Basic elements of the theory of accretion

The paper reviews what we consider to be the essential aspects of the theory of accretion. A general overview is given in Section I. The treatments of accretion of cold gas onto a moving star by Hoyle and Lyttleton and of spherical adiabatic accretion by Bondi are summarized in Section II. The formation of spectra in accretion flows onto neutron stars and white dwarfs is described in Section III, with brief mention of the effects of a magnetic field anchored to the star. Spherical accretion onto black holes is treated in Section IV; in particular we underline the differences between classes of models leading to different efficiencies and spectra. We stress the role of a magnetic field entangled in the infalling plasma and the importance of the Comptonization process. The effects of radiation pressure in spherical accretion are then discussed. Thin accretion disks in the standard α approximation are introduced in Section V. The stability for thermal and viscous processes are examined and the bivalued region of the SIGMA, M dot ^ plot is discussed in view of a possible limit cycle behavior. Thick disks and their stability are briefly treated in Section VI. Section VII offers threads between theory and observations mentioning the astrophysical systems where accretion plays a major role (cataclysmic variables, X-ray binaries, Active Galactic Nuclei) pointing out successes and weaknesses of the present models.