Astronomy and Astrophysics – Astrophysics
Scientific paper
1996-07-09
Astronomy and Astrophysics
Astrophysics
20 pages, 12 Postscript figures. Uses aas2pp4.sty. Accepted in ApJ
Scientific paper
10.1086/178200
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.
Passot Thierry
Pouquet Annick
Vázquez-Semadeni Enrique
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