Structures in a class of magnetized scale-free discs |
| |
| Authors: | Yue Shen Xin Liu Yu-Qing Lou |
| |
| Institution: | Physics Department and Tsinghua Centre for Astrophysics (THCA), Tsinghua University, Beijing 100084, China;National Astronomical Observatories, Chinese Academy of Sciences, A20, Datun Road, Beijing 100012, China;Department of Astronomy and Astrophysics, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA |
| |
| Abstract: | We construct analytically stationary global configurations for both aligned and logarithmic spiral coplanar magnetohydrodynamics (MHD) perturbations in an axisymmetric background MHD disc with a power-law surface mass density Σ0∝ r ?α , a coplanar azimuthal magnetic field B 0∝ r ?γ , a consistent self-gravity and a power-law rotation curve v 0∝ r ?β , where v 0 is the linear azimuthal gas rotation speed. The barotropic equation of state Π∝Σ n is adopted for both MHD background equilibrium and coplanar MHD perturbations where Π is the vertically integrated pressure and n is the barotropic index. For a scale-free background MHD equilibrium, a relation exists among α, β, γ and n such that only one parameter (e.g. β) is independent. For a linear axisymmetric stability analysis, we provide global criteria in various parameter regimes. For non-axisymmetric aligned and logarithmic spiral cases, two branches of perturbation modes (i.e. fast and slow MHD density waves) can be derived once β is specified. To complement the magnetized singular isothermal disc analysis of Lou, we extend the analysis to a wider range of ?1/4 < β < 1/2 . As an illustrative example, we discuss specifically the β= 1/4 case when the background magnetic field is force-free. Angular momentum conservation for coplanar MHD perturbations and other relevant aspects of our approach are discussed. |
| |
| Keywords: | MHD stars: formation ISM: magnetic fields galaxies: kinematics and dynamics galaxies: spiral galaxies: structure |
|
|
