Modelling the photosphere of active stars for planet detection and characterization

TL;DR

This paper introduces StarSim, a comprehensive tool for modeling stellar photospheres with active regions to improve exoplanet detection by characterizing and mitigating stellar activity jitter effects across different wavelengths.

Contribution

The work presents StarSim, a detailed simulation model of active stellar photospheres that accurately reproduces photometric and RV signals, aiding in the analysis and correction of stellar activity in exoplanet studies.

Findings

StarSim accurately models photometry and RVs of active stars.

Chromatic effects of faculae dominate for low-contrast spots.

Spot activity can mimic exoplanet atmospheric signals.

Abstract

Stellar activity patterns are responsible for jitter effects that are observed at different timescales and amplitudes. These effects are currently in the focus of many exoplanet search projects, since the lack of a well-defined characterization and correction strategy hampers the detection of the signals associated with small exoplanets. Accurate simulations of the stellar photosphere can provide synthetic time series data. These may help to investigate the relation between activity jitter and stellar parameters when considering different active region patterns. Moreover, jitters can be analysed at different wavelength scales in order to design strategies to remove or minimize them. In this work we present the StarSim tool, which is based on a model for a spotted rotating photosphere built from the integration of the spectral contribution of a fine grid of surface elements. The model includes all significant effects affecting the flux intensities and the wavelength of spectral features produced by active regions and planets. A specific application for the characterization and modelling of the spectral signature of active regions is considered, showing that the chromatic effects of faculae are dominant for low temperature contrasts of spots. Synthetic time series are modelled for HD 189733. Our algorithm reproduces both the photometry and the RVs to good precision, generally better than the studies published to date. We evaluate the RV signature of the activity in HD 189733 by exploring a grid of solutions from the photometry. We find that the use of RV data in the inverse problem could break degeneracies and allow for a better determination of some stellar and activity parameters. In addition, the effects of spots are studied for a set of simulated transit photometry, showing that these can introduce variations which are very similar to the signal of an atmosphere dominated by dust.