Rationale

PHOST : PHysics of Oscillating STars

What physics can we learn from oscillating stars?

Most of what we know of the universe comes from studying stars. From the birth and evolution of galaxies to the nature of planets, obtaining reliable results needs precise knowledge of the stellar outer parameters, as well as their internal structure and evolution. Until recently, this information only came from the observations and analysis of stellar light, by either spectroscopy or photometry. From these observations, we could derive the atmospheric temperature and pressure, the surface gravity, the magnetic field and the surface abundances of the chemical elements, by using model atmospheres. It was possible to measure the radius of nearby stars by using interferometry and we had access to surface rotation and activity from the study of spectral lines. However, the stellar internal structure could only be derived through evolutionary models.

The advent of helioseismology, in the 1970s, and asteroseismology, twenty years later, represented a revolution for stellar studies. The detection and analysis of stellar oscillation modes led to direct insight of the deep layers of the Sun and the stars. This gave access to their internal structure, depth of the convection zones, internal temperature, pressure and density, internal rotation, and more. Asteroseismology provided tools to distinguish hydrogen shell-burning from helium core burning red giant stars, as well as the evolution of their rotating cores. It also yielded precise values of the radii, masses, and ages of exoplanet-host stars, needed for a good determination of the parameters of the planets and a good characterization of their internal structure. Now, asteroseismic masses and ages for red giants, coupled to GAIA positions and space velocities, are fundamental to galactic archeology.

The study of the internal structure of the stars was initiated at the beginning of the 20^th century by Sir A.S. Eddington and collaborators. The equations needed to describe self-gravitating spheres were solved several decades later with the help of the first computers. This enabled one to build approximate stellar models, with no rotation, no magnetic fields, no mass loss, no internal motions other than dynamical convection. The stellar medium was introduced as a unique gas, with an average molecular mass and average opacity. These models were then considered as “standard”. Later on, the precise constraints brought by helioseismology and asterosismology led stellar physicists to improve these models considerably by adding a number of “non-standard” effects.

Studying and improving stellar physics is important for a better understanding of the stars themselves and their environment. Furthermore, stars represent laboratory sites for physical processes that cannot be tested experimentally on Earth. They help understanding basic physics, such as nuclear physics, particle physics, statistical physics, hydrodynamics, magnetic processes, atomic physics and opacities, and more. High performance computer networks presently available allow numerical simulations, which help to understand these physical processes. They are used in symbiosis with the most recent observations of stellar oscillations, for a better understanding of stellar internal structure and evolution, from pre-main-sequence T Tauri stars to the end states of White Dwarfs.

This conference honours the work of Professor Hiromoto Shibahashi, who devoted most of his scientific life to the study of oscillating stars. He was one of the co-authors of a textbook, "Nonradial Oscillations of Stars", published in the 1970s when the field of asteroseismology was in its infancy and had yet to be named. Over more than 40 years, Hiromoto Shibahashi has been in the forefront of both theory and observation of many related topics. This conference will celebrate his contributions by discussing how the latest research in the oscillations of stars is advancing our understanding of the physics of stars, as well as informing diverse fields from galactic archaeology to habitable exoplanets.