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

We present results from two-dimensional numerical simulations of a sup ersonic turbulent flow with parameters characteristic of the interstel lar medium at the 1 kpc scale in the plane of the Galactic disk, incor porating shear, thresholded and discrete star formation (SF), self-gra vity, rotation, and magnetic fields. A test of the model in the linear regime supports the results of the linear theory presented by Elmegre en in 1991. At low shear, a weak azimuthal magnetic field stabilizes t he medium by opposing collapse of radial perturbations, while a strong field is destabilizing by preventing Coriolis spin-up of azimuthal pe rturbations (magnetic braking). At high shear, azimuthal perturbations are sheared into the radial direction before they have time to collap se, and the magnetic field becomes stabilizing again. In the fully non linear turbulent regime, while some results of the linear theory persi st, new effects also emerge. The production of turbulent density fluct uations appears to be affected by the magnetic field as in the linear regime: moderate field strengths cause a decrease in the time-integrat ed star formation rate, while larger values cause an increase. A resul t not predicted by the linear theory is that, for very large field str engths, a decrease in the integrated SFR obtains again, indicating a ' 'rigidization'' of the medium due to the magnetic field. Other exclusi vely nonlinear effects are the following: (1) Even though there is no dynamo in two dimensions, the simulations are able to maintain or incr ease their net magnetic energy in the presence of a seed uniform azimu thal component. (2) A well-defined power-law magnetic spectrum and an inverse magnetic cascade are observed in the simulations, indicating f ull MHD turbulence. Thus, magnetic field energy is generated in region s of SF and cascades up to the largest scales. (3) The field has a sli ght 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 w hich the magnetic field prevents H II regions from expanding freely, a s 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 p resent 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 magne tic and kinetic energy. There is no clear domination of either the mag netic or the inertial forces. (8) A median value of the magnetic field strength within clouds is similar to 12 mu G, while for the interclou d medium a value of similar to 3 mu G is found. Maximum contrasts of u p to a factor of similar to 10 are observed.