2025A&A...695A.277T

Context. The ESA space mission Ariel requires bright sources that are stable at the level of 100 ppm over 6 hours in order to accurately measure exoplanet atmospheres through transmission spectroscopy. To ensure this, in-flight instrument calibration can be performed by observing stellar calibrators.
Aims. In this study, a stellar calibrator candidate list distributed over the sky is created and a flux variability analysis is performed to identify the best stellar calibrators for transit spectroscopy of exoplanet atmospheres with Ariel.
Methods. A starting candidate sample of 1937 solar-type stars is created using the all-sky surveys Two Micron All Sky Survey and Gaia. Using stellar light curves from the Transit Exoplanet Survey Satellite (TESS), the flux variability of each star is characterised by computing its Lomb-Scargle periodogram and reduced chi-squared. This enables the elimination of stars with detectable variability from the sample.
Results. Approximately 22.2% of stars from the starting sample pass the selection as potential calibrators. These do not all necessarily meet Ariel's stability requirement, although some will. No correlation between flux stability and stellar properties is found, as long as the correct value ranges for the parameters are chosen, like a surface temperature between 5000 and 6300 K. The only exception is stellar magnitude: Noise in TESS data increases as stars get dimmer, so, a high percentage of faint stars passes the selection since their variability is more likely hidden within the inherent TESS noise. Contrarily, stars brighter than 5 mag cannot be used as calibrators.
Conclusions. A list of 430 promising bright calibration targets distributed over the sky has been selected. These can potentially be used as stellar calibrators for the Ariel mission. Targets from this list will have to be further studied to determine which ones possess a flux stability better than 100 ppm over 6 hours.