Session 20 - Molecular Clouds, Dense Cores, and Protostars.
Display session, Monday, January 15
North Banquet Hall, Convention Center

## [20.02] Low-Column Density Clouds in Numerical Simulations of the Interstellar Medium

E. Vázquez-Semadeni, J. Ballesteros-Paredes, L. F. Rodr\'\iguez (IAUNAM)

We present preliminary results from a statistical study of clouds in two-dimensional numerical simulations of the interstellar medium (ISM). Clouds are defined as connected sets of pixels above an arbitrary density threshold. Similarly to real interstellar clouds, the clouds in the simulations exhibit a differential mass spectrum dN(M)/dM \sim M^-1.5 and a velocity dispersion-size relation \Delta v \sim l^0.4, although with correlation coefficients of only 0.6--0.7. The maximum-sized clouds at each velocity dispersion seem to be better described by \Delta v \sim l^0.5. However, the clouds do not follow a unique density-size relation. Instead, at a given mean density, clouds span a wide range of sizes from the smallest resolved scale up to a maximum given by a Larson-type relation l_max \sim \rho^-1, implying non-constant column densities. These results suggest that the Larson relations define the clouds of maximum size at a given mean density or velocity dispersion that can exist before collapsing gravitationally. However, low-column density clouds out of balance between gravity and velocity dispersion are apparently not uncommon, probably being transients induced by turbulence. Such clouds might tend to be missed observationally because of their low column density, as has been suggested by Scalo. Finally, we compare these results with recent observational data. We suggest that a result by Wood and collaborators in which nearly constant column densities for dark clouds are inferred from IRAS 100-\mum images may be an artifact of a temperature gradient within the clouds induced by external radiative heating.