The shapes of molecular cloud cores in Orion

We investigate the intrinsic shapes of starless cores in the Orion GMC, using the prestellar core sample of Nutter & Ward-Thompson (2007), which is based on submillimeter SCUBA data. We employ a maximum-likelihood method to reconstruct the intrinsic distribution of ellipsoid axial ratios from observations of the axial ratios of projected plane-of-the-sky core ellipses. We find that, independently of the details of the assumed functional form of the distribution, there is a strong preference for oblate cores of finite thickness. Cores with varying finite degrees of triaxiality are a better fit than purely axisymmetric cores although cores close to axisymmetry are not excluded by the data. The incidence of prolate starless cores in Orion is found to be very infrequent. We also test the consistency of the observed data with a uniform distribution of intrinsic shapes, where oblate and prolate cores of all degrees of triaxiality occur with equal probability. Such a distribution is excluded at the 0.1% level. These findings have important implications for theories of core formation within molecular clouds.