Thermal Instability behind a Shock Wave in HI and Molecular Clouds

Abstract

We performed one-dimensional hydrodynamic simulations with detailed cooling, heating and chemical processes to examine the thermal stability of shocked gas in cold neutral medium (CNM) and molecular clouds. We find that both CNM and molecular clouds can be thermally unstable in the cooling layer behind the shock wave. The characteristic wavelength of the thermal instability ranges from 10^-5 pc to 0.1 pc in the CNM, and from 10^-7 pc to 0.1 pc in the molecular clouds. This coincides with the size of observed tiny scale structures in the CNM and molecular clouds, indicating that the thermal instability in the shocked gas could be a formation mechanism of these tiny structures in the interstellar medium. We have also calculated the e-folding number of the thermal instability to estimate the amplification of the density fluctuation in the shocked gas. Density perturbations in the CNM grow by a factor of exp(5)~150, whereas the perturbations in the molecular clouds grow only by a factor of a few behind a high Mach number shock. The amplification factor is larger at lower densities and higher velocities. Formation of very small-scale structures by thermal instability in shocked gas is more effective in lower densities.

30 page, 4 figure, accepted in ApJ

References in corpus (9)

• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• Cooling, Gravity and Geometry: Flow-driven Massive Core Formation
• Formation of Turbulent and Magnetized Molecular Clouds via Accretion Flows of HI Clouds
• Cold CO in circumstellar disks: On the effects of photodesorption and vertical mixing
• Chemistry in the First Hydrostatic Core Stage By Adopting Three-Dimensional Radiation Hydrodynamic Simulations
• The Role of Ambipolar Diffusion in the Formation Process of Moderately Magnetized Diffuse Clouds
• Toward Understanding the Origin of Turbulence in Molecular Clouds: Small Scale Structures as Units of Dynamical Multi-Phase Interstellar Medium
• Formation of Fluctuations in the Molecular Slab via Isobaric Thermal Instability

Cited by in corpus (4)

• Warm CO gas generated by possible turbulent shocks in a low-mass star-forming dense core in Taurus
• Thermal instability and multi-phase interstellar medium in the first galaxies
• An ALMA Glimpse of Dense Molecular Filaments Associated with High-mass Protostellar Systems in the Large Magellanic Cloud
• Interactions of a shock with a molecular cloud at various stages of its evolution due to thermal instability and gravity