GMC Collisions As Triggers of Star Formation. IX. Chemical Evolution

TL;DR

This study uses 3D magnetohydrodynamics simulations to explore the chemical evolution and astrochemical signatures of giant molecular cloud collisions, revealing how such events influence molecular abundances, physical conditions, and star formation indicators.

Contribution

It provides detailed chemical and physical modeling of colliding versus non-colliding GMCs, highlighting the impact of collisions on molecular abundances and cloud properties, with implications for interpreting IRDC observations.

Findings

Colliding GMCs produce denser, warmer clumps with higher CO depletion.

Low cosmic ray ionization rates best match observed IRDC properties.

Chemical signatures differ significantly between colliding and non-colliding clouds.

Abstract

Collisions between giant molecular clouds (GMCs) have been proposed as a mechanism to trigger massive star and star cluster formation. To investigate the astrochemical signatures of such collisions, we carry out 3D magnetohydrodynamics simulations of colliding and non-colliding clouds exposed to a variety of cosmic ray ionization rates (CRIRs), $\zeta$, following chemical evolution including gas and ice-phase components. At the GMC scale, carbon starts mostly in $\rm{C^+}$, but then transitions into C, CO, followed by ice-phase CO and $\rm{CH_3OH}$ as dense, cooler filaments, clumps and cores form from the clouds. The oxygen budget is dominated by O, CO and water ice. In dense regions, we explore the gas phase CO depletion factor, $f_D$, that measures the extent of its freeze-out onto dust grains, including dependence on CRIR and observables of mass surface density and temperature. We also identify dense clumps and analyze their physical and chemical properties, including after synthetic line emission modeling, investigating metrics used in studies of infrared dark clouds (IRDCs), especially abundances of CO, $\rm HCO^+$ and $\rm N_2H^+$. For the colliding case, we find clumps have typical densities of $n_{\rm H}\sim10^5\:{\rm{cm}}^{-3}$ and temperatures of $\sim20\:$K, while those in non-colliding GMCs are cooler. Depending on $\zeta$ and GMC dynamical history, we find CO depletion factors of up to $f_D\sim10$, and abundances of HCO$^+\sim 10^{-9}$ to $10^{-8}$ and $\rm{N_2H^+}\sim10^{-11}$ to $10^{-10}$. Comparison with observed IRDC clumps indicates a preference for low CRIRs ($\sim10^{-18}\:{\rm{s}}^{-1}$) and a more quiescent (non-colliding), cooler and evolved chemodynamical history. We discuss the general implications of our results and their caveats for interpretation of molecular cloud observations.