Stellar Granulation in Photometric Data: Properties, Predictions an an Analysis Tool for CHEOPS

Short Description

Starting point / motivation

Extrasolar planets that transit their host stars are key objects for the study of planets and planetary systems. As the planet passes in front of its host star, the observed flux drop reveals the planetary radius and bulk density. High-precision transit light curves (time sequence flux measurements) allow us to obtain a wealth of information on the planet, its interior structure, atmospheric composition, and the planetary system's dynamics.

The CHEOPS satellite (launch 2018) is the first ESA mission dedicated fully to the study of extrasolar planets and will obtain highest-precision transit light curves for many bright planetary systems. It will be followed by PLATO 2.0 (launch 2024), which will use the transit method to create an inventory of small transiting planets orbiting bright stars. At the precision level reached by CHEOPS, the mission is designed to be capable of detecting the minuscule signatures of planets the size of Earth, transiting Solar analogues.

Contents and goals

However, granulation processes on the host star contribute substantial correlated noise (“Flicker”), largely limiting the attainable photometric precision. From an exploratory project, we measure a flicker amplitude of 40 parts-per-million on the Sun, which amounts to roughly 50% of the transit depth of an Earth transiting a Solar analogue. For other stars, amplitudes of up to 350 parts-per-milion have been measured.

So far, no study has been carried out in order to characterize flicker noise at high temporal resolution for a wide range of stellar types. For the full success of CHEOPS, the properties of flicker must be known at their full resolution, and strategies for its modelization developed. This is the goal of the project proposed here.

Methodische Vorgehensweise

We will study the properties of flicker across stellar types, quantify its impact on the achieved precision, create an adequate data analysis procedure, and identify the optimal observing strategy accounting for flicker.

We will proceed by first assessing the properties of flicker from currently existing observations from the Solar SDO instrument, and the NASA Kepler satellite. We will then compare these results to improve theoretical models of granulation, and create a predictive set of theoretical model flicker light curves. Combining these light curves with simulated CHEOPS data, we will obtain realistic predictions for CHEOPS, and use these to find the optimal data analysis approach for modelling flicker-induced correlated noise.

We will create a dedicated software tool to be included in the CHEOPS data analysis toolbox. Finally, we will estimate the effects of flicker on the performance of CHEOPS, and make recommendations for an efficient observing strategy.

Expected results

The nature of this project is timely and critical for the full exploitation of CHEOPS data, and will place the proposing team in the front line of scientific research with CHEOPS. Further, the characterization of flicker across stellar types and the techniques developed to account for it will have a lasting impact thanks to their applicability on the upcoming ESA M-class PLATO 2.0 mission.

Project Partners


Österreichische Akademie der Wissenschaften

Contact Address

Austrian Academy of Sciences
Space Research Institute
Dr. Monika Lendl
Schmiedlstraße 6
A-8042 Graz