ABOUT THOT, Stellar Dating
Building AN unified tool for scientific stellar dating
THOT is a European Union Marie Skłodowska-Curie Action. Dr. Andrés Moya Bedón leads the project at the University of Birmingham under the supervision of Prof. William Chaplin.
THOT is framed in the field of stellar dating. It is planned as the first step in a more general research project making the most updated stellar dating techniques accessible to all the scientific community.
Knowledge of how stars are born, evolve and die underpins much of our understanding of the Universe. In this effort, stellar dating is critical. It helps to interpret observational data and to understand the underlying physics of very different astrophysical issues, such as stellar structure and evolution, exoplanetary systems, astrobiology, Galactic evolution, and eventually cosmology. Although there are a number of techniques in the literature for estimating stellar ages, none is suitable for every star, since there are dependencies with mass, evolutionary stage, chemical composition or unknown initial conditions. In addition, the uncertainties are very large in general. Each technique has its own problems, details and uncertainties 1, 2 and references therein; see also Table 1). Most methods are strongly model-dependent and there is a subset that can only provide relative but not absolute ages. There are no research groups in the world dating stars using all the techniques available. What we find is a number of research groups dating stars using a subset of these techniques, for which they are specialized. Therefore, there is a lack of a universal, absolute and self-consistent age scale and of a general tool for automated stellar dating. In addition to offer a missing and useful tool to the scientific community, it is critical and timely to develop it as current and future space missions such as Kepler/K2 and TESS (NASA), Gaia and Plato (ESA), JWST (NASA-ESA-CSA), etc., will provide a large amount of very accurate observational data (luminosity, rotation velocity, chemical composition, pulsational frequencies, etc.). Gaia will observe 1 billion stars, for example. The automated analysis of these data will be a must, and stellar dating is far from being automated nowadays.
The work its done at the University of Birmingham (UoB) supervised by Prof. William Chaplin and in close collaboration with Drs. A. Miglio and G. R. Davies and Msc. Federico Zuccarino.
1 Soderblom, D.R., Annu. Rev. Astron. Astrophys. 2010. 48:581–629
2 Barrado, D., EAS Publications Series, Volume 80-81, 2016, pp.115-175
Table 1: Summary of the stellar dating techniques in the literature
| Technique | Main observational data | Suitable for | Dependence on models | Absolute scale |
| Gyrochronology | Rotational period | Clusters | None | No |
| Stellar Activity | Activity index | Individual stars and cluster | None | No |
| Stellar Abundances | Relative abundances | Individual stars | Strong | No |
| Lithium (FGK) | Lithium content | Individual F, G and K stars | Strong | No |
| LDB | Lithium content | Young stellar clusters | Slight | Yes |
| Isochrones | Magnitude, colour index and metallicity | Individual stars and clusters | Strong | Yes |
| Asteroseismology | Magnitude, colour index, pulsational frequencies and metallicity | Individual pulsating stars | Strong | Yes |
| Nucleocosmochronology | Relative abundances of rare elements | Pop II and low metallicity stars | None | Yes |
| Kinematics | Space motions (astrometry and radial velocities) | Moving groups | None | No |
In view of the above,
the overarching aims of THOT are
Unify Techniques
Create Software
User Friendly Program
Packages for pipelines
Since a global update of all techniques cannot be achieved in two years, THOT will focus on a selection of those techniques having huge potential for application to data from new and upcoming missions.
Table 2: Summary of the stellar dating techniques in the literature. Planned updates in order.
| Technique | Update proposed | Why and benefits |
| Isochrones | To use of the new M-L relations and asteroseismic results | An accurate mass determination is a critical constraint for isochrone fitting. These relations and Gaia’s data will decrease mass uncertainty. The additional constraint of fitting stellar pulsations removes even more modeling uncertainties |
| Gyrochronology | To use of the new M-L relations and asteroseismic results | The equations describing rotational spin-down are mass dependent. The accurate knowledge of the stellar mass will help to understand the physical basics of gyrochronology. The additional constraint of stellar pulsations removes some modeling uncertainties |
| Asteroseismology | To use the Δν-ρ relation recently found for δ Scuti stars and the new M-L relations | The first relation, as in the solar-like case, is a hard constraint for models, increasing the accuracy of model fitting. The same happens with the M-L relation, since stellar mass is a main uncertainty source when interpreting pulsational modes. |
| Kinematics | To use asteroseismic results to recalibrate the results | Some of the most recent results are based on poor age estimations. The very accurate asteroseismic ageing of stars in the solar neighbourhood can drives to more accurate conclusions |
In Table 2, under the description “M-L relations”, what we want to use is empirical relationS for the estimation of the stellar massES (M) and radii (R). In THOT, we are going to revise all these relations, that is, linear combinations of different observables providing the most precise estimation possible of M and R.
We are going to gather the most updated data sample possible with accurate stellar masses, radii, effective temperatures, luminosities, metallicities and/or densities. These are usually obtained via asteroseismology, detached eclipsing binaries and/or interferometry. With this sample, we are going to analyze every possible linear combination of observables giving M or R. We will use the most precise and accurate ones.
In addition, we will use machine-learning techniques for the most efficient development of the computational tools proposed in the context of THOT.
RESEARCH OBJECTIVES
1) Gathering all the stellar aging techniques in the literature and update some of them (see Table 2 above). Since the requirements and accuracies of the different techniques are very heterogeneous, and the new and upcoming space mission’s data will affect some of them, this initial step is mandatory. This will also help Dr. Moya to resume his research line at the point he stopped.
2) Revising empirical relations for the estimation of M and R. Using these relations for improving age determinations. These relations have not been exploited for stellar dating and they are potentially very useful for improving the accuracy of asteroseismology, isochrones and gyrochronology techniques especially in the era of Gaia’s data.
3) Develop a program in the languages “R” and Apache Spark, taking advantage of their Big Data facilities, to provide automated stellar dating using all the techniques described in Table 2.
4) Develop a user-friendly Program (online & offline) for the use by non-specialists, and the inclusion of this code into data analysis pipelines, mainly focused on ESA space missions.