A Turbulent Model for the Interstellar Medium. II. Magnetic Fields and Rotation

We present results from two-dimensional numerical simulations of a supersonic turbulent flow 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 of Elmegreen (1991). In the fully nonlinear turbulent regime, while some results of the linear theory persist, new effects also emerge. Some exclusively nonlinear effects are: a) Even though there is no dynamo in 2D, the simulations are able to maintain or increase their net magnetic energy in the presence of a seed uniform azimuthal component. b) 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. c) The field has a slight but noticeable tendency to be aligned with density features. d) The magnetic field prevents HII regions from expanding freely, as in the recent results of Slavin \& Cox (1993). e) A tendency to exhibit {\it less} filamentary structures at stronger values of the uniform component of the magnetic field is present in several magnetic runs. f) 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. g) A median value of the magnetic field strength within clouds is $\sim 12\mu$G, while for the intercloud medium a value of $\sim 3\mu$G is found. Maximum contrasts of up to a factor of $\sim 10$ are observed.Comment: Plain TeX file, 25 pages. Gzipped, tarred set of Tex file plus 17 figures and 3 tables (Postscript) available at ftp://kepler.astroscu.unam.mx/incoming/enro/papers/mhdgturb.tar.g

Highly Compressible MHD Turbulence and Gravitational Collapse

We investigate the properties of highly compressible turbulence and its ability to produce self-gravitating structures. The compressibility is parameterized by an effective polytropic exponent gama-eff. In the limit of small gama-eff, the density jump at shocks is shown to be of the order of e^{M^2}, and the production of vorticity by the nonlinear terms appears to be negligible. In the presence of self-gravity, we suggest that turbulence can produce bound structures for gama-eff < 2(1-1/n), where 'n' is the typical dimensionality of the turbulent compressions. We show, by means of numerical simulations, that, for sufficiently small gama-eff, small-scale turbulent density fluctuations eventually collapse even though the medium is globally stable. This result is preserved in the presence of a magnetic field for supercritical mass-to-flux ratios.Comment: 4 pages, 3 postscript figures. Latex, uses aipproc.sty Contribution to the Conference Proc. of the 7th Annual Astrophysics Conference in Maryland, STAR FORMATION, NEAR AND FAR, eds. Stephen S. Holt and Lee G. Mund

Influence of Cooling-Induced Compressibility on the Structure of Turbulent Flows and Gravitational Collapse

We investigate the properties of highly compressible turbulence, the compressibility arising from a small effective polytropic exponent $\gamma_e$ due to cooling. In the limit of small $\gamma_e$, the density jump at shocks is shown to be of the order of $e^{M^2}$. Without self-gravity, the density structures arising in the moderately compressible case consist mostly of patches separated by shocks and behaving like waves, while in the highly compressible case clearly defined long-lived object-like clouds emerge. When the forcing in the momentum equation is purely compressible, the rotational energy decays monotonically in time, indicating that the vortex-stretching term is not efficient in transferring energy to rotational modes. This property may be at the origin of the low amount of rotation found in interstellar clouds. Vorticity production is found to rely heavily on the presence of additional terms in the equations. In the presence of self-gravity, we suggest that turbulence can produce bound structures for $\gamma_e < 2(1-1/n)$, where $n$ is the typical dimensionality of the turbulent compressions. We support this result by means of numerical simulations in which, for sufficiently small $\gamma_e$, small-scale turbulent density fluctuations eventually collapse even though the medium is globally stable. This result is preserved in the presence of a magnetic field for supercritical mass-to-flux ratios. At larger polytropic exponents, turbulence alone is not capable of producing bound structures, and collapse can only occur when the medium is globally unstable. This mechanism is a plausible candidate for the differentiation between primordial and present-day stellar-cluster formation and for the low efficiency of star formation.Comment: 20 pages, 12 Postscript figures. Uses aas2pp4.sty. Accepted in Ap