The stellar surface trated as a puzzle. The study: “PAStar: a model for stellar surface from the Sun to active stars” of A. Petralia (INAF – OAPA) appeared on A&A

In stars, a continuous tug-of-war unfolds between stellar plasma and the magnetic field. At stake is control over the local dynamics: the winner sets the rules that govern the local stellar properties. For instance, the photosphere of stars is generally quite homogeneous, except for granulation. However, in certain regions of the photosphere, the magnetic field can concentrate, intensify, and produce phenomena known as “magnetic activity”, including sunspots and faculae. Sunspots are areas where the magnetic field inhibits convective heating, making them cooler and darker than the rest of the photosphere. Faculae, on the other hand, are regions where deeper, hotter plasma is exposed to the sight, making them brighter than the surrounding photosphere.

While sunspots and faculae on the Sun can be observed in great detail, all other stars are too distant for us to resolve their photospheric details. Nevertheless, these magnetic phenomena have a significant impact on our ability to determine the properties of stars and their planets. For example, recent studies suggest that the presence of sunspots and faculae can substantially influence measurements of stellar age and radius. Regarding exoplanets, stellar magnetic activity can interfere with both planet detection (e.g., by producing signals that can mimic those caused by a planet’s orbital motion—so-called radial velocity signals) and the characterization of planetary properties and atmospheres during transits (i.e., when the planet passes across the stellar disk from our perspective).

In recent years, various methods have been developed to quantify the impact of magnetic activity on measurements of stellar and planetary properties. A team of astronomers at INAF – Osservatorio Astronomico di Palermo, led by astrophysicist A. Petralia, has developed the first model, called PAStar, where the photosphere, sunspots, and faculae are treated as separate components. These components can be combined in modular configurations to simulate any situation. Additionally, the model accounts for effects on each component due to stellar rotation, the inclination of the rotation axis relative to our line of sight, and geometric phenomena like “limb darkening”, which causes a non-uniform brightness distribution across the stellar disk.

The model has been successfully tested, particularly using solar observations, demonstrating that PAStar can provide accurate predictions of the latitude, longitude, and radius of sunspots. This research is detailed in the article “PAStar: a model for stellar surface from the Sun to active stars”, recently published in Astronomy & Astrophysics, with the collaboration of astrophysicists J. Maldonado and G. Micela from INAF – Osservatorio Astronomico di Palermo.

The cover image (click here to view it in full) shows the comparison between the best-fit model with two sunspots and real observations. The panels on the left represent the light curves (i.e., the variation of flux over time) both observed and simulated. The cross, in particular, indicates the values corresponding to the images on the right.