The project "Stellar GRAnulation in Photometric Data: Properties, Predictions
and an Analysis Tool for CHEOPS (GRAPPA)" is funded by the Austrian Research
Promotion Agency (FFG) under the ASAP13 scheme and as part of the Austrian
contribution to the CHEOPS mission.
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.
However, granulation processes on the host star contribute substantial correlated
noise ("Flicker"), largely limiting the attainable 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-million have been measured.
Within GRAPPA, we aim to characterize flicker noise at high temporal resolution
for a wide range of stellar types. We investigate the properties of flicker at
their full resolution, and develop strategies for its modelization. To do so, we
use currently existing observations of the sun and other stars and compare these
to theoretical simulations to create a predictive set of theoretical model
flicker light curves. Based on this understanding, we obtain realistic predictions
for CHEOPS, and use these to find the optimal data analysis approach for modelling
flicker-induced correlated noise. Within GRAPPA, 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.
Monika Lendl (PI)
Sophia Sulis (main project postdoc/engineer)
Space Research Institute, Austrian Academy of Sciences. Schmiedlstrasse 6, 8042 Graz, Austria.
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