H2 distribution during formation of multiphase molecular clouds

Abstract

H2 is the simplest and the most abundant molecule in the ISM, and its formation precedes the formation of other molecules. 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. We perform high resolution MHD colliding flow simulations with the AMR code RAMSES in which the physics of H2 has been included. We compare the simulation results with various observations including the column densities of excited rotational levels. Due to 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, specially for the HI-to-H2 transition. The abundances of excited rotational levels, calculated at equilibrium in the simulations are 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. Our simulations 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 likely revealing the complex intermix between the warm and the cold gas in molecular clouds. This suggests that the 2-phase structure of molecular clouds is an essential ingredient to fully understand molecular hydrogen in these objects.

16 pages, 19 figures. Accepted for publication in A&A

References in corpus (22)

• The Statistics of Supersonic Isothermal Turbulence
• The Density Probability Distribution in Compressible Isothermal Turbulence: Solenoidal versus Compressive Forcing
• A High Order Godunov Scheme with Constrained Transport and Adaptive Mesh Refinement for Astrophysical MHD
• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• Simulating the formation of molecular clouds. II. Rapid formation from turbulent initial conditions
• On the Density Distribution in Star-forming Interstellar Clouds
• Environmental Dependence of the Kennicutt-Schmidt Relation in Galaxies
• From the warm magnetized atomic medium to molecular clouds
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows
• Cooling, Gravity and Geometry: Flow-driven Massive Core Formation
• Models of turbulent dissipation regions in the diffuse interstellar medium
• Spitzer observations of HH54 and HH7-11: mapping the H2 ortho-to-para ratio in shocked molecular gas
• Surface chemistry in the Interstellar Medium II. ${\mathrm{H}}_2$ formation on dust with random temperature fluctuations
• Fragmentation of Shocked Flows: Gravity, Turbulence and Cooling
• Effects of turbulent diffusion on the chemistry of diffuse clouds
• Modeling of diffuse molecular gas applied to HD 102065 observations
• First spectrally-resolved H${}_2$ observations towards HH 54 / Low H${}_2$O abundance in shocks
• A fast tree-based method for estimating column densities in Adaptive Mesh Refinement codes Influence of UV radiation field on the structure of molecular clouds

Cited by in corpus (43)

• Photodissociation and X-Ray Dominated Regions
• SILCC-Zoom: The dynamical and chemical evolution of molecular clouds
• The molecular cloud lifecycle
• The Cloud Factory I: Generating resolved filamentary molecular clouds from galactic-scale forces
• Analytic HI-to-H2 Photodissociation Transition Profiles
• Thermal Phases of the Neutral Atomic Interstellar Medium -- from Solar Metallicity to Primordial Gas
• SILCC-Zoom: H${}_2$ and CO-dark gas in molecular clouds -- The impact of feedback and magnetic fields
• From diffuse gas to dense molecular cloud cores
• Structure distribution and turbulence in self-consistently supernova-driven ISM of multiphase magnetized galactic discs
• The SILCC project - V. The impact of magnetic fields on the chemistry and the formation of molecular clouds
• Surround & Squash: The Interstellar Medium around Scorpius Centaurus OB2
• The HI-to-H2 Transition in a Turbulent Medium
• Understanding star formation in molecular clouds IV. Column density PDFs from quiescent to massive molecular clouds
• Chemistry and radiative shielding in star forming galactic disks
• The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds
• Origin of CH+ in diffuse molecular clouds warm H2 and ion-neutral drift
• Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks
• 3D chemical structure of diffuse turbulent ISM. I. Statistics of the HI-to-H${}_2$ transition
• DHIGLS: DRAO H I Intermediate Galactic Latitude Survey
• HI-to-H2 Transition Layers in the Star-Forming Region W43
• A simple model for molecular hydrogen chemistry coupled to radiation hydrodynamics
• On the resolution requirements for modelling molecular gas formation in solar neighbourhood conditions
• The in situ formation of molecular and warm ionised gas triggered by hot outflows
• Synthetic Observations of 21cm HI Line Profiles from Inhomogeneous Turbulent Interstellar HI Gas with Magnetic Field
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
• On the accuracy of HI observations in molecular clouds -- More cold HI than thought?
• The Early Stage of Molecular Cloud Formation by Compression of Two-phase Atomic Gases
• The role of neutral hydrogen in setting the abundances of molecular species in the Milky Way's diffuse interstellar medium. I. Observational constraints from ALMA and NOEMA
• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition
• Mid-J CO Shock Tracing Observations of Infrared Dark Clouds III: SLED fitting
• Bimodal Behavior and Convergence Requirement in Macroscopic Properties of the Multiphase Interstellar Medium Formed by Atomic Converging Flows
• The evolution of HCO${}^{+}$ in molecular clouds using a novel chemical post-processing algorithm
• Synthetic CO emission and the $X_{\mathrm{CO}}$ factor of young molecular clouds: a convergence study
• Mapping the Thermal Condensation of Diffuse H I in the North Celestial Pole Loop
• Compact dusty clouds and efficient H${}_2$ formation in diffuse ISM
• CO enhancement by magnetohydrodynamic waves; Striations in the Polaris Flare
• Intertwined Formation of ${\mathrm{H}}_2$, Dust, and Stars in Cosmological Simulations
• The unusual ISM in Blue and Dusty Gas Rich Galaxies (BADGRS)
• Physical Properties and HI-to-H2 Transition Across Taurus Linear Edge
• H2 distribution during 2-phase Molecular Cloud Formation
• One H2 molecule per ten million H-atoms reveals sub-pc scale cold overdensities at z~4
• Hydrogen in diffuse molecular clouds in the Milky Way: Atomic column densities and molecular fraction along prominent lines of sight