On the existence of three-dimensional hydrostatic and magnetostatic equilibria of self-gravitating fluid bodies

Abstract

(Abridged) We develop an analytical spectral method to solve the equations of equilibrium for a self-gravitating, magnetized fluid body, under the only hypotheses that (a) the equation of state is isothermal, (b) the configuration is scale-free, and (c) the body is electrically neutral. All physical variables are represented as series of scalar and vector spherical harmonics of degree l and order m, and the equilibrium equations are reduced to a set of coupled quadratic algebraic equations for the expansion coefficients of the density and the magnetic vector potential. The method is general, and allows to recover previously known hydrostatic and magnetostatic solutions possessing axial symmetry. A linear perturbation analysis of the equations in spectral form show that these basic axisymmetric states, considered as a continuos sequence with the relative amount of magnetic support as control parameter, have in general no neighboring nonaxisymmetric equilibria. This result lends credence to a conjecture originally made by H. Grad and extends early results obtained by E. Parker to the case of self-gravitating magnetized bodies. The only allowed bifurcations of this sequence of axisymmetric equilibria are represented by distortions with dipole-like angular dependence (l=1) that can be continued into the nonlinear regime. These new configurations are either (i) azimuthally asymmetric (m=+-1) or (ii) azimuthally symmetric but without reflection symmetry with respect to the equatorial plane (m=0). To the extent that interstellar clouds can be represented as isolated magnetostatic equilibria, the results of this study suggest that the observed triaxial shapes of molecular cloud cores can be interpreted in terms of weakly damped Alfven oscillations about an equilibrium state.