Chemistry as a diagnostic of prestellar core geometry

Abstract

We present a new method for assessing the intrinsic 3D shape of prestellar cores from molecular column densities. We have employed hydrodynamic simulations of contracting, isothermal cores considering three intrinsic geometries: spherical, cylindrical/filamentary and disk-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 $\le$ 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 disk-like cores. Specifically, if a disk-like core is observed as a filamentary object in dust emission, then it will be observed as two parallel filaments in $\rm{N_2H^{+}}$; b) for cores with higher aspect ratios (i.e. 0.15 $\sim$ 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 ($\rm{CN}$) 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 $\sim$ 1), we have identified three molecules ($\rm{OH}$, $\rm{CO}$ and $\rm{H_2CO}$) 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.