On the intrinsic shape of molecular clouds

Abstract

Knowledge of the intrinsic shape of molecular clouds and molecular cloud cores provides useful information on both the formation and collapse mechanisms associated with the objects and on the initial conditions for star formation. We compare the shapes of molecular clouds as determined by the Heyer et al. (2001, HCS01) and Brunt et al. (2003, BKP03) catalogues of 12 CO(J = 1 − 0) emission in the outer Galaxy. The catalogues are based upon different versions of the FCRAO Outer Galaxy Survey and also utilize different techniques for defining both the extent and shape of the clouds, which allows us to examine the effects of using different cloud definition and shape-fitting algorithms. In order to compare the two catalogues we use a subset of the cloud population where the clouds are well-defined in both data sets. We model the clouds in terms of triaxial ellipsoids and use a Monte Carlo technique to determine the best-fit intrinsic shape distribution which matches the observed axis ratio distributions. Our analysis shows that the observed shapes of molecular clouds can be best described in terms of an intrinsic distribution of triaxial ellipsoids that are intermediate between near-oblate and near-prolate ellipsoids. The lack of high axis ratio clouds seen in the HCS01 catalogue is shown to be an artifact of the cloud definition algorithm and not an intrinsic property of the molecular clouds.