H₂ distribution during the formation of multiphase molecular clouds

¹ Laboratoire de radioastronomie, LERMA, Observatoire de Paris, École Normale Supérieure, PSL Research University, CNRS, UMR 8112, 75005 Paris, France
e-mail: valeska.valdivia@lra.ens.fr
² Laboratoire AIM, Paris-Saclay, CEA/IRFU/SAp – CNRS – Université Paris Diderot, 91191 Gif-sur-Yvette Cedex, France
e-mail: patrick.hennebelle@lra.ens.fr
³ Sorbonne Universités, UPMC Université Paris 06, UMR 8112, LERMA, 75005 Paris, France

Received: 8 September 2015
Accepted: 16 December 2015

Abstract

Context. H₂ is the simplest and the most abundant molecule in the interstellar medium (ISM), and its formation precedes the formation of other molecules.

Aims. Understanding the dynamical influence of the environment and the interplay between the thermal processes related to the formation and destruction of H₂ and the structure of the cloud is mandatory to understand correctly the observations of H₂.

Methods. We performed high-resolution magnetohydrodynamical colliding-flow simulations with the adaptive mesh refinement code RAMSES in which the physics of H₂ has been included. We compared the simulation results with various observations of the H₂ molecule, including the column densities of excited rotational levels.

Results. As a result of a combination of thermal pressure, ram pressure, and gravity, the clouds produced at the converging point of HI streams are highly inhomogeneous. H₂ molecules quickly form in relatively dense clumps and spread into the diffuse interclump gas. This in particular leads to the existence of significant abundances of H₂ in the diffuse and warm gas that lies in between clumps. Simulations and observations show similar trends, especially for the HI-to-H₂ transition (H₂ fraction vs. total hydrogen column density). Moreover, the abundances of excited rotational levels, calculated at equilibrium in the simulations, turn out to be very similar to the observed abundances inferred from FUSE results. This is a direct consequence of the presence of the H₂ enriched diffuse and warm gas.

Conclusions. Our simulations, which self-consistently form molecular clouds out of the diffuse atomic gas, show that H₂ rapidly forms in the dense clumps and, due to the complex structure of molecular clouds, quickly spreads at lower densities. Consequently, a significant fraction of warm H₂ exists in the low-density gas. This warm H₂ leads to column densities of excited rotational levels close to the observed ones and probably reveals the complex intermix between the warm and cold gas in molecular clouds. This suggests that the two-phase structure of molecular clouds is an essential ingredient for fully understanding molecular hydrogen in these objects.