On the structure of molecular clouds

TL;DR

The paper demonstrates that the observed Larson scaling relation in molecular clouds is an artifact of observational thresholds and does not reflect their true structure or universal properties.

Contribution

It reveals that the Larson scaling relation arises from observational biases and clarifies the actual structural implications for molecular clouds based on the column density probability distribution.

Findings

Larson relation is an observational artifact.

Thresholds influence perceived cloud structure.

Mass-radius relation depends on Npdf properties.

Abstract

We show that the inter-cloud Larson scaling relation between mean volume density and size $\rho\propto R^{-1}$, which in turn implies that mass $M\propto R^2$, or that the column density $N$ is constant, is an artifact of the observational methods used. Specifically, setting the column density threshold near or above the peak of the column density probability distribution function Npdf ($N\sim 10^{21}$ cm\alamenos 2) produces the Larson scaling as long as the Npdf decreases rapidly at higher column densities. We argue that the physical reasons behind local clouds to have this behavior are that (1) this peak column density is near the value required to shield CO from photodissociation in the solar neighborhood, and (2) gas at higher column densities is rare because it is susceptible to gravitational collapse into much smaller structures in specific small regions of the cloud. Similarly, we also use previous results to show that if instead a threshold is set for the volume density, the density will appear to be constant, implying thus that $M \propto R^3$. Thus, the Larson scaling relation does not provide much information on the structure of molecular clouds, and does not imply either that clouds are in Virial equilibrium, or have a universal structure. We also show that the slope of the $M-R$ curve for a single cloud, which transitions from near-to-flat values for large radii to $\alpha=2$ as a limiting case for small radii, depends on the properties of the Npdf.