MAGicSTar

Cool stars with outer convective zones exhibit various manifestations of activity: chromospheric emission and X-ray radiation, flares, as well as changes in spectra and brightness. All of these phenomena are essentially caused by magnetic fields that emerge from the stellar interior and reach the stellar surface, thereby altering the structure of the stellar atmosphere.

Interest in stellar activity phenomena is by no means limited to stellar and solar astrophysics. For example, variations in stellar brightness represent a limiting factor in the detection and characterization of exoplanets using transit photometry, and the quantitative assessment of these fluctuations is essential for the upcoming PLATO mission. Magnetic fluctuations in radial velocity complicate the spectroscopic detection of exoplanets, and the flickering of stellar brightness hampers their astrometric discovery (for instance, with the help of the Gaia space telescope or the planned TOLIMAN mission).

Recent studies have also shown that magnetic activity can interfere with the characterization of the chemical composition of exoplanet atmospheres via transmission spectroscopy. Quantifying stellar contamination in transmission spectroscopy is of utmost importance for the interpretation of data from the James Webb Space Telescope.

In this context, the primary goal of MAGicSTar is the modeling of stellar magnetic activity in broadband observations of stars — specifically:
a) transit photometry,
b) broadband transit spectroscopy, and
c) astrometry.

Until recently, the greatest obstacle in modeling active regions was the lack of reliable information about their brightness contrasts compared to the magnetically inactive surroundings. This situation has now changed thanks to advances in three-dimensional magnetohydrodynamic simulations of radiative transfer. In particular, simulations using the MURaM code, developed in-house, have achieved a high degree of realism, enabling solar observations to be reproduced in great detail.

The significant success of the MURaM simulations has led to their extension — by the applicant and her home institute — to active regions on stars with different effective temperatures and metallicities. These simulations provide the latest insights, allowing MAGicSTar to make substantial progress in modeling magnetically active regions.

As a first step, this information will be used to model stellar brightness variations, which have been extensively recorded by transit photometry missions such as Kepler and TESS. In the next phase, the brightness variation model will be expanded to account for intrinsic stellar signals in transmission spectra and astrometric measurements.