The structure, formation and evolution of low-mass stars and brown dwarfs

Abstract

The development of a self-consistent theoretical description of the internal structure of stars, and the consequent construction of models which trace evolutionary behaviour, represents one of the major achievements of twentieth-century astrophysics. Most studies have centred on intermediate-and high-mass stars, with little consideration of M dwarfs with masses below 0.6 M. In part, this re¯ects the availability of more precise observational constraints, and also greater analytic tractability in modelling higher-mass stars. Recently, however, the lower main sequence has attracted more attention, with a series of detailed models extending past the hydrogen-burning limit to the boundary between low-mass brown dwarfs and giant planets. The aim of the present chapter is to concentrate on issues relevant to the structure of low-mass stars. The following section provides an historical overview of the development of stellar structure theory, allowing modern analyses to be placed in the proper context, while the succeeding section outlines the general principles. Thorough discussion of stellar structure theory can be found in textbooks devoted speci®cally to that subject, including Clayton [C4], Goldberg and Scadron [G3], Kippenhahn and Weigert [K9] and Hansen and Kawaler [H1], while the classic monographs by Chandrasekhar [C3] and Schwarzschild [S7] remain strikingly relevant in many areas. 3.2 A BRIEF HISTORY The foundations of the modern theory of stellar structure were laid during the later stages of the nineteenth century in a series of analyses of the equilibrium con®guration of gaseous spheres. The ®rst such study ± by the American