Our objectives

To optimise the science outcome of Ariel it is pivotal to know the fundamental properties of the host stars in the Ariel Mission Candidate Sample, their chemistry, their level of activity, their orbital properties, their galactic origins, their age.  

Our goal is to provide an accurate knowledge of such properties, well before the mission launch in 2029. This is essential to determine the final target list to be observed with Ariel.

Why is it important to study the host stars to gain a comprehensive understanding of the planets in its system?

The characterisation of planet-hosting star(s) is an important step for understanding the nature of their transiting planetary companion(s). On a general level, characterising the star is salient for the following reasons : 

• The determination of the planetary radius, mass, and bulk composition -- they are directly dependent by the accurate and precise determination of the stellar radius and mass. In turn, the inference of both stellar radius and mass usually hinges on the stellar atmospheric parameters through the use of stellar models.

• The exact identification of atmospheric features -- stellar variability, caused by the interaction between magnetic fields and turbulent plasma, is responsible for intrinsic variations of the stellar spectrum that can be confused with planetary features when occurring on transit (or phase-curve) timescales. It is therefore essential to acquire an exact knowledge of the stellar spectrum to at least the same level of the planetary signal, for accurately determining the chemical composition and molecular abundances in a planetary atmosphere. Furthermore, information on long-term stellar activity is important for studying the effect of the stellar irradiation onto the planet atmosphere and for constraining the evolution of the atmosphere itself.

• The investigation of planetary formation and migration processes  -- determining the precise stellar age and, by extension, of the planetary system is critical as it tells us how much time was available for the system to dynamically evolve. This, in turn, allows us to constrain the pathways that produced its current architecture. Knowing the age of the system provides the temporal dimension of the problem. In parallel, the atmospheric composition of the host star is itself a cipher key to decode the compositional signatures on planets left by their formation and migration histories."

• The knowledge of the galactic origin of the planetary systems -- protoplanetary discs formed in different galactic environments will present different properties, such as amount of refractory vs volatiles, or the level of irradiation. A disc located in a overdensity region would suffer higher stellar irradiation, and hence could present different chemical properties than a disc in a field region. Furthermore, their dissipation timescales will be very different, hinging for instance the formation of gas giants in the first case. Knowing the stellar kinematics, and hence where in the Galaxy the planetary systems formed, is extremely important to understand trends and correlations found between stellar and planetary properties, as well as having contraints on the planetary classes occurrence rates. 

 Building a public catalogue of homogeneously characterised  Ariel exoplanet host stars

For Ariel the concept of homogeneity is crucial: fundamental parameters of host stars are usually found in the literature as the result of a case-by-case analysis performed by different teams, resulting in an inhomogeneous census of stars with planets.

In order to push the state-of-the-art of the field forward, and prepare the ground for the success of the Ariel, the stellar parameters need to be homogeneous and self-consistent.  A sample of uniform and self-consistent stellar parameters will establish a robust reference frame that will enable us to perform comparative planetary studies for the thousand planets, and hence to shed light on their formation and evolution on a global scale. "