A simple perspective on the mass-area relationship in molecular clouds

Despite over 30 yr of study, the mass-area relationship within and among clouds is still poorly understood both observationally and theoretically. Modern extinction data sets should have sufficient resolution and dynamic range to characterize this relationship for nearby molecular clouds, although recent papers using extinction data seem to yield different interpretations regarding the nature and universality of this aspect of cloud structure. In this paper we try to unify these various results and interpretations by accounting for the different ways cloud properties are measured and analysed. We interpret the mass-area relationship in terms of the column density distribution function and its possible variation within and among clouds. We quantitatively characterize regional variations in the column density probability distribution function (PDF). We show that structures both within and among clouds possess the same degree of 'universality', in that their PDF means do not systematically scale with structure size. Because of this, mass scales linearly with area.