On the probability distribution function of the mass surface density of molecular clouds. II

TL;DR

This paper analyzes the probability distribution function of mass surface density in molecular clouds, focusing on the effects of pressure, self-gravity, and cloud geometry, extending previous models to ensembles of structures.

Contribution

It applies an analytical model of density profiles to study the PDF of self-gravitating, pressurized structures and their ensembles, considering pressure variations and inclination effects.

Findings

The PDF shape depends on external pressure and overpressure.

Ensemble effects modify the overall PDF distribution.

Pressure ratio distributions influence the high-density tail.

Abstract

The probability distribution function (PDF) of the mass surface density of molecular clouds provides essential information about the structure of molecular cloud gas and condensed structures out of which stars may form. In general, the PDF shows two basic components: a broad distribution around the maximum with resemblance to a log-normal function, and a tail at high mass surface densities attributed to turbulence and self-gravity. In a previous paper, the PDF of condensed structures has been analyzed and an analytical formula presented based on a truncated radial density profile, $\rho(r) = \rho_c/(1+(r/r_0)^2)^{n/2}$ with central density $\rho_c$ and inner radius $r_0$, widely used in astrophysics as a generalization of physical density profiles. In this paper, the results are applied to analyze the PDF of self-gravitating, isothermal, pressurized, spherical (Bonnor-Ebert spheres) and cylindrical condensed structures with emphasis on the dependence of the PDF on the external pressure $p_{ext}$ and on the overpressure $q^{-1} =p_c /p_{ext}$, where $p_c$ is the central pressure. Apart from individual clouds, we also consider ensembles of spheres or cylinders, where effects caused by a variation of pressure ratio, a distribution of condensed cores within a turbulent gas, and (in case of cylinders) a distribution of inclination angles on the mean PDF are analyzed. The probability distribution of pressure ratios $q^{-1}$ is assumed to be given by $P(q^{-1}) \propto q^{-k_1}/(1+(q_0/q)^{\gamma})^{(k_1+k_2)/{\gamma}}$, where $k_1$, ${\gamma}$, $k_2$, and $q_0$ are fixed parameters.