Contributions
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The titles and abstracts of the posters are listed below the talks in the lower half of this page.
Talks
Automated Analysis Pipelines for Spectroscopic Surveys of Stars
Carlos Allende Prieto
On-going and planned spectroscopic surveys target samples in the range 1e5-1e8 stars. These large data sets not only demand a fully automated data reduction pipeline, but also automated data analysis, including the derivation of the fundamental atmospheric parameters, radial velocities, and chemical abundances. In this talk, I will review the different approaches followed by a number of projects (Gaia, Gaia-ESO, SDSS SEGUE and APOGEE), highlighting their strengths and weaknesses.
Galactic Archaeology with CoRoT and APOGEE
Friedrich Anders
We will present the first coherent dataset obtained from newly-reduced CoRoT lightcurves for 700 red giant stars in two fields of the Galactic disc which have been observed within an ancillary campaign of the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Using the Bayesian parameter estimation code PARAM to calculate distances, extinctions, masses and ages for these stars, we have measured chemical abundance patterns and ages of field stars over a large radial range of the Milky Way's disc, and present recent results on the age dependence of the radial metallicity distribution in the thin disc. Another intriguing results is that the tight correlation between age and chemical abundances previously seen in the Solar vicinity does not seem to be valid for the inner Galactic disc: [alpha/Fe]-rich stars are not necessarily old. Our results also promote the inclusion of dynamical effects of the Galactic bar in future chemical evolution models of the inner Galactic disc.
Some input for ages of cool main sequence stars from stellar rotation
Sydney Barnes
Recent results from space (Kepler) and from the ground seem to be telling us that the measured rotation periods of cool stars provide a reasonable way of determining the ages of cool (late-F to early M) main sequence stars, i.e. those that are particularly difficult to obtain by other methods. I could present a few highlights from this area of research.
Modelling the Galaxy with DFs and EDFs
James Binney
There are compeling reasons to modelling galaxies with equilibrium dynamical models. By Jeans' theorem such a model can be specified by giving a function f(J) on action space for each component of the galaxy (stellar disc, stellar halo, bulge, dark halo). Models built in this way have proven predictive power, which is the scientific gold-standard. Strictly every class of observationally distinguishable star forms an independent component and needs its own DF f(J). We achieve this by adding age and metallicity to the argument list of f(). I will show results obtained with such an Extended Distribution Function (EDF).
Galaxia: a holistic view of the Galaxy
Joss Bland-Hawthorn
In an era of massive stellar surveys and numerical simulations that purport to describe the Galaxy, there is a pressing need for a software tool that brings these together. This software must cope with survey incompleteness, the lumpiness of N-body simulations, statistical methods to fit analytic functions, made to measure and action-angle optimisation. We describe our progress to date and show what can be learnt from the RAVE and Kepler surveys.
SP_Ace: a new code to estimate stellar parameters and chemical abundances
Corrado Boeche
I present the code SP_Ace (Stellar Parameters and Chemical abundances Estimator) which derives stellar parameters Teff, log g, and chemical abundances [El/H] for 10 elements from stellar spectra with spectral resolution between R=2,000 and R=20,000. SP_Ace relies on a library of General Curve-Of-Growth (GCOG) of a list of 4500 absorption lines identified in the wavelenght ranges 5212-6900A and 8400-8900A (the present wavelength coverage can be extended). All the transition probabilities of these line have been astrophysically calibrated. The present GCOG library has been built on 1D stellar models and under LTE assumption but it can be extended to 3D stellar models and NLTE assumptions. The code works particularly well at low-medium resolution (R<10,000), where the line profile is dominated by the instumental profile and where the instrumental blending of the lines become severe. SP_Ace does not need user intervention, providing at the output Teff, log g, and chemical abundances [El/H]. The code will be registered under General Public License and distributed soon.
Testing Asteroseismic Scaling Relations using Eclipsing Binaries
Karsten Brogaard
The accuracy level of asteroseismic scaling relations for mass and radius can only be tested using when independent measures of these quantities are available. We demonstrate how classical high-precision measurements of eclipsing binary stars in star clusters and the field can be used to test the accuracy of the scaling relations. With enough measurements we should be able to obtain a calibration to improve the accuracy beyond the current level. We discuss the current status and future plans.
Gaia's contribution to reconstructing the history of the Milky Way
Carla Cacciari
As stated in the Concept and Technology Study Report (ESA-SCI 2000-4): ``The primary objective of Gaia is the Galaxy: to observe the physical characteristics, kinematics and distribution of stars over a large fraction of its volume, with the goal of achieving a full understanding of the MW dynamics and structure, and consequently its formation and history.'' A review of the Gaia mission and its science performance after the first 6 months of normal operations will be presented, and the contribution to reconstructing the history of the Milky Way will be discussed.
Towards 21st century stellar models: Star Clusters, Supercomputing, and Asteroseismology
Simon Campbell
Stellar models provide a vital basis for many aspects of astronomy and astrophysics. For example they provide key inputs for models of Galactic evolution and are central to age determinations of stars, both of which are important for deciphering the processes behind the formation and evolution of the Milky Way. Recent advances in observational astronomy -- through asteroseismology, precision photometry, high-resolution spectroscopy, and large-scale surveys -- are placing stellar models under greater quantitative scrutiny than ever. The model limitations are being exposed and the next generation of stellar models is needed as soon as possible. Here I review the evolution and importance of core helium burning stars and then discuss a range of methods, both theoretical and observational, that we are using to advance the models. Known observationally as SdB, red clump, horizontal branch, RR Lyrae and second clump stars, core helium burning stars are numerous and relatively luminous and therefore contribute disproportionately to Galactic emission. Our best models of these stars are very uncertain, and poorly constrained by observations. These uncertainties propagate to the later phases of stellar evolution, undermining the accuracy of models of supernova explosions and red giants - both vital to the chemical evolution of the Galaxy.
The vertical age structure of the Milky Way disc
Luca Casagrande
We derive vertical stellar mass, age and metallicity gradients in the Galactic disc using Kepler red giants. Photometry from the SAGA survey is used to gauge the Kepler selection function, and to complement seismic stellar parameters with classical ones. We find that old stars dominate at increasing Galactic heights, whereas stars closer to the plane exhibit a wide range of ages and metallicities, resulting into clear vertical gradients. The smooth age distribution of our stars over the last 10 Gyr, together with their flat age-metallicity relation is consistent with a quiescent evolution for the Milky Way disc since a redshift of about two.
Welcome
Cristina Chiappini
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The ages of very metal-poor stars
Norbert Christlieb
The unprecedented datasets that will be collected in this decade by wide-angle spectroscopic surveys, and astrometry measured by the Gaia satellite, will boost our knowledge of the formation history and chemical evolution of our Galaxy. An important variable in this effort is age. In my talk, I will review past attempts, and future opportunities for determining the ages of individual, metal-poor field stars of the halo of the Milky Way.
Chemical Abundance Gradients from Open Clusters using APOGEE data
Katia Cunha
The Open Cluster Chemical Analysis and Mapping (OCCAM) Survey aims to produce a comprehensive, uniform, infrared-based dataset for hundreds of open clusters. We present chemical abundance gradients obtained from 30 open clusters, using SDSS/DR12 data from the SDSS-III/Apache Point Observatory Galactic Evolution Experiment (APOGEE). The sample includes open clusters with R_{GC} ranging from 7 to 17 kpc and a range in ages. Results for 13 chemical elements Fe, C, N, O, Ca, Mg, Ti, Si, S, Ni, K, Na, Al will be discussed.
The oscillations of red giants - what information does the power spectrum contain?
Guy Davies
A star's Individual modes of oscillation contain a wealth of information and this content can be leveraged in order to make inference on stellar properties. By comparing the observed frequencies of modes of oscillation to modeled frequencies, it is possible to constrain even the age of a star. But with the dense information content of red giant oscillations comes difficulty in robustly extracting oscillation properties. In this talk I will discuss some of the techniques available that allow us to make decisions on the nature of a power spectrum of a red giant. We will see that care is required when studying complicated power spectra but that theory can guide the observer. By considering the shape of the structure in the power spectrum and the physics that determines this, we will explore the parameters and hence information that can be extracted.
4MOST: Spectroscopic host star characterisation for the southern PLATO stars
Roelof de Jong
4MOST is ESO's next generation spectroscopic survey facility for the VISTA 4m-class telescope with first light planned in 2021. In its initial 5 year survey 4MOST is expected to deliver spectra for >20 million objects spread over a large fraction of the southern sky. Of particular interest for PLATO will be that 4MOST can obtain resolution R=20,000 spectra for a large fraction (>300,000) of the southern dwarf and giant stars to be observed by PLATO in the 9-15th mag range. Such a spectroscopic survey would: - Provide exo-planet host-star characterisation yielding birth environment information by identifying stellar families in abundance and dynamics space; - Identify double line spectroscopic binaries; - Support the asteroseismology investigations such that much more accurate masses, ages and distances of stars can be obtained, putting strong constraints on Milky Way formation models; - By repeat observations yield single line radial velocity binaries. However, this would require a modification of the survey strategy from what is currently planned. Also improving the radial velocity accuracy requirement from 1km/s to <200 m/s would then be desirable.
Models of rotating stars constrained by asteroseismic measurements of red giants
Patrick Eggenberger
Solar-like oscillations have now been characterized for a large number of stars, thanks to asteroseismic data obtained recently by space missions. This has led to the determination of the global and internal properties of these stars. In particular, core rotation rates have been obtained for red-giant stars, which is of prime importance to progress in the modelling of the dynamical processes at work in stellar interiors. In this presentation, we discuss which constraints can be brought by these asteroseismic measurements on stellar models that includes rotational effects.
Non-LTE iron abundances in red giants with asteroseismic gravities : the role of hydrogen collisions
Rana Ezzeddine
We determine NLTE iron abundances in a sample of benchmark stars, using well determined asteroseismic fundamental parameters. Our aim is to empirically calibrate H-collision cross-sections with iron, as no quantum mechanical calculations are available yet. We have developed a new iron model atom including collision cross-sections for both excitation and charge transfer. We show that collisions with hydrogen leading to charge transfer are important for an accurate NLTE modeling. We also use our model atom to derive NLTE stellar parameters for a larger number of red giant and dwarf stars as part of the GES-CoRoT effort, and we compare them to parameters derived using the LTE approximation.
Dynamical constraints on non-axisymmetries in the Galaxy
Benoit Famaey
Current Galactic dynamical models still often rely on the assumptions of a smooth time-independent and axisymmetric gravitational potential, while more involved ab initio simulations in a cosmological context are not very flexible. First order models then try to isolate the effects of one main perturber, such as the bar or the spiral arms, but non-linear couplings can be present when both are taken into account simultaneously. Here we show that, when models take both the bar and the spiral arms into account, characteristic features in all three stellar velocity components (radial, tangential and vertical) do appear, and that different regions of the Galaxy and different velocity components are predominantly affected by one or the other of these non-axisymmetric components. We also demonstrate that the combined effect of the bar and spiral pattern is different from the linear combination of both in isolation. We argue that, in future dynamical modeling of the Galaxy, one should simultaneously fit all these effects without too strong priors on the shape of the potential of the fiducial axisymmetric background. Such modeling should allow us to put stringent constraints on the nature and amplitude of non-axisymmetries in the Milky Way with future and ongoing large surveys, which will be very important in order to quantitatively ascertain secular evolutionary processes driven by these non-axisymmetries.
Milky Way stellar populations with TRILEGAL
Leo Girardi
I will overview the status of the TRILEGAL population synthesis model of the Milky Way. Started as a tool to simulate the photometry of MW fields in several passbands, it has grown into a more elaborate tool where we can turn on the simulation of additional quantities such as the astroseismic parameters and kinematics, and the inclusion of "stellar exotica" such as interacting binaries, hot white dwarfs, optically-obscured AGB stars, etc. As such, the code can be efficiently used both in the interpretation of observed star counts, and as a platform to test available grids of stellar models and atmospheres. A few applications to recent wide-are surveys will be presented.
The effects of radial migration on the evolution of simulated galaxies
Robert Grand
In recent years, radial migration has been highlighted to be an important evolutionary process capable of significantly affecting galaxy evolution, and therefore must be understood in order to achieve a complete understanding of galactic archeology. We analyse the evolution of several high resolution simulations of Milky Way-sized galaxies, which include both isolated idealised simulations and cosmological zoom simulations performed with state of the art chemodynamics codes GCD+ and Arepo with the latest feedback models developed from the Illustris simulation. We analyse the effect of radial migration on the galaxy metal distribution of stars and gas, and find that it significantly increases the scatter in the stellar metallicity distribution function while maintaining a negative radial metallicity gradient. We find also that the gas undergoes significant radial migration, however the metallicity distribution function is kept narrow owing to efficient metal mixing. We analyse also the effect of radial migration on the structure of the galactic disc(s) - including the extent to which a thickened component may be formed, and breaks in the surface brightness profiles of galaxy discs, and discuss how these properties are related to bar and spiral structure.
Discussion Section Leader
Arlette Grotsch-Noels
Discussion Section - Uncertainties
Peakbagging in the open cluster NGC 6819: Opening a treasure chest or Pandora's Box?
Rasmus Handberg
In Kepler Red Giant stars, detailed peakbagging, meaning the extraction of individual oscillation modes and all their characteristics, have only been performed in a handful of stars. The discipline has proven itself extensively for main-sequence and sub-giant stars, but has not been applied to a large extent to evolved Red Giants. This has mainly been due to complications introduced by the many mixed dipole modes. In this talk I will report on an extensive peakbagging effort on the evolved red giant stars of the open cluster NGC 6819. This consists of around 50 stars spanning all the way up the red giant branch (RGB) and down to and including the red clump (RC). These stars represent a unique sample, because of their common distance, metallicity, age and length of observation. By employing sophisticated pre-processing of the time series and Markov Chain Monte Carlo (MCMC) techniques we have extracted individual frequencies, heights and line widths for hundreds of individual oscillation modes in the sample of stars. For some of the stars, rotational splittings could also be obtained. I will show that average asteroseismic parameters derived from these can be used to distinguish the stellar evolutionary state between RGB and RC stars, without having to measure the often difficult dipole modes. Furthermore, I will show how the fitting of some of these dipole modes can improve the detectability of acoustic glitches arising from the Helium II ionization zone in these stars and show results on how this can be used to constrain the Helium content in the cluster. I will also discuss some of the difficulties facing similar studies in the future, where it seems that detailed studies of clusters like this is facing some difficult times ahead.
Finding the twins of the Gaia benchmark stars in the Gaia-ESO Survey
Paula Jofre
We have provided the set of so-called "Gaia benchmark stars", which are stars with very precise stellar parameters that cover a wide range in the HR diagram at various metallicities. They are supposed to be good representative of typical FGK stars in the Milky Way. Currently, they are used by several spectroscopic surveys to validate and calibrate the methods that analyse the data. In this talk I will review our recent activities done for these stars. Additionally, by applying our new method to find stellar twins (Jofre et al in prep) in the Gaia-ESO Survey, I will discuss how good representatives of Milky Way stars the benchmark stars are and how they distribute in space.
Galactic Stellar Populations with APOGEE and Kepler
Jennifer Johnson
The APOGEE spectrocopic survey and the Kepler/K2 photometric mission provide complementary data on the properties of stars. Combining these dataset results in a powerful problem of Galactic stellar populations. I will discuss what we can learn about Galactic evolution from the combination of masses, metallicities, abundance ratios, and evolutionary states, both in the original Kepler field and in the new windows on the Galaxy opened by K2.
Dynamical and chemical evolution of the thin disc
Andreas Just
Our detailed semi-analytical local disc model quantifies the interrelation between kinematic properties (verlocity dispersion, mean tangential speed, ...) and spatial parameters (scale lengths, scale heights, mass fractions, ...) of stellar subpopulations with different age, metallicity and alpha-enhancement. Large survey observations (SEGUE, APOGEE, RAVE, Gaia-ESO) allow significant constraints on the long-term evolution of the extended solar neighbourhood. We can also characterize, which populations suffer most from radial migration.
Discussion Lead
Daisuke Kawata
Stellar Motion around Spiral Arms and the Evolution of the Galactic Disk: We compare the stellar motion around a spiral arm created in two different scenarios, transient/co-rotating spiral arms and density-wave-like spiral arms. We generate Gaia mock data from snapshots of the simulations following these two scenarios using our stellar population code, SNAPDRAGONS, which takes into account dust extinction and the expected Gaia errors. We compare the observed velocity around a spiral arm similar in position to the Perseus arm, and find that there is a clear difference in the velocity features around the spiral arm between the co-rotating spiral arm and the density-wave-like spiral arm. Our result demonstrates that the volume and accuracy of the Gaia data are sufficient to clearly distinguish these two scenarios of the spiral arms. In addition, we discss how the spiral arms and the bar formation affect the current properties of the thin and thick disks.
Chemodynamical properties of the Milky Way disc from RAVE and Gaia-ESO
Georges Kordopatis
Multi-object spectrographs have opened a new window on the analyses of the chemodynamical properties of old stars of the Milky Way. These analyses allow us to trace back the internal evolution and the external factors that influenced our Galaxy, and therefore understand fundamental aspects of galaxy evolution. In this talk, I will present recent results from RAVE and Gaia-ESO, exploring the Milky Way properties in different ways in terms of number of targets, magnitude range, and spectral resolution. In particular, I will focus on the detection of radial migration evidence within the thin disc, which puts among others constraints on the spiral history of the milky Way, and on the characterisation of the metal-weak thick disc, for which models are still having difficulties reproducing.
Hydrodynamical model atmospheres: Their impact on stellar spectrocopy and asteroseismology
Hans-Günter Ludwig
I shall give an overview of the physics implemented in the hydrodynamical model atmophere code CO5BOLD emphasizing its capabilities and limitations. Thereafter I will describe an application in the field of stellar abundance analysis, with the idea to relate this to the broader context of massive spectrocopic surveys. The multi dimensional and time-dependent nature of the hydrodynamical models allow us to study dynamical phenomena of relevance for asteroseismology. As a particular example, I discuss the so-called granulation background and its variation across the Hertzsprung-Russell diagram.
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Its Successor, APOGEE-2
Steven Majewski
The Apache Point Observatory Galactic Evolution Experiment (APOGEE), one of the programs in Sloan Digital Sky Survey III (SDSS-III), has now completed its three year survey of our galaxy. APOGEE has produced a large catalog of high resolution (R = 22,500), high quality (S/N > 100), infrared (H-band) spectra for stars throughout all stellar populations of the Milky Way, and including time series information via repeat visits to stars. With some half million spectra collected on 146,000 unique stars, APOGEE data are being applied to numerous problems in stellar populations, Galactic astronomy, and stellar astrophysics. I will summarize some of the early results of the APOGEE project, including measurements of Galactic dynamics, multi-element chemical maps of the disk and bulge, new views of the interstellar medium, explorations of stellar companions, the chemistry of star clusters and dSph satellites, and the discovery of rare stellar species. I will then describe plans for the recently initiated successor to APOGEE, the dual-hemisphere APOGEE-2 project. Together, the APOGEE-1 and -2 databases will constitute a large, comprehensive, systematic and homogeneous high resolution stellar spectroscopic survey across the entire Milky Way.
Young alpha-enriched stars in the solar neighbourhood
Marie Martig
We derive age constraints for 1639 red giants in the APOKASC sample for which seismic parameters from Kepler, as well as effective temperatures, metallicities and [alpha/Fe] values from APOGEE DR12 are available. We investigate the relation between age and chemical abundances for these stars, using a simple and robust approach to obtain age. While the overall trend between maximum age and chemical abundances is a declining fraction of young stars with increasing [alpha/Fe], at least 14 out of 241 stars with [alpha/Fe] > 0.13 are younger than 6 Gyr. Such unusual alpha-rich young stars have also been detected by other surveys, but defy simple explanations in a galaxy evolution context.
Impacts of stellar rotation on stellar evolution models
Georges Meynet
We shall discuss the various kinds of stellar models with rotation presently discussed in the literature and show the consequences on the outputs of the stellar models as the lifetimes and the stellar yields. Particular emphasis will be given on the effects of rotation at very low metallicity. Strengths and weaknesses of the present day stellar models will be discussed.
Basics of asteroseismology
Andrea Miglio
This introductory talk will be a guided tour through the rudiments of asteroseismology. With the aid of analytical approximations I will recall the direct relation between properties of stellar interiors and frequencies of global oscillation modes, with emphasis on the pulsation modes detected in red-giant stars. I will then emphasise the wider significance of asteroseismology as a tool for testing stellar physics and for stellar population studies, with examples on how seismic predictions depend on our (often poor) knowledge of the relevant physics, such as transport of chemical elements and angular momentum.
Milky Way Chemo-dynamics in the era of Gaia
Ivan Minchev
I will discuss our current understanding of the Milky Way chemo-dynamical evolution. The available range of chemo-kinematic constraints will be presented, considering data from recent and ongoing spectroscopic surveys, e.g., RAVE, SEGUE, Gaia-ESO, and APOGEE. A particular focus will be put on the importance of accurate stellar ages, such as those obtained through asteroseismology by the CoRoT and Kepler missions, for breaking the degeneracy among different Galactic evolution scenarios. Finally, I will discuss how the unprecedented accuracy of Gaia data proper motions, distances, and ages, combined with ground-based spectroscopic observations, may change our current understanding of the formation of the Galaxy.
Seismic parameters
Benoit Mosser
The detection of oscillations with a mixed character in red giants allows us to probe the physical conditions in their cores. Kepler data were selected to map various evolutionary stages and stellar masses. This allows us to monitor stellar evolution from the main sequence to the asymptotic giant branch. The seismic information can now be used for stellar modelling, especially for studying the energy transport or for specifying the inner properties of stars entering the red or asymptotic giant branches. Mass loss can be derived from precise mass determination.
Effective Temperatures for Metal-Poor Giants from ugriz Photometry RCP SETI
Ruth Peterson
Reliable stellar effective temperatures are critical to establishing stellar luminosities and abundances. Under favorable circumstances, temperatures for metal-poor giants may be derived to about 100K from ugriz photometry alone, by comparison with appropriately-adjusted model-atmosphere color indices. The photometry itself must be accurate, and the reddening moderate to low and well known. We demonstrate the regime over which this applies, and illustrate its validity in optimal and less optimal domains.
A general treatment of accretion and radial flows in chemodynamical models of spiral galaxies
Gabriele Pezzulli
The metal-poor gas, known to be continuously accreting onto the discs of spiral galaxies, is unlikely to arrive from the IGM with exactly the same rotation velocity of the galaxy itself. Even a small angular momentum mismatch inevitably drives gas radial flows that significantly impact the final distribution of metals. It is therefore crucial, for modern chemodynamical models, to include this effect in a fully consistent way. We present a simple analytic method that allows us to decompose a generic gas flow, in the presence of a generic angular momentum mismatch, into its two components: external accretion and a radial gas flow. We show that the predicted accretion and metallicity profiles for the Milky Way are very sensitive to the details of angular momentum accretion. Furthermore, we propose a novel method to compute the metallicity evolution in the presence of radial flows. Our strategy, based on characteristic lines, is extremely effective to trace the crucial impact of boundary conditions on the outer slope of abundance profiles. This is of great importance to properly understand the outskirts of spirals, including our Galaxy, where models often disagree with each other and with observations.
Discussion Leader
Bertrand Plez
As part of the GES-CoRoT effort, we use our new iron model atom to derive NLTE stellar parameters for red giant and dwarf stars with asteroseismic gravities. We compare these to parameters derived using the LTE approximation.
Uncertainties in grid-based estimates of stellar ages
Pier Giorgio Prada Moroni
The determination of the age of single stars by means of grid-based techniques is a well established method. I will discuss the impact on these estimates of the uncertainties in several ingredients routinely adopted in stellar computations. More in detail, the systematic bias on age determination caused by varying the assumed initial helium abundance, the mixing-length and convective core overshooting parameters, the microscopic diffusion, and the input physics will be quantified and compared with the statistical error caused by the current uncertainty in the observations.
On the impact of radial migration on the chemical evolution of the MW disk
Nikos Prantzos
I will discuss recent results concerning the impact of the radial migration of stars on the chemical evolution of the Milky way disk. In particular, I will discuss the impact on the dispersion of the local age-metallicity relation, the chemical properties of the thin and thick disks and the radial abundance profiles.
The PLATO 2.0 Mission
Heike Rauer
The PLATO Mission (Rauer et al. 2014) has been selected for ESA's M3 launch opportunity. PLATO will discover and bulk characterise extrasolar planets around hundreds of thousands of stars. With launch foreseen in early 2024. PLATO will follow the very successful space missions CoRoT and Kepler, as well as ESA's first small mission CHEOPS and NASA's mission TESS. PLATO will carry out high-precision, long-term photometric and astroseismic monitoring of up to a million of stars covering over 50% of the sky, and significantly increase the number of charactherized small planets around bright stars in comparison with previous missions. Its exquisite sensitivity will ensure that it detects hundreds of small planets at intermediate distances, up to the habitable zone around solar-like stars. PLATO will characterize planets for their radius, mass, and age. It will provide the first large-scale catalogue of well-characterized small planets at intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and providing targets for future atmosphere spectroscopy. This data base of bulk characterized small planets will provide a solid basis to put the Solar System into a wider contex and allow for compartative exo-planetology. The talk will provide an overview of the PLATO mission and its science goals.
Clues on the Milky Way thick disc from population synthesis simulations
Annie Robin
In recent years the stellar populations of the Milky Way have been investigated from large scale surveys in different ways, from pure star count analysis, to detailed studies based on spectroscopic surveys. While in the former case the data can constrain the scale height and scale length thanks to completeness, they suffer from high correlation between these two values. On the other hand, spectroscopic surveys suffer from complex selection functions which hardly allow to derive accurate density distributions. The scale length in particular has been difficult to constrain, resulting in discrepant values in the literature. We here investigate the thick disc characteristics from comparison of simulations with large scale data sets. The simulations are done from a population synthesis model of Besançon. We explore the parameters of the thick disc (shape, local density, age, metallicity) using a Monte Carlo Markov Chain method to constrain the model free parameters. Correlations between parameters are limited due to the vast spatial coverage of the used surveys (SDSS + 2MASS). We show that the thick disc was created during a long phase of formation, starting about 12 Gyr ago, finishing about 10 Gyr, during which gravitational contraction occurred, both vertically and radially. Moreover in its early phase the thick disc was flaring in the outskirts. We conclude that the thick disc has been created prior to the thin disc during a gravitational collapse phase, slowed down by turbulence related to a high star formation rate, as explained for example in Bournaud et al (2009). Our result does not favor a formation from an initial thin disc thickened later by merger events or by evolution of the thin disc.
Distances and extinctions for giants observed by Kepler and APOGEE
Thaíse Rodrigues
I will present a first determination of distances and extinctions for individual stars in the first release of the APOKASC catalogue, built from the joint efforts of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the Kepler Asteroseismic Science Consortium (KASC). Our method takes into account the spectroscopic constraints derived from the APOGEE Stellar Parameters and Chemical Abundances Pipeline, together with the asteroseismic parameters from KASC. These parameters are then employed to estimate stellar properties, such as masses, radii, surface gravities, ages, and absolute magnitudes, using the Bayesian tool PARAM. We then find the distance and extinction that best fit the observed photometry in Sloan Digital Sky Survey (SDSS), 2MASS, and WISE passbands. The first 1989 giants targeted by APOKASC are found at typical distances between 0.5 and 5 kpc, with individual uncertainties of just 2 per cent. Our extinction estimates are systematically smaller than provided in the Kepler Input Catalogue and by the Schlegel et al. maps. Distances to individual stars in the NGC 6791 and NGC 6819 star clusters agree to within their credible intervals. Comparison with the APOGEE red clump and SAGA catalogues provide another useful check, exhibiting agreement with our measurements to within a few per cent. Overall, present methods seem to provide excellent distance and extinction determinations for the bulk of the APOKASC sample. Uncertainties are expected to be reduced in future releases of the catalogue, when a larger fraction of the stars will have seismically determined evolutionary status classifications.
Age-dating combining stellar models, asteroseismology and spectroscopy. Challenges for stellar evolution calculations
Maurizio Salaris
Age-dating of stars lies at the heart of much of astrophysics, and provides crucial information for understanding the physical processes driving formation and evolution of galaxies, and more specifically our Milky way. There are several techniques that can be applied to estimate ages of stars in clusters and in the field of the various Galactic components. All these techniques more or less directly rely on information coming from stellar evolution calculations. In recent years, results from asteroseismology and spectroscopic surveys have been combined with stellar modeling to provide new promising ways to estimate stellar populations ages. The accuracy of all these estimates obviously depends not only on the observational data, but also on the accuracy of stellar evolution calculations. This talk will discuss the major outstanding uncertainties in stellar evolution modeling that impact age-dating methods, with emphasis on age estimates obtained in synergy with asteroseismology and spectroscopic observations.
Carbon-enhanced stars in the Milky Way and dwarf spheroidal galaxies
Stefania Salvadori
The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. I propose to discuss this topic by showing the most recent findings for CEMP stars that have been obtained by using cosmological chemical-evolution models for the formation of the Milky Way and its dwarf satellites. I will clarify why the fraction of carbon-enhanced to carbon-normal stars is observed to vary in dwarf galaxies with different luminosity, and discuss the origin of the first carbon enhanced star found in the Sculptor dwarf spheroidal galaxy.
Testing of Kepler Asteroseismic results against predictions of stellar population synthesis based model of the Milky Way
Sanjib Sharma
The NASA Kepler mission provides asteroseismic data for a large number of giants and sub-giants. Asteroseismic frequencies contain information about the mass and radius of the stars which in principle can be used to test the theories of both the stellar evolution and the formation of the Milky Way. We employ a population synthesis based model of the Milky Way and test its predictions against the asteroseismic results provided by the Kepler mission. First we compare the distribution of frequencies and they match well with the models. However, when scaling relations are employed the derived masses are found to be systematically offset.We discuss some of the causes for this. Unlike previous studies, for sub-giants, we find no differences in observed distributions of mass and radius to predictions from models.
Age determination for dwarfs and giants from Asteroseismology
Victor Silva Aguirre
One of the major challenges in the theory of galaxy formation is setting the timeline in the series of events that shaped the Milky Way into what we observe today. Stellar ages are the keystone in this endeavour, and progress has been strongly limited by our inability of determining reliable stellar ages for large samples of stars. We can now break this impasse using Asteroseismology, a technique sensitive to the deep layers of stars that allows precise determination of fundamental stellar parameters. I present different approaches for extracting stellar ages for dwarfs and giants depending on the available seismic data, discuss their uncertainties, and the prospects for improving the age determination of large samples of stars suitable for galactic archaeology studies.
Discussion Lead
Verne Smith
Probing the s-Process across the Milky Way with APOGEE Spectra: Heavy-element, neutron-rich elements identified in APOGEE spectra have been used to probe s-process abundances across the various stellar populations of the Milky Way. The s-process is a sensitive indicator of AGB star yields and is also a useful chemical tracer of stellar streams, which originated from disrupted dwarf galaxies but now reside within the Galactic halo, thick disk, or thin disk.
The chemo-dynamical evolution of the Milky Way as seen by RAVE
Matthias Steinmetz
The Radial Velocity Experiment (RAVE) is a large wide-field spectroscopic stellar survey of the Milky Way. Over the period 2003-2013, 574,630 spectra for 483,330 stars have been amassed at a resolution of R=7500 in the Ca-triplet region of 8410-8795 AA. Wavelength coverage and resolution are thus comparable to that anticipated from the Gaia RVS. Derived data products of RAVE include radial velocities, stellar parameters, chemicals abundances for Mg, Al, Si, Ca, Ti, Fe, and Ni, and absorption measures based on the diffuse interstellar bands (DIB) at 8620 AA. Since more than 290000 RAVE targets are drawn from the Tycho-2 catalogue, RAVE will be an interesting prototype for the anticipated full Gaia data releases, in particular when combined with the early Gaia data releases, which contain astrometry but not yet stellar parameters and abundances.
The K2 Galactic Archaeology Program
Dennis Stello
Recent space missions have transformed our ability to study vast numbers of stars using asteroseismology. This advance has opened up for exploration of the structure and evolution of the Galaxy using oscillating red giant stars as distant tracers of stellar populations including the halo, the bulge and the thin and thick disks. Initial results from the CoRoT and Kepler missions have demonstrated the enormous potential there is in the marriage between asteroseismology and contemporary Galactic Archaeology based on single-epoch spectroscopy, photometry, and parallax measurements. The scope for this research received a significant boost last year on several fronts. This includes the re-purposed Kepler telescope, K2, which started observing tens of thousands of red giants along the ecliptic covering all main constituents of the Galaxy. In this talk I will give an overview of the K2 Galactic Archaeology Program and show our most recent results from the K2 data analysis.
Cumulative theoretical uncertainties in lithium depletion boundary age
Emanuele Tognelli
We performed a detailed analysis of the main theoretical uncertainties affecting the age at the lithium depletion boundary (LDB). To do that we computed almost 12 000 pre-main sequence models with mass in the range [0.06, 0.4] Msun by varying input physics (nuclear reaction cross sections, plasma electron screening, outer boundary conditions, equation of state, and radiative opacity), initial chemical elements abundances (total metallicity, helium and deuterium abundances, and heavy elements mixture), and convection efficiency (mixing length parameter, ML). As a first step we studied the effect of varying these quantities individually within their extreme values. Then, we analysed the impact of simultaneously perturbing the main input/parameters without an a priori assumption of independence. Such an approach allowed us to build for the first time the cumulative error stripe, which defines the edges of the maximum uncertainty region in the theoretical LDB age.
The CoRoT-GES Collaboration: the joint action of asteroseismology and spectroscopy in the field of Galactic Archeology
Marica Valentini
Nowadays large spectroscopic surveys, like the Gaia-ESO Survey, provide unique stellar databases for better investigating the formation and evolution of our Galaxy. Great attention must be paid to the accuracy of the basic stellar properties derived: large uncertainties in stellar parameters lead to large uncertainties in abundances, distances and ages. Asteroseismology has a key role in this context: when seismic information is combined with information derived from spectroscopic analysis, highly precise constraints on distances, masses, extinction and ages of Red Giants can be obtained. In the light of this promising joint-action, we started the CoRoT-GES collaboration. GES has observed a set of 1,718 CoRoT Red Giants located in the inner disk (CoRoT field LRc01) and among these stars, 979 have reliable global seismic parameters. We present precise stellar parameters, ages, kinematical and orbital parameters and detailed element abundances for this sample of stars. We also show that, thanks to asteroseismology, we are able to obtain an higher precision than what can be achieved by the standard spectroscopic means. This sample of CoRoT Red Giants, spanning Galactocentric distances from 5 to 8 kpc and a wide age interval, provides us a unique dataset for testing modern chemo-dynamical models, by investigating the age-metallicy relation, chemical gradients and kinematics.
Posters
GALAH + CoRoT: where high-resolution spectroscopy meet asteroseismology
Borja Anguiano
GALAH collaboration with the CoRoT group provides an outstanding opportunity to get better stellar ages as Asteroseismology opens a new era for dating the Galactic disk. Without abundance information, the uncertainty of the inferred ages for the CoRoT giants is about 30%. With abundances information from the GALAH spectra, the uncertainty on the ages drops to less than 15% (Miglio et al. 2013). With 15% age errors, a precise age-metallicity and age-velocity relations can be measured. I will give an update of the status of this project.
The age-velocity relation in the solar neighbourhood as revealed by a sample of hydrogen-rich SDSS white dwarfs
Borja Anguiano
In this study we use a sample of hydrogen-rich white dwarf from the DR 10 SDSS catalogue to investigate the age-velocity relation in the Galactic disk. White dwarfs (WD) are natural clocks and they can be used to compute accurate ages. The total age of a WD can be expressed as the sum of its cooling time (Salaris et al. 2000) plus the main sequence lifetime of its progenitor, adopting a initial-final mass relationship (Dominguez et al. 1999; Weidemann 2000). I will present some results about how WDs can help for dating the Galactic disk. Combining RV, proper motions, distances and ages I will also present the space motions of these objects in a temporal diagram together with a discussion on the possible disk heating mechanism.
Mapping the Galactic gravitational potential with RAVE data
Olivier Bienaymé
We determine the Galactic potential in the solar neighbourhood from RAVE spectroscopic observations. We select red clump stars for which accurate distances, radial velocities, and metallicities have been measured in the range of 200 pc to 2000 pc distances towards the South Galactic Pole. Our results are in agreement with previously published Kz determinations up to 1 kpc, while the extension to 2 kpc shows evidence for an unexpectedly large amount of dark matter.
Single lined spectroscopic binary stars in the RAVE survey
Danijela Birko
The RAdial Velocity Experiment is a medium resolution spectroscopic survey of the Milky Way which has obtained over half a million stellar spectra of randomly selected stars. Although the number of stars observed in the RAVE survey is much smaller than the total number of stars in the Galaxy, it is possible to make a good study of the star population. In the case of single lined spectroscopic binary stars (SB1) repeated spectroscopic observations of the same system are needed. First task is to identify SB1 candidates from the RAVE data release. Probability function needs to be defined in order to establish quantitative criterion for RV variability. Using synthetic spectra it is possible to derive the primary parameters of the stellar atmosphere: temperature, metallicity, gravity and rotational velocity. It can be checked if residuals from the spectral fitting of SB1 indicate the existence of secondary set of spectral lines. The most pronounced residuals are expected to be on the calcium triplet lines. Fitting residuals SB2 can be detected.
A chemodynamical model of the Milky Way stellar halo
Payel Das
Halo stars may have formed in situ, been flung out from further in, or accreted from neighbouring systems. The particular combination of processes is embedded in the phase-space and metallicity distributions of the stars, and therefore they serve as fossils of the halo's formation history. Mutual investigation of these distributions is paramount to make the most of the observations available to us. We examine the existence of a double power-law density profile found in several recent studies of stellar populations from a full chemodynamical perpective by extending a recently proposed action-based distribution function (Posti et al., 2015, Williams et al., 2015) to also vary continuously as a function of metallicity. We infer the parameters of the extended distribution function by maximising the likelihood of phase-space and metallicity observations of a sample of K giants, blue horizontal branch stars, and RR Lyrae that trace the halo out to about 120kpc. We assume the stars to be moving in a potential consisting of thin, thick, and gas disc, bulge, and dark matter halo components. A distribution function is recovered that is able to satisfactorily fit the data, implying that there is a smooth component in the halo exhibiting equilibrium dynamics. The best-fit parameters describe a double-power law density profile similar to that found in past studies. In particular, we learn that the break is supported dynamically by apocentres preferentially being located there (a mechanism proposed to explain the Milky Way's break radius from accretion only). The halo is found to be radially anisotropic, and more so in the metal-poorer stars, supporting the contribution of accretion events to the build-up of the halo.
No breakdown of the radiatively-driven wind theory in low-metallicity environments
Eric Depagne
Hot star winds are radiatively driven by the transfer of photospheric photons momentum to atmospheric material through absorption and scattering. The mass-loss rate of massive hot stars is thus dependent on the metal content. We present an analysis of the mass loss rate - metallicity relation, supported by observations made with HST/COS.
Progenitors of Supernovae Type Ia in SPH galaxy simulations: Can we constraint the progenitor's identity by the study of chemical patterns and scaling relations of the host galaxy?
Noelia Jimenez
The nature of the Type Ia supernovae (SNIa) progenitors remains stil uncertain. This is a major issue for galaxy evolution models since both chemical and energetic feedback play a major role in the gas dynamics, star formation and therefore in the overall stellar evolution. The progenitor models for the SNIa available in the literature propose different distributions for regulating the explosion times of these events. These functions are known as the Delay Time Distributions (DTDs). In this talk i will present a study of five different DTDs for SNIa implemented in SPH galaxies dominated by a rapid quenching of the star formation, displaying the majority of the stars concentrated in the bulge component. The simulations are compared with observables such as the present observed SNIa rates in spheroidal dominated galaxies, and to the [O/Fe] ratios shown by the bulge of the Milky Way. We find that not every proposed scenario (DTD) is able to reproduce the observations. Additionally, some of the DTDs can reproduce simultaneously the SNIa rates, the [O/Fe] ratios and an observed correlation between the specific SNIa rate and the specific star formation rate. Our results suggest that SNIa observations in galaxies with very low and very high specific star formation rates can help to impose more stringent constraints on the DTDs and therefore on SNIa progenitors.
Bulge RR Lyrae Radial Velocity Assay
Andrea Kunder
Using mainly bulge red clump (RC) stars and giants as tracers, a picture has emerged of the Galactic bulge consisting of a rotating peanut-shaped structure made up largely of old and metal-rich stars (around 10 Gyr, [Fe/H] falling between -0.5 and 0 dex). However, there is a more metal-poor population of stars present in the bulge, the dynamics of this population has largely gone unexplored. We are carrying out a spectroscopic survey of bulge RR Lyrae variables, tracers of possibly the oldest and most metal-poor stars in the bulge ([Fe/H] peaking at -1.0 dex). To date, we have obtained radial velocities of 90 OGLE bulge RR Lyrae stars in a 2 degree window at (l,b) = (3,-3) for which NIR-photometry from the VVV surveys exists. Therefore, our RR Lyrae stars have accurate (3%) distances based on near infrared light curves. A subsample of our RR Lyrae stars have OGLE proper motions, so orbits of this population have been obtained. Our results indicate that the kinematic properties of RR Lyrae stars at (l,b) = (3,-3) are similar to that of the other components of the Galactic bulge. This may depend either on a relatively short relaxation time or on the fact that they belong to the same population. Because the RR Lyrae stars have kinematics consistent with the barred structure formed of the intermediate-age RC stars, the bulge RRL are likely in the same gravitational potential together with the more massive Galactic bar.
Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints - The open cluster NGC 6633 and field stars
Nadege Lagarde
The availability of asteroseismic constraints for a large sample of red giant stars from the CoRoT and Kepler missions paves the way for various statistical studies of the seismic properties of stellar populations. We use the first detailed spectroscopic study of 19 CoRoT red-giant stars (Morel et al 2014) to compare theoretical stellar evolution models to observations of the open cluster NGC 6633 and field stars. In order to explore the effects of rotation-induced mixing and thermohaline instability, we compare surface abundances of carbon isotopic ratio and lithium with stellar evolution predictions. These chemicals are sensitive to extra-mixing on the red-giant branch. We estimate mass, radius, and distance for each star using the seismic constraints. We note that the Hipparcos and seismic distances are different. However, the uncertainties are such that this may not be significant. Although the seismic distances for the cluster members are self consistent they are somewhat larger than the Hipparcos distance. This is an issue that should be considered elsewhere. Models including thermohaline instability and rotation-induced mixing, together with the seismically determined masses can explain the chemical properties of red-giants targets. However, with this sample of stars we cannot perform stringent tests of the current stellar models. Tighter constraints on the physics of the models would require the measurement of the core and surface rotation rates, and of the period spacing of gravity-dominated mixed modes. A larger number of stars with longer times series, as provided by Kepler or expected with Plato, would help for ensemble asteroseismology.
Origin of the Metallicity Distribution in the Thick Disc
Maider Miranda
We examine the origins of (a) the inverted, or positive, radial metallicity gradients observed in the thick disc of the Milky Way; (b) the steep (and negative) vertical metallicity gradients observed through the thick disc; and (c) the vertical gradient of the stellar rotation velocity through the thick disc. All this using a suite of cosmological chemodynamical disc galaxy simulations and observational data from a nearby edge-on spiral galaxy. We analyse five simulated spiral galaxies with masses comparable to the Milky Way. The simulations span a range of star formation and energy feedback strengths, particle- and grid-based hydrodynamical implementations, as well as initial conditions/assembly history. We also consider RSS longslit spectroscopy of the edge-on disc galaxy IC2531. Consistently, we find that the steeper, negative, radial metallicity gradients seen in the mid-plane flatten with increasing height away from the plane. In simulations with stronger (and/or more spatially-extended) feedback, the negative radial gradients invert, be- coming positive for heights in excess of 1 kpc. Such behaviour is consistent with that inferred from recent observations. Simulations employing stronger/more extended feedback prescriptions possess radial and vertical metallicity and kinematic gradients more in line with recent observations. The inverted, positive, radial metallicity gradients seen in the simulated thick stellar discs originate from a population of younger, more metal-rich, stars formed in-situ, superimposed upon a background population of older migrators from the inner disc; the contrast provided by the former increases radially, due to the inside-out growth of the disc.
Kine-chemical tagging of FGK stars: discerning between field-like stars and real physical structures of coeval stars with a common origin
David Montes
Using several high resolution spectroscopic surveys of nearby FGK stars (including the Gaia-ESO survey) we have information about kinematics, rotational velocities, age (Lithium abundance and chromospheric activity) and fundamental stellar parameters (Teff, log g, metallicity, [Fe/H], and microturbulent velocity) that allowed us to ascribe these stars to moving groups and associations of different ages. By applying the chemical tagging technique (differential abundance analysis of 20 elements) we are testing the membership of these stars to these kinematic groups. All this information allowed us to better understand the star formation history in the solar neighborhood discerning between field-like stars (associated with dynamical resonances (bar) or spiral structure) and real physical structures of coeval stars with a common origin (debris of star-forming aggregates in the disk). We summarize the results obtained for FGK candidates stars to the Hyades supercluster (Tabernero et al. 2012) the Ursa Mayor moving group (Tabernero et al. 2015) and the Castor moving group.
Lithium abundance in spectroscopic surveys: application of 3D stellar model atmospheres
Alessandro Mott
Determining chemical abundances from stellar spectra plays a fundamental role in improving our understanding of stellar, galactic and cosmic evolution. Nowadays it is possible to derive accurate chemical abundances by using realistic 3D model atmospheres, and even account for departures from LTE of selected elements. The results are in general more reliable compared to those obtained with classical 1D models, but the 3D hydrodynamical simulations and spectrum synthesis calculations are computationally rather demanding. We present an analytical fitting function for deriving the lithium abundance in metal-poor stars, together with some practical applications. Based on sophisticated 3D-NLTE line formation calculations, the fitting function eliminates the need of actually computing 3D stellar atmosphere models and a grid of synthetic spectra necessary for deriving the 3D-NLTE curve-of-growth. This analytic formula provides a simple means of taking into account 3D and non-LTE effects in the analysis of a large number of observed spectra, as in the case of the ongoing spectroscopic surveys (e.g. Gaia-ESO), where current abundance determinations rely on classical 1D models. We also describe our plans of updating the fitting function for lithium to cover a wider range of Teff, logg and [Fe/H], extending its application to stars of lower gravity and up to solar metallicity. With the help of such tool, the use of realistic 3D model atmospheres in the framework of big surveys would become more feasible.
Padua Galaxy model in preparation for Gaia: asymmetries and action space in star count modelling techniques
Stefano Pasetto
The concept of stellar population as a group of stars that share common properties in a multidimensional age-metallicity-phase space is presented in an extended mathematical formalism. Star count techniques are popular framework where to study the complicate inter-dimensional coupling of these stellar populations (e.g., Besancon model) and are especially valuable in those situations where the system under exam is mapped with fine data. In this context, Gaia will expand greatly the quality of the phase-space description required in this kind of modelling approach to the Milky Way and a new version of the Padua star count Galaxy model is undergoing major update to coupe with MW phase space asymmetries. We present the first results of phase-space portrait we drawn to account for spiral arms and an application to APOGEE data. Extensive comparison with Besancon Galaxy model will be presented.
The alchemy of tracing the prevalence of massive stars
Jan Rybizki
Using Bayesian inference together with a local Milky Way disk model we map the effect of a universal initial mass function (IMF) into the space of observables and constrain its parameters using high quality data. In a prove of concept we exploited Hipparcos starcounts resulting in tight constraints of the IMF up to 8 solar masses. Now including a model of chemical enrichment we want to reliably determine the high mass slope of the IMF.
Stellar multiplicity in high-resolution spectroscopic surveys
Edita Stonkute
Stellar multiplicity is a key parameter for many astrophysical questions. Several interesting astronomical phenomena, such as gravitational waves and gamma-ray bursts, arise from binary stars, and the knowledge of multiplicity could provide constraints on possible channels of star formation and evolution in the Galaxy. For ongoing and coming large spectroscopic surveys, such as RAVE, SEGUE, LAMOST, Gaia-ESO, GALAH and 4MOST, it is important to identify the binaries to clean the survey products from potentially faulty results. However, little is known about the binary frequency in Milky Way field stars, particularly outside the Solar neighbourhood. We present our models of the effect of binaries on high-resolution spectroscopic surveys, in order to determine how many binaries will be observed, whether unresolved binaries will contaminate measurements of chemical abundances, and how we can use spectroscopic surveys to better constrain the population of binaries in the Galaxy.
This seminar is generously funded by the Wilhelm und Else Heraeus-Stiftung.
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