Chemistry as a diagnostic of prestellar core geometry
  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 $\Delta$ that quantifies whether a
molecular column density profile is centrally peaked, depressed or flat. We
have identified one molecule ($\rm{CN}$) for which $\Delta$ 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.
