A TURBULENT MODEL FOR THE INTERSTELLAR-MEDIUM .2. MAGNETIC-FIELDS AND ROTATION

We present results from two-dimensional numerical simulations of a supersonic turbulent flow with parameters characteristic of the interstellar medium at the 1 kpc scale in the plane of the Galactic disk, incorporating shear, thresholded and discrete star formation (SF), self-gravity, rotation, and magnetic fields. A test of the model in the linear regime supports the results of the linear theory presented by Elmegreen in 1991. At low shear, a weak azimuthal magnetic field stabilizes the medium by opposing collapse of radial perturbations, while a strong field is destabilizing by preventing Coriolis spin-up of azimuthal perturbations (magnetic braking). At high shear, azimuthal perturbations are sheared into the radial direction before they have time to collapse, and the magnetic field becomes stabilizing again. In the fully nonlinear turbulent regime, while some results of the linear theory persist, new effects also emerge. The production of turbulent density fluctuations appears to be affected by the magnetic field as in the linear regime: moderate field strengths cause a decrease in the time-integrated star formation rate, while larger values cause an increase. A result not predicted by the linear theory is that, for very large field strengths, a decrease in the integrated SFR obtains again, indicating a ''rigidization'' of the medium due to the magnetic field. Other exclusively nonlinear effects are the following: (1) Even though there is no dynamo in two dimensions, the simulations are able to maintain or increase their net magnetic energy in the presence of a seed uniform azimuthal component. (2) A well-defined power-law magnetic spectrum and an inverse magnetic cascade are observed in the simulations, indicating full MHD turbulence. Thus, magnetic field energy is generated in regions of SF and cascades up to the largest scales. (3) The field has a slight but noticeable tendency to be aligned with density features. This appears to be as much a consequence of the gas pushing on the magnetic field as due to constraints on gas motions because of the presence of the magnetic field. (4) A ''pressure-cooker'' effect is observed in which the magnetic field prevents H II regions from expanding freely, as in the recent results of Slavin and Cox. (5) The orientation of the large-scale azimuthal field appears to follow that of the large-scale Galactic shear. (6) A tendency to exhibit less filamentary structures at stronger values of the uniform component of the magnetic field is present in several magnetic runs. Possible mechanisms that may lead to this result are discussed. (7) For fiducial values of the parameters, the flow in general appears to be in rough equipartition between magnetic and kinetic energy. There is no clear domination of either the magnetic or the inertial forces. (8) A median value of the magnetic field strength within clouds is similar to 12 mu G, while for the intercloud medium a value of similar to 3 mu G is found. Maximum contrasts of up to a factor of similar to 10 are observed.