Correcting EChO data for stellar activity by direct scaling of activity signals

Abstract

Stellar activity patterns are responsible for jitter effects that are observed at different timescales and amplitudes in the measurements currently obtained from photometric, spectroscopic and astrometric time series observations. These effects are usually considered just noise, and the lack of a characterization and correction strategy represents one of the main limitations for exoplanet sciences. Precise simulations of the stellar photosphere based on the most recent available models for main sequence stars can provide synthetic time series data. These may help to investigate the relation between activity jitters and stellar parameters when considering different active region patterns. Moreover, jitters can be analysed at different wavelength scales in order to design strategies to remove or minimize them from the passbands defined for specific instruments or space missions. In this work we present a model for a spotted rotating photosphere built from the contribution of a fine grid of surface elements, including all significant effects affecting the flux intensities and the wavelength of spectral features. The resulting time series data generated with this simulator are used in order to design strategies to minimize the effects of activity jitter from EChO infrared observations. An approach is presented to remove activity signals from the full infrared light curve by using the simultaneously obtained data in the visible range. Then, a similar strategy is used to correct data focused ontransit observations, and a methodology to remove the effects of non-occulted spots from transit depth measurements is presented. These procedures provide a correction of the transit data to a few times 10−5 for significantly active stars, which is within EChO noise standards.