On the structure of molecular clouds
  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.
