Why do starless cores appear more flattened than protostellar cores?

Abstract

We evaluate the intrinsic three-dimensional shapes of molecular cores, by analysing their projected shapes. We use the recent catalogue of molecular line observations of Jijina et al. and model the data by the method originally devised for elliptical galaxies. Our analysis broadly supports the conclusion of Jones et al. that molecular cores are better represented by triaxial intrinsic shapes (ellipsoids) than biaxial intrinsic shapes (spheroids). However, we find that the best fit to all of the data is obtained with more extreme axial ratios (1:0.8:0.4) than those derived by Jones et al. More surprisingly, we find that starless cores have more extreme axial ratios than protostellar cores — starless cores appear more ‘flattened’. This is the opposite of what would be expected from modelling the freefall collapse of triaxial ellipsoids. The collapse of starless cores would be expected to proceed most swiftly along the shortest axis — as has been predicted by every modeller since Zel'dovich — which should produce more flattened cores around protostars, the opposite of what is seen.