[Paper Review] On the structure of the interstellar atomic gas

P. Hennebelle, E. Audit|arXiv (Cornell University)|Jan 24, 2007

Cold Atom Physics and Bose-Einstein Condensates3 citations

TL;DR

This study uses high-resolution 2D numerical simulations to model turbulent atomic hydrogen in the interstellar medium, revealing that cold neutral medium (CNM) fragments naturally form dense, compact structures (down to ~400 AU and 10⁴ cm⁻³) via high-Mach-number collisions. These structures closely resemble observed tiny small-scale atomic structures (TSAS), suggesting turbulence and two-phase physics alone can explain their origin without requiring external triggers.

ABSTRACT

The interstellar atomic hydrogen is known to be a 2-phase medium in which turbul ence plays an important rôle. Here we present high resolution numerical simulations describing the gas from tens of parsec down to hundreds of AU. This high resolut ion allows to probe numerically, the small scale structures which naturally arises from the turbulence and the 2-phase physics.

Motivation & Objective

• To investigate the formation and structure of interstellar atomic hydrogen at high spatial resolution, from tens of parsecs down to hundreds of AU.
• To understand how turbulence and two-phase (CNM/WNM) physics generate small-scale structures in the interstellar medium.
• To determine whether observed tiny small-scale atomic structures (TSAS) can arise naturally from the dynamics of a turbulent, two-phase gas.
• To assess the role of high-Mach-number collisions between CNM fragments in forming dense, compact structures.

Proposed method

• Numerical simulations solve the compressible hydrodynamic equations for a radiatively cooling, thermally conductive gas with γ = 5/3 and μ = 1.4mH.
• The cooling function includes Lyman-α, C⁺, O line cooling and photoelectric heating from dust grains.
• Thermal conductivity is modeled using a temperature-dependent expression based on Spitzer-like diffusion.
• A 2D simulation with 10,000² cells and a 20 pc box achieves a spatial resolution of 0.002 pc, enabling resolution of small-scale structures.
• Boundary conditions include converging flows at the left and right, with outflow conditions on top and bottom to allow mass escape.
• Initial conditions consist of a uniform, low-density warm neutral medium (WNM) at equilibrium (n ≈ 0.8 cm⁻³), with no initial CNM present.

Experimental results

Research questions

• RQ1 Can high-resolution simulations reproduce the formation of dense, compact atomic gas structures observed as TSAS in the interstellar medium?
• RQ2 How do turbulence and two-phase physics (CNM/WNM) contribute to the fragmentation and confinement of cold gas?
• RQ3 What physical mechanisms, such as high-Mach-number collisions, lead to the formation of structures with densities up to 10⁴ cm⁻³ and sizes of ~400 AU?
• RQ4 To what extent do the physical parameters of simulated structures match those inferred from observations of TSAS and low-column-density CNM clouds?
• RQ5 Does the 2D simulation setup qualitatively capture the dynamics seen in 3D, given the resolution limitations?

Key findings

• The CNM is highly fragmented into long-lived, pressure-confined cloudlets bounded by contact discontinuities, with densities up to 100 cm⁻³ and sizes as small as a few thousand AU.
• Dense structures with peak densities of ~10⁴ cm⁻³ and sizes of ~400–800 AU form at stagnation points of converging flows, consistent with high-Mach-number shocks.
• These dense structures are dynamically maintained by ram pressure from large-scale converging flows and have lifetimes of approximately 10⁴ years.
• The pressure in these dense structures is strongly correlated with density, as expected for high-Mach-number shocks, confirming their origin in supersonic collisions.
• The simulations reproduce the observed anti-correlation between density peaks and pressure in CNM structures, except at stagnation points where pressure and density are correlated.
• 3D simulations with 1200³ cells confirm that the qualitative behavior—fragmentation, confinement, and formation of dense structures—persists, validating the 2D results despite lower resolution.