Understanding signatures of magnetic activity in broadband stellar observations

• Applicant: Alexander Shapiro, Ph.D.
• Subject Area: Astrophysics and Astronomy
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 520307354

Cool stars with outer convection zones show various manifestations of activity: chromospheric and X-ray emission, flares, spectroscopic and brightness variations. All these phenomena are essentially driven by magnetic fields emerging from below the stellar surface and affecting the structure of the stellar atmosphere. The interest in stellar activity is not limited to solar and stellar astrophysics. For example, stellar brightness variability is a limiting factor for detecting and characterising exoplanets with transit photometry, and its quantitative assessment is important for the upcoming PLATO mission. The magnetic jitter in radial velocity affects the spectroscopic detection of planets, while stellar wobbles caused by magnetic activity can impede the astrometric detection of planets (e.g. with the Gaia space observatory or anticipated TOLIMAN mission). Recent studies have also shown that magnetic activity can interfere with identifying the chemical composition of exoplanetary atmospheres with transmission spectroscopy. Quantifying such magnetic contamination of transmission spectroscopy is urgently needed for the interpretation of data from the James Webb Space Telescope In this context, the main goal of MAGicSTar is to understand and model signatures of stellar magnetic activity in broadband stellar observations, namely a) transit photometry, b) broadband transit spectroscopy, and c) astrometry. Until recently, the main hurdle in such modelling was the absence of reliable information about the brightness contrasts of magnetic features with respect to the quiet stellar regions. The situation has now changed due to the progress in 3D radiative-MHD simulations of stellar atmospheres. In particular, simulations with the MURaM code developed in the host institute of the applicant have reached a high degree of realism reproducing solar observations in great detail. The success of solar MURaM simulations initiated their extension to stars other than the Sun with the applicant and the applicant’s host institute simulating magnetic features on stars with various effective temperatures and metallicities. These simulations provide MAGicSTar with a key information needed for making a substantial step forward in modelling broadband signatures of stellar magnetic activity. They will first be utilized for modelling of stellar brightness variations observed by transit photometry missions (such as Kepler and TESS). In the next step, the model of stellar brightness variability will be extended to also calculate intrinsic stellar signals in transmission spectra and astrometric measurements. MAGicSTar will benefit both stellar and exoplanetary research by a) boosting our understanding of stellar magnetic activity; b) bringing us closer to effectively mitigating stellar signal in observations needed for a more accurate characterisation of exoplanets.

• DFG Programme: Research Grants
• International Connection: Australia, France
• Co-Investigators: Dr. Robert Cameron; Professor Dr. Stefan Dreizler; Professor Dr. Laurent Gizon
• Cooperation Partners: Professor Dr. Allan Sacha Brun; Dr. Ben Montet