Chemistry as a diagnostic of prestellar core geometry

TL;DR

This paper introduces a novel method to determine the 3D shape of prestellar cores using molecular emission maps and chemistry simulations, enabling shape inference from observational data.

Contribution

The study combines hydrodynamic simulations with non-equilibrium chemistry to identify molecular signatures that reveal core geometry, advancing core shape diagnostics.

Findings

Molecular emission maps distinguish cylindrical from disk-like cores at high elongation.

The metric Δ using CN probes core geometry for moderate aspect ratios.

OH, CO, and H2CO profiles help determine shape in nearly circular cores.

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 $\Delta$ 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.