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

The formation of stars and planetary systems is a complex phenomenon that relies on the interplay multiple physical processes. Nonetheless, it represents crucial stage for our understanding Universe, in particular conditions leading to key molecules (e.g. water) comets planets. Herschel observations demonstrated form gaseous filamentary structures which main constituent molecular hydrogen (H 2 ). Depending its nuclear spin H can be found two forms: ‘ortho’ with parallel spins ‘para’ where are anti-parallel. relative ratio among these isomers, ortho-to-para (OPR), plays role variety processes related thermodynamics star-forming gas fundamental chemistry affecting deuteration water clouds, commonly used determine origin Solar System bodies. Here, first time, we assess evolution OPR starting from warm neutral medium by means state-of-the-art 3D magnetohydrodynamic simulations turbulent clouds. Our results show clouds exhibit low (≪0.1) already at moderate densities (∼1000 cm −3 We also constrain cosmic-ray ionisation rate, finding 10 −16 s −1 lower limit required explain diffuse represent step forward star planet providing robust determination chemical initial both theoretical observational studies.