Abstract

Recent developments in astrophysical spectropolarimetry have significantly expanded our understanding of stellar magnetism. Using modern instrumentation and data analysis methods it is becoming possible to detect and characterise surface magnetic fields not just for a few peculiar objects but for essentially all types of stars, ranging from massive OB stars to brown dwarfs. The intention of this chapter is to give a general overview of the stellar magnetic field diagnostic methods and to present results of the application of these methods to different classes of stars. First, the chapter discusses the physical processes that lead to appearance of magnetic signatures in stellar spectra. Based on this discussion, the most commonly used magnetic observables are introduced for the intensity and polarisation spectra. The methods of interpretation of magnetic measurements, starting with robust field detection techniques and leading to reconstruction of the physically realistic vector maps of surface magnetic fields, are discussed. Finally, the current knowledge about the incidence, geometries and the origin of magnetic fields in stars of different masses and ages is reviewed and the astrophysical significance of magnetism in the context of stellar structure and evolution is discussed.

Introduction

This chapter presents an introduction to the studies of stellar magnetic fields. We explore the questions of how magnetic fields can be detected on the stellar surfaces, what information about magnetic field topologies can be extracted from different observations, and what are the observed properties of magnetic fields in stars of different masses and ages. The origin of magnetism in different types of stars and its relation to other physical processes operating in stellar interiors and atmospheres are briefly discussed.

The following sections start by presenting the essential theoretical foundations of the stellar magnetic field diagnostic and modelling. We consider the Zeeman effect in spectral lines and information contained in the polarisation characteristics of stellar radiation. A connection between the local magnetic field parameters and morphology of the stellar polarisation profiles is established using both detailed numerical calculations and simplified analytical considerations. Based on this discussion, we introduce different methodologies suitable for the detection and characterisation of stellar magnetic fields.

Magnetic fields play an important role in many astrophysical processes. They are difficult to detect and characterize because often their properties have to be inferred through interpreting the polarization of the light. Magnetic fields are also challenging to model and understand. Magnetized plasmas behave following highly non-linear differential equations having no general solution, so that every astrophysical problem represents a special case to be studied independently. Hence, magnetic fields are often an inconvenient subject that is overlooked or simply neglected (the elephant in the room, as they are dubbed on posters in the XXV Canary Islands Winter School of Astrophysics). Such a difficulty burdens the research on magnetic fields, which has evolved to become a very technical subject, with many small disconnected communities studying specific aspects and details. The school tried to amend the situation by providing a unifying view of the subject. The students had a chance to understand the behavior of magnetic fields in all astrophysical contexts, from cosmology to the Sun, and from starbursts to AGNs. The school was planned to present a balanced yet complete review of our knowledge, with excursions into the unknown to point out present and future lines of research.

The subject of Cosmic Magnetic Fields was split into seven different topics: cosmic magnetic field essentials, solar magnetic fields, stellar magnetic fields, the role of magnetic fields on AGN feedback, magnetic fields in galaxies, magnetic fields in galaxy clusters and at larger scales, and primordial magnetic fields and magnetic fields in the early Universe. The corresponding lectures were delivered by seven well known and experienced scientists that have played key roles in the major advances of the field during the last years: F. Cattaneo, P. Judge, O. Kochukhov, R. Keppens, R. Beck, K. Dolag, and F. Finelli. Their lectures were recorded and are freely available at the IAC website: http://iactalks.iac.es/talks/serie/19. Together with the reviews included in the present volume, they form a unique resource for both students and professional researchers. They provide a global view of this very compartmentalized, yet fundamental, field of research.