Seminars

In 2020, the iSIMBA group initiated a series of seminars on topics we wanted to know more about. Descriptions and recordings of these seminars running throughout 2020 and 2021 can be found below.


Upcoming seminars


Simulating Structure and Galaxy Formation

Speaker: Mark Vogelsberger
Monday, 23rd August 2021, 14:00 CET

Abstract: Cosmological simulations of galaxy formation have evolved significantly over the last years. In my talk I will describe recent efforts to model the large-scale distribution of galaxies with cosmological hydrodynamics simulations. I will focus on large-scale simulations like IllustrisTNG. After demonstrating the success of these simulations in terms of reproducing an enormous amount of observational data, I will also talk about their limitations and directions for further improvements over the next couple of years. Furthermore, I will also talk briefly about our new simulation campaign, the THESAN project, to study the epoch of re-ionization and the early Universe.


The properties of faint dwarf galaxies around the Milky Way

Speaker: Eline Tolstoy
Monday, 27th September 2021, 14:00 CET

Abstract: I will talk about what we know of the properties of faint, low-mass dwarf galaxies that live close enough to us that they can be studied in detail star by star. It is of interest to understand the detailed properties of individual metal-poor galaxies in their own right and also to trace ancient star-formation in the early Universe. Careful studes of these very common dwarf-type galaxies are also important to constrain our models of their contribution to the build up of larger galaxies, like the Milky Way.


Past seminars


Uncertain Times: The Redshift-Time Relation from Cosmology and Stars

Speaker: Michael Boylan-Kolchin
Tuesday, 18th August 2021, 14:00 CET
Recording available here

Abstract: Although the ‘Hubble Tension’ — a discrepancy between the Hubble expansion rate measured locally versus inferred from large-scale cosmological observations — is usually framed as an issue for cosmology, it also has important implications for other areas of astronomy. One example is stellar ages: many popular solutions to the Hubble Tension result in a Universe that is 5% younger than the canonical 13.8 Gyr that originates from the LCDM model as constrained by Planck data. I will discuss the Hubble Tension in this context, with an emphasis on prospects for using stellar ages to constrain cosmology. I will also discuss a related point of intersection of stellar and cosmological physics, the relative timing of cosmic reionization and the formation of the first galaxies and globular clusters.


Galactic Globular Clusters: Multiple Stellar Populations and Galaxy formation process

Speaker: Santi Cassisi
Monday, 28th June 2021, 14:00 CET
Recording available here

Abstract: Galactic globular clusters have always been at the crossroad of several investigations in both Stellar and Galactic Astrophysics. For long time, they have been considered the prototypes of Simple Stellar Populations, and hence used for testing and calibrating stellar evolutionary models as well as population synthesis tools. Nowadays, after the discovery of the presence of multiple stellar populations in almost all Galactic GCs, we know that this assumption is no longer valid. The process(es) of formation and early evolution of these star clusters is (are) very far to be understood, and any scenario so far envisaged is severely challenged by the pletora of empirical evidence collected till now. In the same time, thanks to the availability of an impressive observational framework – collected by combining kinematic measurements from Gaia mission, with data provided by large spectroscopic and photometric surveys -, GCs are playing a crucial role for our understanding of the assembly history of the Milky Way.
We will review our present knowledge about these important stellar systems, discussing the several, open issues related to their formation/evolution, and discuss how we can use them in our effort to depict the Milky Way assembly history.


Moving to the low-frequency regime: gravito-inertial asteroseismology in action

Speaker: Conny Aerts
Monday, 15th June 2021, 14:00 CET
Recording available here

Abstract: Space asteroseismology is currently triggering tremendous progress in our understanding of stellar interiors. In this seminar, we focus on dwarfs of intermediate and high mass covering all fractions of rotation up to the critical rate.  We recall the different restoring forces at play and along with them the time scales of various types of nonradial modes. We then highlight several breakthroughs since the birth of gravity-mode asteroseismology and its expansion towards the gravito-inertial regime. We focus on new insights into the internal rotation and levels of chemical mixing of dwarfs throughout their main sequence evolution.  We also illustrate how Kepler photometry remains underexplored in terms of measuring internal magnetism for slow and fast rotatoring dwarfs.  We end with the potential to expand space asteroseismology to higher mass and more evolved stages from the current TESS data and look out for opportunities brought by the future ESA PLATO space mission.


The VINTERGATAN project: towards understanding the origins of the Milky Way

Speaker: Oscar Agertz
Monday, 3rd May 2021, 14:00 CET
Recording available here

Abstract: In this talk I will highlight recent advances in the field of cosmological galaxy formation simulations. I will present a new cosmological ‘zoom’ simulation of a Milky Way-mass galaxy called VINTERGATAN, and how this can be used to improve our understanding of how chemically, kinematically and structurally distinct stellar discs form. The role of gas accretion, mergers and secular processes, in establishing features of the simulated thin and thick disc, will be presented and contrasted to Milky Way observations. Finally, I will discuss a novel way of performing modified cosmological simulation, in which details of the merger history can be controlled, and how this technology can better our understanding of the Milky Way’s origins.


The GRIFFIN project – towards realistic simulations of galactic star cluster populations

Speaker: Thorsten Naab
Monday, 19th April 2021, 10:00 CET
Recording available here

Abstract: I will present an overview of the GRIFFIN project (https://wwwmpa.mpa-garching.mpg.de/naab/griffin-project/index.html) which aims at an “as realistic as possible” modelling approach for entire star cluster populations from a galactic multi-phase ISM at high spatial ( sub-parsec), mass ( solar mass, individual massive stars), and time ( 100 yr) resolution at low metallicity. The underlying model approach will be briefly presented together with highlights on the evolution of dwarf galaxies, the formation of molecular clouds and star clusters as well as the self-consistent formation of globular cluster progenitors in a metal poor starburst environment.


Gaia & the fingerprints of the Galactic bar – A dynamical Milky Way mystery

Speaker: Wilma Trick
Monday, 22nd March 2021, 10:00 CET

Abstract: The Milky Way’s disk around the Sun is interspersed by 7 streams of stars, amongst them the famous “Hercules”, “Sirius”, and “Hyades” moving groups. These streaming motions are caused by some powerful perturber. Suspects are the spiral arms, satellites, and/or the Galactic bar. So far, the true culprit has been notoriously elusive. This talk shows how stars whose orbits are in resonance with the Galactic bar exhibit distinctive patterns in action-angle space. Based on these fingerprints, the exquisite Gaia data narrows down the search for the bar’s Outer Lindblad Resonance (OLR) to two likely candidates: the “Hat” and the “Sirius” moving groups. Finally pinning down the true OLR (and therefore the pattern speed of the Galactic bar) will be a milestone in uncovering the Milky Way’s structure and evolution.


Recent disturbances in the Milky Way disc with Gaia DR2 and EDR3

Speaker: Teresa Antoja
Monday, 8th March 2021, 10:00 CET
Recording available here

Abstract: Thanks to the large amount of data delivered by the Gaia satellite and their excellent precision, in the last two years we have experienced a vertiginous progress in the studies of our Galaxy. I will talk about the discovery of unexpected structures in the Milky Way phase-space such as the phase spiral, ridges in the in-plane velocities (now seen at much larger distances from the Sun with EDR3), and waves in the angular momentum and vertical space. Finally, I will discuss how we are trying to figure out whether all these disturbances are consistent with the effects of internal mechanisms such as the bar or the spiral arms, or are produced by the interaction with satellite galaxies.


Rethinking the Surface Term

Speaker: Joel Ong
Thursday, 11th February 2021, 14:00 CET
Recording available here

Abstract: Localised modelling error in the near-surface layers of stellar models causes the frequencies of their normal modes of oscillation to differ from those of actual stars with matching interior structures. These frequency differences are referred to as the surface term. While some global stellar properties of main-sequence stars, estimated via detailed constraints on individual mode frequencies, have previously been thought to be robust with respect to different parametrisations of this surface term, this robustness is less well-studied with respect to nonparametric treaments. Including such treatments in this comparison, we demonstrate that the choice of surface correction does affect the estimated initial helium abundance, with implications for the use of asteroseismology in studying Galactic archaeology. We also show that nonparametric surface corrections return dramatically different results for a sample of evolved red giants, compared to parametric ones. This suggests that the surfaces of such evolved stars behave differently than main-sequence stars.

While the study of subgiants might shed light on the transition between these two regimes, these nonparametric methods cannot be applied directly to strongly mixed modes. We derive semi-analytic expressions by which the pure p-like and g-like wave operators may be decoupled, and show that this permits nonparametric characterisation of the surface term for subgiants undergoing isolated avoided crossings. Such a decomposition also illuminates the regimes of validity for more traditional parametric surface corrections, originally intended for p-modes, when applied to mixed modes in general.


Age-metallicity relations of stellar populations from HR diagram inversion

Speaker: Christian Sahlholdt
Monday, 22nd February 2021, 10:00 CET
Recording available here

Abstract: With the combination of large spectroscopic, photometric, and astrometric surveys, we have the data necessary to estimate stellar ages for millions of field stars in the Galaxy based on their positions in the HR diagram. Most commonly, this is achieved by Bayesian isochrone fitting which gives a probability density function (PDF) of the age for each star. From this age PDF, a single age estimate and an associated uncertainty can be determined. However, rather than reducing the age PDF to a point estimate, studies have shown that it is beneficial to use the entire age PDF of each star when estimating the age distribution of a sample. In this talk I will give an overview of existing methods utilising this principle and present a new method extending it to the use of age-metallicity PDFs. I will show how this method can give a more precise estimate of the age-metallicity distribution of a stellar sample and thereby, potentially, reveal new details about the formation history of the Galaxy.


Constraints on the nature of thermonuclear supernovae from the chemical composition of Galactic stars

Speaker: Maria Bergemann
Thursday, 26th November 2020, 10:00 CET
Recording available here

Abstract: One of the most exciting questions in modern astrophysics is what are the progenitors of Supernovae Ia (SN Ia). As endpoints of binary stellar evolution, SNIa’s are primarily responsible for the nucleosynthesis of Fe-group elements, they influence the chemical enrichment and star formation history of galaxies, and – through their role as geometric probes of cosmic expansion – they underpin observational cosmology.

I will describe our long-term program to constrain the physical properties of SN Ia using the chemical abundances of galactic stars. This field has radically changed, as massive spectroscopic surveys and large facilities equipped with high-resolution spectrographs now allow us to probe the previously inaccessible regions of the Milky Way, its dwarf satellites, and other large star-forming galaxies in the Local Group and beyond. Also, major improvements in spectroscopic analysis methods and physical models of stellar spectra have made it possible to determine the chemical abundances of stars with unprecedented precision.

I will talk about 3D Non-LTE chemical abundances of the Sun, main-sequence and RGB stars, and I will show how we can use them to trace the chemical evolution of the Galactic disc. I will then focus on the connections between the enrichment history of the Galaxy and the properties of SNIa’s, which have dominated its evolution over the past 8 billion years. Finally, I will discuss the new evidence in the context of the environmental diversity of SN Ia’s and relate it to the problem of residuals in the Hubble diagram.


Galactic Archaeology to its limits

Speaker: Else Starkenburg
Wednesday, 2nd December 2020, 10:00 CET
Recording available here

Abstract: The lowest metallicity stars that still exist today probably carry the imprint of very few supernovae. As such, they represent a window into the early Universe. In this talk I will review what we know about the early (chemical) evolution of the Milky Way system. In particular, I will present results of the Pristine survey, a narrow-band photometric survey of the Milky Way designed to get metallicity information for millions of stars very efficiently. I will show how we can use this great discriminatory power to hunt for the very rare extremely metal-poor stars (bearers of the chemical imprint of the first stars) and to better understand the formation of the Milky Way.


The History of the Galactic disc and halo from Gaia DR2 HR diagram fitting

Speaker: Carme Gallart
Thursday, 19th November 2020, 10:00 CET
Recording available here

Abstract: Gaia has provided distances and photometry, and thus colour-magnitude diagrams in the absolute plane, for stars over an unprecedented vast volume in the Milky Way, encompassing significant fractions of the thin and thick disk, and halo. This has allowed us, for the first time, to derive unprecedentedly detailed star formation histories from direct modelling of these colour-magnitude diagrams, using the same techniques that have been proven successful for external galaxies in the Local Group. Our first results for a volume of 2 Kpc radius from the Sun are extraordinarily promising. They have allowed us to date the first events involved in the formation of the inner Milky Way halo (Gallart et al. 2019, NatAstro) and to determine the presence of epochs of enhanced star formation well constrained in time, that can be associated to the various pericentric passages of the Sagittarius dwarf galaxy (Ruiz-Lara et al. 2020, NatAstro). I will discuss these results as well as future prospects to reach a larger Milky Way volume, and to combine chemodynamical information from spectroscopic surveys with this new approach to study the Milky Way evolutionary history.


Asteroseismology and inferring stellar ages

Speaker: Guy Davies
Thursday, 12th November 2020, 10:00 CET
Recording available here

Abstract: The true ages of stars would be useful in lots of contexts, for example gyrochronology, exoplanet studies, and galactic archeology. But stars do not divulge their ages willingly. Asteroseismology can give us a unique perspective on stellar ages by probing the interior structure of a star but this does not reveal stellar age in itself. Stellar ages, for all stars but the Sun, require some kind of model in order to infer age. In this talk I will introduce (very quickly) asteroseismology of solar-type pulsators as a tool for studying stars. I will discuss some of the successes and limitations of the current methodology for inferring ages using asteroseismology. And then I will present our ongoing work to address and overcome limitations. Much of this ongoing work focuses on the construction and execution of large multi-level models (Hierarchical Bayesian Models) and I hope to demonstrate the utility, and potential to scale, of these models as a common place method of the future.


Highlights from the dawn of million-star spectroscopy

Speaker: Karin Lind
Wednesday, 4th November 2020, 10:00 CET

Abstract: The light from stars in our Galaxy can trace the origin of the elements and the formation of stellar populations back to the earliest epochs and throughout cosmic time. Ground-based stellar surveys all over the world are following Gaia’s footsteps, collecting observational samples of steadily increasing size and quality. As I will demonstrate throughout this talk, insights can be made both by finding the needles in the haystack, the chemically peculiar or pristine stars, and by analyzing the statistical properties of large samples. I will further touch upon the progress in building realistic stellar models and efficient spectrum analysis techniques, both which are crucial for the success of any survey. The talk will focus on the GALAH survey (Galactic Archaeology with HERMES) in particular its forthcoming third data release, containing data and accompanying science papers for a sample of more than half a million stars.


Seismology for Galactic Archaeology with K2 and TESS

Speaker: Dennis Stello
Thursday, 22nd October 2020, 10:00 CEST
Recording available here

Abstract: In this talk I will show new results from the K2 Galactic Archaeology Program and a recently initiated project aimed at benchmarking the TESS performance in the Kepler field.


The impact of evolved stars on the chemical evolution of the Universe

Speaker: Amanda Karakas
Thursday, 15th October 2020, 09:00 CEST
Recording available here

Abstract: The chemical evolution of the Universe is governed by the nucleosynthesis contribution from stars, which in turn is determined primarily by the initial stellar mass. In this talk I will discuss results from projects that test the impact of evolved stars on this topic including the chemical evolution of our Galaxy up until the present time, and implications for chemical evolution in the distant future, using results from very metal-rich stellar models.


Stellar parameters from global seismology. A deeper look at grid based modeling

Speaker: Aldo Serenelli
Thursday, 24th September 2020, 10:00 CEST
Recording available here

Abstract: In this talk, I will present and discuss some results on stellar parameter determinations of red giant stars through grid based modeling (GBM), done within the APOKASC collaboration. This will include comparison of results obtained with different GBM pipelines, based on different sets of stellar evolutionary tracks and treatment of seismic quantities (large frequency separation, mostly). In the talk, I will also discuss previous work based on an alternative approach (as proposed by Pinsonneualt et al. 2018), and validation tests based on radii determined from astrometric (Gaia) data as well.


How fast is the Universe expanding?

Speaker: Thomas Tram
Wednesday, 16th September 2020, 13:00 CEST
Recording available here

Abstract:  The current expansion-rate of the Universe is often parametrised by the Hubble constant H0. This parameter can be measured in the local Universe by observations of type 1a supernovae and by analysing strongly lensed quasars. The Hubble constant can also be inferred from the Cosmic Microwave Background (CMB) anisotropies in the context of the flat LCDM model. However, the value inferred from the CMB is significantly smaller than the one inferred from observations of the local Universe. This discrepancy is approaching the 5 sigma mark, and the list of possible systematic effects explaining this difference is becoming shorter every month. Now the arrow is pointing towards the LCDM model: could the discrepancy be due to a missing component in our cosmological model?


Small-Scale Structure of the Milky Way’s Stellar Orbit Distribution

Speaker: Johanna Coronado
Thursday, 3rd September 2020, 10:00 CEST
Recording available here

Abstract: The exact processes behind the formation and evolution of galaxies are interesting puzzles in modern astrophysics. Our Galaxy offers us the unique opportunity to be studied in detail, as we can obtain the 3D positions, 3D velocities and also the chemical information on a star-by-star basis. By combining the astrometric survey Gaia with chemical information from spectroscopic surveys, we can obtain a detailed physical picture of our Galaxy. In this work we investigate the stellar orbit distribution of the Milky Way, while also adding their chemical information ([Fe/H]) in a chemical tagging generalization approach. We first make use of the spectroscopic information from LAMOST, in combination with parallaxes and proper motions from Gaia. We develop a method to obtain improved spectrophotometric distances (with errors less than 6%) for 150 000 main sequence stars. With more precise distances at hand, we investigate the small-scale structure in the orbit distribution of the Galactic disc for ∼ 600 000 main sequence stars in LAMOST × Gaia. Most stars disperse from their birth sites and siblings, in orbit and orbital phase, becoming ‘field stars’. We explore and provide direct observational evidence for this process in the Milky Way disc, by quantifying the probability that orbit similarity among stars implies indistinguishable metallicity. We define the orbit similarity among pairs of stars through their distance in action-angle space ∆(J, θ) and their abundance similarity by ∆[Fe/H]. By grouping such star pairs into associations with a friend-of-friends algorithm linked by ∆(J,θ), we find that hundreds of mono-abundance groups –some clusters, some spread across the sky– are over an order-of-magnitude more abundant than expected for a smooth phase-space distribution, suggesting that we are witnessing the ‘dissolution’ of stellar birth associations into the field.


3D model atmospheres and the PLATO mission: two case studies

Speaker: Hans-Günter Ludwig
Thursday, 27th August 2020, 10:00 CEST
Recording available here

Abstract: Three-dimensional time-dependent model atmospheres of late-type stars are nowadays routinely applied in analyses where high fidelity is of primary importance. In the context of the PLATO mission, 3D models are add-ons to standard 1D models supporting the spectral as well as asteroseismic analysis of PLATO targets. I will report on two ongoing projects at LSW in Heidelberg applying 3D CO5BOLD model atmospheres which are linked to the PLATO mission: i) a determination of the solar silicon abundance, and ii) the center-to-limb variation of solar-like stars from KEPLER photometry of systems exhibiting planetary transits. I will embed my comments in a wider landscape pointing to needs concerning 3D model grids, and future technical developments.


The SkyMapper view of the Sausage, Sequoia, and Splash

Speaker: Diane Feuillet
Thursday, 20th August 2020, 10:00 CEST
Recording available here

Abstract: Gaia DR2 has shown the Milky Way to have several previously unknown stellar populations; notably the Enceladus/Sausage, the Splash and the Sequoia. These could only be detected thanks to Gaia’s exquisite astrometry. However, the nature of these kinematic structures and their origin needs a deeper characterisation of their stars than possible with Gaia DR2 data alone. We present the SkyMapper view of these structures using 900,000 stars with photometric [Fe/H] measurements from SkyMapper and Gaia RVS measurements. We examine the kinematic properties, metallicity distribution functions, and population ages of stars possibly belonging to the Sausage, Sequoia, and the Splash. The Sausage has a mean [Fe/H] of -1.2 and is most apparent at high JR. We show that the selection of Sausage stars is quickly contaminated by disk stars below JR of 900 km/s. Using a population age analysis, we estimate the Splash to be old, with a peak older than 9 Gyr. With the full kinematic information available for such a large sample, we explore the prospects of robustly selecting stars belonging to the Sausage, Splash, and Sequoia.


Streams, substructures and the early history of the Milky Way

Speaker: Amina Helmi
Thursday, 13th August 2020, 10:00 CEST
Recording available here

Abstract: Since the advent of 2nd data release of the Gaia mission our understanding of the Milky Way and its constituents is undergoing a revolution. In this talk, I will highlight a few of the results stemming from the analysis of this truly spectacular dataset. In particular, I will focus on what we have learned about the assembly of the Milky Way thus far.


Better Stellar Modeling: Numerical Tools and Techniques for the Modern Observational Landscape

Speaker: Meridith Joyce
Monday, 10th August 2020, 10:00 CEST
Recording available here

Abstract:In the era of Gaia and TESS, astronomers often assert that better observational data will lead to improvements in stellar modeling, but we hear less frequently about the translation of better data to better theoretical formalisms in practice.My research concerns the development and use of stellar structure and evolution (SSE) modeling tools, especially the Modules for Experiments in Stellar Astrophysics (MESA) software suite, the Dartmouth Stellar Evolution Program (DSEP), and the GYRE stellar oscillation code. As our observing capabilities continue to improve, my work ensures that stellar models reflect observational reality and improve in their scope and capacity as predictive tools.In this talk, I will highlight my recent work using these stellar structure and evolution programs in novel ways to build predictive models of variable stars and make improved stellar parameter determinations using a combination of classical and asteroseismic techniques.


Hydrodynamical simulations of galaxy formation

Speaker: Volker Springel
Wednesday, 24th June 2020, 10:00 CEST
Recording available here

Abstract: Simulations of cosmic structure formation have come a long way. Nowadays, they are not only accurately predicting the dark matter backbone of the cosmic web and the internal structure of halos and their satellites far into the non-linear regime, but are also capable of following the baryonic sector with rapidly improving physical fidelity. In my talk, I will review the methodology and selected successes of recent hydrodynamical galaxy formation simulations, and critically discuss some of the primary uncertainties in modelling strong, scale-dependent feedback processes. I will also highlight predictions for the structure of magnetic fields in galaxies and the importance of cosmic rays in galaxy evolution. Finally, I discuss some of the challenges lying ahead in this field in the coming years.


Chemodynamics of the Milky Way

Speaker: Michael Hayden
Thursday, 18th June 2020, 10:00 CEST
Recording available here

Abstract: Galactic Archaeology is in the midst of a revolution due to the combination of large-scale spectrscopic surveys such as APOGEE and GALAH, which provide detailed chemical abundances for hundreds of thousands of stars, and precise astrometry from the Gaia satellite. Using APOGEE and GALAH, I will describe the chemical structure of the Galaxy, from the bulge to the edge of the disk. We find that the chemical structure of the Galaxy is highly dependant on location within the Galaxy, and that for example the high-alpha stellar populations making up the chemical thick disk truncates just beyond the solar circle. Using the precise astrometry from Gaia, I will also characterize kinematic properties of the disk, such as the velocity dispersion, and how the kinematics of the Galaxy vary with age, chemistry, and location within the Milky Way. Using these observational constraints, I will present new distribution functions and chemical evolution models that are able to reproduce chemodynamic observations across the Galaxy, giving insight into the possible origin of structures like the chemical thin and thick disks, as well as highlighting the importance of dynamical processes such as radial migration in the evolution of the Milky Way.


Seeing the Milky Way Disk Evolve with Red Clump Stars

Speaker: Neige Frankel
Monday, 8th June 2020, 10:00 CEST
Recording available here

Abstract: Disk galaxies present a great regularity in their stellar bodies but reveal complex structures in their young stars and gas. What connects the highly structured birth conditions to the overall regularity of disk galaxies? To tackle this question, we can use the Milky Way, for which we have star-by-star data, as a model organism to study the processes setting the evolution of its disk. The recent technical advances in spectroscopic and astrometric surveys have made it possible to collect 6D phase-space information, abundances and ages for a large number of stars. I will present a statistical model for the evolution of the Milky Way disk, describing when and where its stars were born, with what metallicity, and how their orbits subsequently evolved until their present state. APOGEE red clump stars and Gaia constrain that model well and implies strong radial migration: over the age of the low-alpha disk, the typical migration distance is about the disk’s half-mass radius. This extensive radial migration seems to happen with only modest radial orbit heating. Consequently, the disk looks very regular without losing its radial gradients.