Ariel Tier 3 Observations: An Opportunity to Explore both Stellar and Planetary Variability through Broad Wavelength Time-domain Spectroscopy

Abstract

Ariel's Tier 3 observing strategy offers a unique opportunity to explore planetary and stellar variability in the spectral and time domain. However, without the correct toolkit, temporal variability of the host star may complicate combined analysesof observations obtained at different epochs. We investigate the impact of stellar variability on Ariel Tier 3 observations and establish three different mitigation approaches of increasing physical complexity. Using simulated repeated observations of a favourable system contaminated by different spot and facula configurations, we compare a naive, unmodified stacking approach, separate analysis of the infrared region through progressive removal of the most strongly contaminated optical wavelengths, and the correction of individual visits using combined star-planet retrievals. We find that stacking inconsistent visits can produce substantial biases in the retrieved planetary properties under strong stellar variability, although a single activity correction can still recover the planetary parameters reasonably well. Applying progressive optical data cuts substantially improves the accuracy of retrievals up to a point, without requiring any in-retrieval correction and causing only a modest loss of precision. Correcting individual visits prior to stacking produces more consistent spectra and generally smaller uncertainties, but requires greater computational effort. These results demonstrate that stellar variability should be considered when interpreting repeated Ariel observations, but need not preclude atmospheric characterisation of active systems. A flexible suite of mitigation approaches, supported by preparatory and contemporaneous stellar monitoring, can allow the treatment of stellar variability to be adapted to individual targets while maintaining the efficiency required to maximise Ariel's scientific return.