Activity indexes sub-WG

Context and objectives

Stellar activity is a crucial property of an exoplanet host. On one hand, it provides valuable information about the star; on the other hand, it can significantly affect our ability to detect planets around stars with moderate to high levels of activity.

Stellar activity, primarily driven by the presence of magnetic fields, encompasses various phenomena occurring on different timescales and in distinct regions of the stellar atmosphere, chromosphere, and corona. Among others, these include flares, granulation, starspots, faculae, and activity cycles. Moreover, stellar activity varies depending on the star's evolutionary stage and spectral type, and it can be observed across a wide range of wavelengths, from XUV to the infrared. For all these reasons, there is no single way to define the level of stellar activity or a universal diagnostic method applicable to all types of stars. However, photometric and spectroscopic observations can provide valuable insights into the main properties of the phenomena related to stellar activity.

In the research field of exoplanets, stellar activity represents a major limitation in the detection and characterization of planets. It can obscure or, conversely, mimic the signal of a planet, complicating the search for such companions. In the context of the future Ariel observations, stellar activity may also impact planetary spectra and lead to misinterpretations of planetary atmospheres

By using archival data or requesting new observations, the Activity Indexes sub Working Group is dedicated to the measurement of stellar activity indexes for the stars included in the Ariel Candidate Sample. These indexes are derived by analyzing spectral lines sensitive to stellar activity. Specifically, we focus on obtaining indexes related to the Calcium H&K lines (by using e.g. HARPS and HARPS-N data, see Danielski et al. 2022) or the Calcium IIIRT by exploiting the data from the Gaia Radial Velocity Spectrometer (see Lanzafame et al. 2023). Another key parameter useful for characterizing stars, which is also related to activity indexes, is the stellar rotation period. It can be determined through the analysis of photometric and spectroscopic time series, when available, or through theoretical estimates based on its correlation with the Calcium H&K index.

The final goal of the subWG is to provide a comprehensive characterisation of the host stars in synergy with the other subWG of the Stellar Characterisation WG.