Projection-angle effects when “observing” a turbulent magnetized collapsing molecular cloud

I. Chemistry and line transfer

¹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
² Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada
³ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George, St., Toronto, ON M5S 3H8, Canada
⁴ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁵ Australian Research Council Centre of Excellence in All Sky Astrophysics (ASTRO3D), Canberra, ACT 2611, Australia

^★ Corresponding author; aris.tritsis@epfl.ch

Received: 27 August 2024
Accepted: 10 February 2025

Abstract

Context. Most of our knowledge regarding molecular clouds and the early stages of star formation stems from molecular spectral-line observations. However, the various chemical and radiative-transfer effects, in combination with projection effects, can lead to a distorted view of molecular clouds and complicate the interpretation of observations.

Aims. Our objective is to simultaneously study all of these effects by creating synthetic spectral-line observations based on chemo- dynamical simulations of a collapsing molecular cloud.

Methods. We performed a three-dimensional ideal magnetohydrodynamic simulation of a supercritical turbulent collapsing molecular cloud where the dynamical evolution was coupled to a nonequilibrium gas-grain chemical network consisting of 115 species, the evolution of which was governed by >1600 chemical reactions. We post-processed this simulation with a multilevel nonlocal thermodynamic equilibrium radiative-transfer code to produce synthetic position-position-velocity data cubes of the CO, HCO+, HCN, and N₂H⁺ (J = 1 → 0) transitions under various projection angles with respect to the mean component of the magnetic field. Synthetic polarization maps are presented in a companion paper.

Results. We find that the chemical abundances of various species in our simulated cloud tend to be over-predicted in comparison to observationally derived abundances and attribute this discrepancy to the fact that the cloud collapses rapidly and therefore the various species do not have enough time to deplete onto dust grains. This suggests that our initial conditions may not correspond to the initial conditions of real molecular clouds and cores. We show that the projection angle has a notable effect on the moment maps of the species for which we produced synthetic observations. Specifically, the integrated emission and velocity dispersion of CO, HCO⁺ and HCN are higher when the cloud is observed “face on” compared to “edge on,” whereas column density maps exhibit an opposite trend. Finally, we show that only N₂H⁺ is an accurate tracer of the column density of the cloud across all projection angles studied.