Density profile of a self-gravitating polytropic turbulent fluid in the context of ensembles of molecular clouds

We obtain an equation for the density profile in a self-gravitating polytropic spherically symmetric turbulent fluid with an equation of state p(gas) proportional to rho(Gamma). This is done in the framework of ensembles of molecular clouds represented by single abstract objects as introduced by Donkov et al. The adopted physical picture is appropriate to describe the conditions near to the cloud core where the equation of state changes from isothermal (in the outer cloud layers) with Gamma = 1 to one of 'hard polytrope' with exponent Gamma > 1. On the assumption of steady state, as the accreting matter passes through all spatial scales, we show that the total energy per unit mass is an invariant with respect to the fluid flow. The obtained equation reproduces the Bernoulli equation for the proposed model and describes the balance of the kinetic, thermal, and gravitational energy of a fluid element. We propose as well a method to obtain approximate solutions in a power-law form which results in four solutions corresponding to different density profiles, polytropic exponents, and energy balance equations for a fluid element. One of them, a density profile with slope -3 and polytropic exponent Gamma = 4/3, matches with observations and numerical works and, in particular, leads to a second power-law tail of the density distribution function in dense, self-gravitating cloud regions.

Automated summary

The article presents an equation for the density profile in a self-gravitating polytropic spherically symmetric turbulent fluid, along with a method for obtaining approximate solutions in a power-law form. The physical conditions near the cloud core and the invariant total energy per unit mass in steady state are discussed, along with the balance of kinetic, thermal, and gravitational energy of a fluid element. Different density profiles, polytropic exponents, and energy balance equations for a fluid element are explored, with one solution matching observations and numerical works, particularly in dense, self-gravitating cloud regions.