predictions for different chemical elements from lithium to europium, we can reconstruct the history of star formation occurred in each component, and thus the history of formation and evolution of the entire Galaxy.
Donatella Romano
INAF, Astrophysics and Space Science Observatory, Bologna
Galactic evolution of lithium from GES iDR6
We use a sample of open cluster and field stars for which high-precision 7Li abundances and stellar parameters are homogeneously derived by the Gaia-ESO Survey to trace the evolution of lithium in the Milky Way thick and thin discs. We confirm previous findings that the abundance of 7Li in the solar neighbourhood does not decrease at super-solar metallicities. The comparison of the data with theoretical predictions from Galactic chemical evolution (GCE) models favours a scenario in which the majority of 7Li in the Galaxy comes from nova outbursts. Current data seem to suggest that the nova rate flattens out at later times. If confirmed, this may have implications for the masses of the white dwarf nova progenitors that deserve further investigations. To conclude, we comment on the possible use of 7Li data gathered by GALAH to further constrain our GCE models.
Boquan Chen
USyd
Multi-Zone Chemical Evolution Model with Updated Yields
Chemical evolution models are crucial to understanding the global chemical trends we observed in large spectroscopic surveys. Adopting stellar yields from Kobayashi et al. (2020, 10.3847/1538-4357/abae65), we created a multi-zone chemical evolution model that incorporates radial inflow and radial migration. The chemical evolution tracks for different radial zones serve as direct comparison to the analytic tracks extracted from Apache Point Observatory Galactic Evolution Experiment (APOGEE) in Sharma et al. (2020, arXiv:2005.03646). Our model found qualitative agreement with key features seen in observations and extends the work of Schoenrich & Binney (2009) and Andrews et. al. (2015). The code will be made public soon.