Mitigating flicker noise in high-precision photometry

Abstract

Context. In photometry, the short-timescale stellar variability (“flicker”), such as that caused by granulation and solar-like oscillations, can reach amplitudes comparable to the transit depth of Earth-sized planets and is correlated over the typical transit timescales. It can introduce systematic errors on the inferred planetary parameters when a small number of transits are observed. Aims. The objective of this paper is to characterize the statistical properties of the flicker noise and quantify its impact on the inferred transit parameters. Methods. We used the extensive solar observations obtained with SoHO/VIRGO to characterize flicker noise. We simulated realistic transits across the solar disk using SDO/HMI data and used these to obtain transit light curves, which we used to estimate the errors made on the transit parameters due to the presence of real solar noise. We make these light curves publicly available. To extend the study to a wider parameter range, we derived the properties of flicker noise using Kepler observations and studied their dependence on stellar parameters. Finally, we predicted the limiting stellar apparent magnitude for which the properties of the flicker noise can be extracted using high-precision CHEOPS and PLATO observations. Results. Stellar granulation is a stochastic colored noise, and is stationary with respect to the stellar magnetic cycle. Both the flicker correlation timescales and amplitudes increase with the stellar mass and radius. If these correlations are not taken into account when fitting for the parameters of transiting exoplanets, this can bias the inferred parameters. In particular, we find errors of up to 10% on the ratio between the planetary and stellar radius (Rp∕Rs) for an Earth-sized planet orbiting a Sun-like star. Conclusions. Flicker will significantly affect the inferred parameters of transits observed at high precision with CHEOPS and PLATO for F and G stars. Dedicated modeling strategies need to be developed to accurately characterize both the star and the transiting exoplanets.

Magnetic Activity of F-, G-, and K-type Stars in the LAMOST–Kepler Field

“…One mechanism might be the suppression of granulation within the boundaries of a spot, which reduces the number of granules contributing to F 8 and thereby increases F 8 . The solar F 8 has been shown to be independent of the solar cycle (Bastien et al 2013;Sulis et al 2020), suggesting that activity levels up to solar levels do not affect F 8 . We have not attempted to model this possibility for greater activity levels in the present work.…”

“…Another motivation for understanding stellar flicker is to constrain the noise present in planetary transit measurements. Studies have shown that stellar granulation produces a non-negligible effect in the noise present during observations of planetary transits, introducing, e.g., uncertainty in planetary radius measurements of a few percent (Chiavassa et al 2017;Morris et al 2020), or even up to 10% (Sulis et al 2020), depending on the observation being modeled. One cause of this noise is that the granulation provides one of many deviations from a perfect black body, affecting signals especially during spectroscopic observations.…”

Analysis of Early Science observations with the CHaracterising ExOPlanets Satellite (CHEOPS) usingpycheops

“…The damping time scale for this process is τ = 2 Q/ω0 and the standard deviation of the process is σGP = √ S0 ω0 Q. The Dataset module includes a function to plot the fast Fourier transform (FFT) of the residuals from the best-fit transit or eclipse model in log-log space so that the user can look for a slope or peaks in the power spectrum due to stellar granulation or oscillations (Sulis et al 2020).…”

A Readily Implemented Atmosphere Sustainability Constraint for Terrestrial Exoplanets Orbiting Magnetically Active Stars

“…This versatile and highly reproducible analysis shows that space weather cannot be left out of considerations for planetary habitability in stellar systems. It carries both value and promise: consider the European Space Agency CHEOPS (Characterizing Exoplanet Satellite) mission, for example ( e.g., Sulis et al 2020). The mission is designed to characterize only selected, confirmed exoplanets, ranging from super-Earth to Neptune sizes, aiming toward studies that extend into their potential atmospheres.…”