Chemistry as a diagnostic of prestellar core geometry

Abstract

We present a newmethod for assessing the intrinsic 3D shape of prestellar cores frommolecular column densities. We have employed hydrodynamic simulations of contracting, isothermal cores considering three intrinsic geometries: spherical, cylindrical/filamentary and disc-like. We have coupled our hydrodynamic simulations with non-equilibrium chemistry.We find that (a) when cores are observed very elongated (i.e. for aspect ratios ≤0.15) the intrinsic 3D geometry can be probed by their 2D molecular emission maps, since these exhibit significant qualitative morphological differences between cylindrical and disc-like cores. Specifically, if a disc-like core is observed as a filamentary object in dust emission, then it will be observed as two parallel filaments in N2H+; (b) for cores with higher aspect ratios (i.e. 0.15-0.9) we define a metric δ that quantifies whether a molecular column density profile is centrally peaked, depressed or flat.We have identified one molecule (CN) for which δ as a function of the aspect ratio probes the 3D geometry of the core; and (c) for cores with almost circular projections (i.e. for aspect ratios ~1), we have identified three molecules (OH, CO and H2CO) that can be used to probe the intrinsic 3D shape by close inspection of their molecular column density radial profiles. We alter the temperature and the cosmic ray ionization rate and demonstrate that our method is robust against the choice of parameters.