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Description
The traditional technique to characterize the structure of molecular clouds is mapping their line emission. We aim to test and apply a sampling technique that can characterize the line emission from a molecular cloud more efficiently than mapping. We have sampled the molecular emission from the Perseus cloud using the H_2_ column density as a proxy. We have divided the cloud into 10 logarithmically- spaced column density bins, and we have selected 10 random positions from each bin. The resulting 100 cloud positions have been observed with the IRAM 30m telescope covering the 3mm-wavelength band and parts of the 2mm and 1mm bands. We focus our analysis on the eleven molecular species (plus isotopologs) detected toward most column density bins. In all cases, the line intensity is tightly correlated with the H_2_ column density. For the CO isotopologs, the correlation is relatively flat, while for most dense gas tracers, the correlation is approximately linear. To reproduce these trends, we have developed a cloud model in which most species have abundance profiles characterized by an outer photo-dissociation edge and an inner freeze-out drop. With this model we determine that the intensity behavior of the dense gas tracers arises from a combination of excitation effects and molecular freeze out, with some modulation from optical depth. The quasi-linear dependence of the dense-gas tracer emission with H_2_ column density makes the gas at low column densities dominate the cloud- integrated emission. It also makes this emission proportional to the cloud mass inside the photodissociation edge. Stratified random sampling is an efficient technique to characterize the emission from molecular clouds. Despite its complex appearance, the molecular emission from Perseus presents a relatively simple behavior that, from a limited comparison with other clouds, seems to reflect a general pattern.
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