Session 20 - Chemistry & Physical Process in the ISM.
Display session, Wednesday, January 07
Exhibit Hall,

## [20.06] On the Density Distribution Function of Interstellar Gas

E. Vázquez-Semadeni (Instituto de Astronomía, UNAM), J. Scalo, D. Chappell (Astronomy Dept., U. Texas at Austin), T. Passot (Obs. Nice)

We investigate the form of the one-point probability distribution function (pdf) for the density field of the interstellar medium using numerical simulations that successively reduce the number of physical processes included. Two-dimensional simulations of self-gravitating supersonic MHD turbulence, of supersonic self-gravitating hydrodynamic turbulence, and of decaying Burgers turbulence, produce in all cases filamentary density structures and evidence for a power-law density pdf with logarithmic slope around -1.7. This suggests that the functional form of the pdf and the general filamentary morphology are the signature of the nonlinear advection operator.

These results do not support previous claims that the pdf is lognormal. A series of 1D simulations of forced supersonic polytropic turbulence is used to resolve the discrepancy. They suggest that the pdf is lognormal only for effective polytropic indices \gamma=1 (or nearly lognormal for \gamma\not=1 if the Mach number is sufficiently small), while power laws develop for densities larger than the mean if \gamma<1.