On the low ortho-to-para H2 ratio in star-forming filaments

TL;DR

This study uses advanced simulations to show that star-forming filaments have a very low ortho-to-para H2 ratio early in their evolution, influencing star and planet formation chemistry.

Contribution

First to assess the evolution of the H2 ortho-to-para ratio from the warm neutral medium using 3D magneto-hydrodynamic simulations of turbulent molecular clouds.

Findings

Star-forming clouds exhibit a low OPR (<0.1) at moderate densities (~1000 cm$^{-3}$).

A cosmic ray ionization rate of at least 10^{-16} s^{-1} is needed to match diffuse cloud observations.

Results provide initial chemical conditions crucial for star and planet formation models.

Abstract

The formation of stars and planetary systems is a complex phenomenon, which relies on the interplay of multiple physical processes. Nonetheless, it represents a crucial stage for our understanding of the Universe, and in particular of the conditions leading to the formation of key molecules (e.g. water) on comets and planets. Herschel observations demonstrated that stars form out of gaseous filamentary structures in which the main constituent is molecular hydrogen (H$_2$). Depending on its nuclear spin H$_2$ can be found in two forms: `ortho' with parallel spins and `para' where the spins are anti-parallel. The relative ratio among these isomers, i.e. the ortho-to-para ratio (OPR), plays a crucial role in a variety of processes related to the thermodynamics of star-forming gas and to the fundamental chemistry affecting the deuteration of water in molecular clouds, commonly used to determine the origin of water in Solar System's bodies. Here, for the first time, we assess the evolution of the OPR starting from the warm neutral medium, by means of state-of-the-art three-dimensional magneto-hydrodynamic simulations of turbulent molecular clouds. Our results show that star-forming clouds exhibit a low OPR ($\ll 0.1$) already at moderate densities ($\sim$1000 cm$^{-3}$). We also constrain the cosmic rays ionisation rate, finding that $10^{-16}\,\rm s^{-1}$ is the lower limit required to explain the observations of diffuse clouds. Our results represent a step forward in the understanding of the star and planet formation process providing a robust determination of the chemical initial conditions for both theoretical and observational studies.