Short abstract
Ongoing, large scale, observational surveys are producing a very detailed view of the chemo-kinematic properties of the Milky Way. To fully exploit the potential of this wealth of information, and constrain Galactic formation and evolution models, it is crucial to retrieve an additional, fundamental information: the age of large samples of stars in Galactic fields and clusters. In such a way, a detailed chrono-chemo-dynamical map of the Galaxy can be obtained.
The main aim of this project is to obtain a homogeneous age scale appropriate for the whole parameter space, i.e., from very young stars to the oldest ones in a wide metallicity range. Various stellar chronometers -based on a range of observables- are applied to stars in different evolutionary stages, all of them calibrated homogeneously on a set of stellar models. To this purpose, the first step is to provide a complete and updated theoretical evolutionary framework -validated on benchmark stars- whose still present uncertainties has to be properly evaluated. To adjust the various stellar clocks to the same age scale, one needs to use suitable observational benchmarks, whose age dating can be simultaneously performed by using different age indicators. To this aim, we will adopt a sample of Galactic open clusters, whose properties are (or will be) carefully investigated in the context of high-precision, observational surveys, such as Gaia and the Gaia-ESO survey.
Another major goal of this project is to derive a complete and reliable characterization – including spectroscopy, kinematics, seismic information and photometry - for a very large sample of Galactic field stars, by combining the wealth of data coming from ongoing and forthcoming observational surveys in a single, homogeneous and self-consistently calibrated catalogue: The Survey of Surveys. The application of the various homogeneously calibrated age indicators to this catalogue will allow us by using statistical methods, to recover a complete chrono-chemo-dynamical map of the Milky Way. This map will answer most long-lasting questions in Galactic astronomy, and by extension, in disk galaxy evolution, such as: Is there a specific star formation episode associated with the thick disk formation and what caused it? Do thin and thick disks have overlapping or distinct ages? What is the relative importance of radial mixing and migration? How did the Galactic assembly occur?
As a legacy, the project will make available to the scientific community, the complete theoretical and observational databases, as well as the chrono-chemo-dynamical characterization for the stars analyzed by the project.
State of the art about Galactic Archaeology
The formation and evolution of galaxies is one of the major outstanding open problems in Astrophysics. Disc galaxies similar to the Milky Way (MW) dominate the stellar mass density in the Universe, yet the details of their formation/evolution are still poorly understood. While observations at high redshift can provide a snapshot of a large number of galaxies at specific times in their evolution, Galactic archaeology provides a unique tool to trace with unparalleled level of details the complete history of our Galaxy, through the study of its successive generations of stars: from old stars, witnesses of the first epochs of Galaxy evolution, to young stars, tracers of more recent star formation episodes. This highly detailed information is fundamental to constrain galaxy formation and evolution models (Nuza+19, Spitoni+19, Agertz+21).
The pivotal importance of Galactic archeology is demonstrated by the impressive amount of resources devoted to it. The ESA Gaia mission is obtaining exquisite parallaxes, proper motions, and photometry for almost 2 billion stars, sampling all MW components: bulge, thick/thin disk, and halo (Gaia collaboration+16, +18, +21). Several ambitious spectroscopic surveys (as APOGEE, Gaia-ESO, GALAH, LAMOST, RAVE) have already been carried out, or are planned (as WEAVE, 4MOST, MOONS) with the aim of securing a detailed chemo-kinematical map of the MW. At the same time, on-going (as Kepler/K2, TESS) and future (as PLATO) seismic surveys are aimed at providing a detailed information about stellar pulsations, that can be used to probe the physical and evolutionary properties of large samples of stars. With all these efforts we are at a turning point in Galactic astronomy.
While these datasets provide a detailed picture of the chemo-dynamical structure of the MW, the crucial chronological information is missing: ages of stars are essential to establish a temporal sequence of events, and hence to interpret the observed properties in the context of galaxy models.
Unlike other properties, the age of stars cannot be measured directly, but only inferred via indirect methods (Soderblom 10; Sahlholdt+19). One of the most common methods is the isochrone fitting technique, in which age is inferred by comparing a set of physical parameters derived from stellar spectra and/or photometry with stellar models. However, this method is problematic for field stars in those evolutionary stages where the age sensitivity of isochrone fitting decreases, such as the red giant branch and the lower main sequence (Sanders & Das 18). In spite of this, the wealth of empirical data has led to ambitious projects exploiting advanced statistical methods to derive ages for large stellar samples by comparison with extended model grids (Silva Aguirre+15, Queiroz+18).
Other age tracers have also been designed for stars in specific evolutionary stages, so suitable only for limited age and/or mass ranges. For instance, lithium and beryllium abundances are excellent tools to estimate the age of young, solar-like stars (Martin+18) and very old ones (Smiljanic+09) respectively; the [C/N] abundance ratio can provide age for giant stars (Casali+19); gyrochronology and chromospheric activity can be also used to estimate the age of main-sequence (MS) late-type stars; asteroseismology, combined with spectroscopy, has turned into the big hope to derive accurate ages, particularly for red giants (Chaplin+14, Miglio+17). However, all these clocks need to be calibrated by using stellar models, and commonly this has been done with heterogeneous model grids, developed by distinct research groups by adopting different input physics and physical assumptions (Cassisi & Salaris 13, Silva Aguirre+17, Pinsonneault+18). The crucial implication is that all these stellar clocks do not provide estimates in a homogeneous and self-consistent scale. However, since a single ideal age indicator, applicable to the whole parameter space, does not exist, we are forced to use all of them to obtain the age of stars in different evolutionary stages. As a consequence, despite its fundamental importance, we still lack a well-calibrated, homogeneous age scale, to obtain a reliable chronological mapping of the MW.
This project is aimed at filling this gap by calibrating an accurate and homogeneous age scale for all the previously mentioned age indicators. To do this, a state-of-the-art stellar theoretical framework, and accurate observational datasets will be used. The overarching goal of this project is the build-up of a self-consistent chrono-chemo-dynamical map of the Galaxy.
Brief description of the project
Age determination for field stars is a very challenging task, mainly due to the lack of a single ideal age indicator, suitable for the whole age range, and covering all evolutionary stages. Distinct stellar clocks, independently and heterogeneously calibrated, have been envisaged and (sometimes uncritically) applied, so that the age distributions of the various Galactic components are affected by systematic effects whose impact is difficult to be assessed. This represents a strong limitation for our understanding of the Galaxy formation/evolution.
A goal of our project is to define and calibrate a reliable and homogeneous age scale suitable over a wide age interval and various stellar evolutionary stages: from the youngest to the oldest MW stars. This is the first attempt to combine in a synergic effort the expertise of researchers working in complementary research fields, with the aim of overcoming this limitation in an era in which large observational surveys are providing a detailed view of the MW.
A homogeneous age scale, combined with the wealth of information from the last generation of spectroscopic, astrometric, and seismic surveys such as APOGEE, Gaia, Gaia-ESO (GES), Kepler/K2, etc., will allow us to finally obtain a complete and accurate chrono-chemo-dynamical map of the Galaxy. To achieve these goals, a methodological approach based on the combination of state-of-the-art stellar models and a front-end, accurate observational framework will be adopted.
Improving the evolutionary scenario
The technique commonly used for cluster age determination is isochrone fitting. However, when dealing with field stars in specific evolutionary stages such as the pre-main sequence (PMS), the faint main sequence (MS), and the red giant branch (RGB), its resolving power dramatically decreases even when using the precise Gaia parallaxes and photometry and accurate chemical abundances. Thus, additional empirical age indicators have been designed, based on the observed relations between a measured property -such as rotation rate, magnetic activity, surface chemical abundance- and the stellar age. However, all these methods need to be calibrated. In recent times, asteroseismology is turning out as a formidable tool to derive precise (at the level of 10-15%) ages (Miglio+17, Montalban+21), but to obtain an age estimate from seismic + spectroscopic data, stellar models are necessary too. Thus seismic ages are also model-dependent and systematic errors can be still large (Pinsonneault+18, Valle+15,+18c,d). Our first objective is to define a model-based age scale as accurate as possible, covering a wide range of ages and chemical compositions.
To obtain a good stellar clock, it is not enough to use the most updated inputs for computing the stellar models, but it is mandatory to know also how precise the clock is. This information can be obtained only by testing how the age-dependent stellar properties – and hence the correlated clocks- are affected by variations -within their estimated errors- of the input physics and of the treatments of the physical processes at work in stars as, e.g., mass loss, mixings and diffusive processes (Cassisi+98, Valle+13).
We have a long standing expertise in the field of stellar evolution, and have hugely contributed to the build-up of accurate, widely used, stellar model libraries: BaSTI-Pietrinferni+04,+06; BaSTI-IAC-Hidalgo+18, Pietrinferni+21; PISA: Dell’Omodarme+12. These models are based on the best, presently available, physical inputs, and on plausible assumptions about the efficiency of diffusive processes and convective core overshooting, but they do not account for rotation or radiative levitation. This framework represents a unique starting point for the definition of an age scale, but to further improve it we shall investigate the effects on stellar models due to:
- the efficiency of diffusive processes.
- Non canonical mixings including rotationally and/or magnetically induced ones, and thermohaline.
- Convection efficiency in the superadiabatic layers.
- Mass loss efficiency.
- Mass accretion in the protostellar.
- Magnetic fields.
Building up the calibration sample
Open Clusters, whose ages are in the range from few Myr to several Gyr, offer a unique opportunity to check the consistency of the age scales of different indicators, and to calibrate them by using the estimates based on isochrone fitting. In this project, we consider a set of age tracers that: i) cover a wide age range from ~10Myr to the age of the oldest MW stars; ii) can be applied to a sizeable number of OCs; iii) have some overlap allowing them to be cross-calibrated. These tracers include: Asteroseismology, Lithium abundance, Surface abundance ratios, Gyrochronology.
Building up the observational framework: the Survey of Surveys
A fundamental data source for CHRONOS will lie in the presently vast panorama of ongoing and planned Galactic surveys -such as MOONS, 4MOST, and WEAVE. However, each individual survey focuses on different stellar samples, portions of the sky, wavelength ranges, or types of stars, and each dataset is analyzed with different methods and models. Experience with APOGEE and GES (Pancino+17) has shown that survey homogenization (internal consistency) and calibration (external reference frame) are fundamental ingredients for the reliability and inter-comparison of the results. We will therefore closely collaborate with the “Survey of Surveys” team (hereafter SoS, Tsantaki+21), who is applying various inter-calibration and machine learning techniques to create a homogeneous and robustly calibrated compilation of stellar parameters and abundance ratios of all the species involved in the chemical clocks described above, for several million stars. The SoS methodology is based on: 1) the official Gaia cross-match algorithm (Marrese+17,+19) to identify stars in common among surveys using the full Gaia astrometric information; 2) a robust statistical model to renormalize the survey uncertainties using stars with multiple observations in each survey and to recalibrate all surveys on a common system using the stars in common; 3) sets of high-quality external calibrators (Pancino+17) to bring the SoS homogenized system into a standardized and reproducible external scale. Within CHRONOS, we shall continue the work to homogenize the astrophysical parameters and abundances, providing an unprecedented dataset for homogeneous age estimates.
A pipeline for age estimates: the age machine
The development of a statistical pipeline allowing simultaneous fitting of different input data to obtain the probabilistic determination of stellar ages with the related uncertainties, taking into account also the different quality of the observations, is one of our main goals.
To do this we will profit from our experience in developing similar tools (SCEPtER: Valle+14; BASTA: Silva-Aguirre+22, PARAM: Rodrigues+17, AIMS: Rendle+19). The calibrated relationships between observed parameters and age can provide an age estimate for any star. The pipeline will accept a wide range of input data, from the basic ones as magnitudes and distance, to more complete data such as abundances, rotation periods, and magnetic activity information. It will perform also an isochrone fitting with our ‘best’ model set, including global seismic parameters, if available. Based on the input data, the pipeline will apply the calibrated relationships to provide age measurements with errors.
This pipeline together with other Population Synthesis tools will allow us to derive the chrono-chemo-dynamical map of the MW, that will be made available to the scientific community.
Scientific exploitation: addressing fundamental science cases
The availability of a chrono-chemo-kinematical map of the MW and of a homogeneous age scale allow to address fundamental open problems concerning Galaxy assembly and evolution and, as byproduct, the characterization of planet-host stars.
Galactic halo assembly: galactic haloes are thought to be the outcome of the merging processes occurring during the assembly of galaxies, when smaller systems accrete onto the main galactic progenitor (Searle & Zinn 78). While the halo only represents a small fraction (3-5%) of the total Galaxy mass, it is key to unravel its merger history. Although debris, including associated globular clusters (GCs), from accreted galaxies (Massari+19; Forbes 20), have been isolated using chemodynamical constraints, the small number of Galactic GCs with robust, and self-consistent age estimates, has hampered the possibility to characterize -in terms of star formation (SF) rate and mass- their parent galaxies (Kruijssen+19). In addition, at large Galactocentric distances [where the debris of the least massive members lie], GCs are the best tracers of accretion processes; yet robust and homogeneous age estimates are needed to identify and characterise the properties of their progenitors.
Galactic disk/bulge formation/evolution: the availability of accurate Gaia distanes allows us to apply the Color-Magnitude diagram (CMD)-fitting to retrieve the SF history (SFH) and the age-metallicity relation (AMR) of various MW disk fields (Gallart+19, Ruiz-Lara+20, Dal Tio+21). The CMD-fitting is a statistical technique to reproduce the observed CMD with synthetic ones (Cignoni+15,+19). This method will take advantage of the updated stellar model library, and the complete chemo-dynamical information collected within CHRONOS. This technique can be applied to a significant fraction of the thick/thin disk (with a resolution of about 1Gyr for the oldest populations within a few Kpc), and to several low extinction windows of the Galactic bulge, revealing the sequence of events that have shaped the geometry, and the chemo-kinematic properties of these Galactic structures (Grand+18).
These science cases can be investigated in the context of CHRONOS thanks to the specific know-how of the team members.
The structure of the Project Team
The team is organized in 3 Units of Research (UdR):
- Istituto Nazionale di Astrofisica (INAF): with researchers from the INAF-Astronomical Observatories of Abruzzo, Bologna and Catania. The INAF UdR members have a long standing, internationally recognized, expertise in key areas: from stellar evolution to observational surveys, from stellar activity to asteroseismology;
- University of Catania (UniCT): it includes researchers with a long-standing expertise in the field of angular mom entum evolution theory, gyrochronology and magnetic activity.
- University of Pisa (UniPI): it gathers people with a huge experience in the fields of stellar evolution, stellar populations, and statistical analysis of large observational datasets.
CHRONOS is organized in 4 working packages (WP) with complementary activities:
WP1 "The stellar framework” is aimed at developing an accurate theoretical scenario used to provide a stellar model-based age scale as precise as possible.
WP2 "The observational framework" is aimed at obtaining a sample of Open Clusterss with a very accurate characterization of their properties, to be used for the calibration of the various empirical chronometers. It has also the goal to make available the global observational dataset for field stars needed to retrieve the chrono-chemo-dynamical map of the MW.
WP3 "Empirical age indicators" is aimed at the calibration of the various empirical chronometers, as well as to improve the reliability of these age indicators.
WP4 “Applying the age machine” is aimed at exploiting the results obtained by the other WPs to estimate the age of large samples of stars in Galactic fields and clusters, and hence to unveil the chrono-chemo-dynamical map of the MW.
Scientific impact of the Project
This project has the following primary expected impacts:
- a relevant number of publications on peer-review journals of scientific results obtained mainly via European space- and ground-based data;
- high-level, homogenized observational data and updated stellar evolutionary models made available through appropriate archives such as SoS - http://gaiaportal.asdc.asi.it, Stellar models - http://basti-iac.oa-abruzzo.inaf.it;
- a complete characterization -including the age estimate- of the sample of MW stars analyzed in this project, fully available for the scientific community;>/li>
- tools developed for the advanced processing of data and determination of homogeneous stellar ages, available via Web-based interfaces.
The public release of the whole theoretical and observational framework developed in the context of CHRONOS, as well as the accurate chrono-chemo-dynamical map of the Milky Way, will constitute a long-term legacy of this project, even in a landscape of a growing number of new large ground- and space-based surveys.
Dissemination, exploitation of the results, and community impact
As already discussed, to provide our results and the developed software pipelines to the scientific community is one of the core goals of the project and our dissemination actions are designed with this aim in mind.
We have identified several thematic groups of potential users that will have a special interest in our products:
- Groups and institutions hosting the data archives that will be used in CHRONOS. The obtained stellar characterization can be seen as an enhanced metadata product to be ingested back to these data archives;
- Researchers involved in the preparation of future European and international ground- and space-based surveys;
- Researchers developing stellar evolution models and population synthesis tools;
- Researchers interested in stellar ages, as those working in the fields of exoplanets, SFH of both Galactic and extra-galactic resolved stellar systems, and even of unresolved extra-galactic stellar populations.
The actions planned to disseminate the project results are thus tailored to reach these research groups. These actions will aim at assisting the community to understanding our methods, stressing the impact and advantages of a homogeneous age scale, and encouraging the use of our pipeline in publications and follow-up researches. The main dissemination action, planned at the end of the project, is the public release of the pipeline. The publication of results in peer-review journals and their presentation at both international and national meetings will also be crucial to inform potentially interested colleagues about our project. Moreover, our team members will also present (intermediate) CHRONOS results at their own research institutions as well as at different ones, also making real-time advertisements at conferences in which they will be taking part.
Outreach activities and social impact
The main goal is to advertise the project and its results. We plan to use the results of our project to raise the interest of specific groups in astronomy and other research areas as (general) science, technology, and engineering. We have identified distinct audiences for our outreach activities: 1) Media - using both the press and internet channels; 2) High-school students and teachers, with the aim of raising the interest in the research areas mentioned before; 3) University students; 4) Governmental and funding agencies; 5) Public audience interested in astronomy and natural sciences.
Several team members have a longstanding experience in outreach activities both for the public in general and for university and high school students, and have been also actively involved in national and local projects – activities strongly affected by the pandemic stop - to popularize astronomy and to promote the choice of STEM disciplines at the University.
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