How Long Can Tiny HI Clouds Survive?

Abstract

We estimate the evaporation timescale for spherical HI clouds consisting of the cold neutral medium surrounded by the warm neutral medium. We focus on clouds smaller than 1pc, which corresponds to tiny HI clouds recently discovered by Braun & Kanekar and Stanimirovi{ć} & Heiles. By performing one-dimensional spherically symmetric numerical simulations of the two-phase interstellar medium (ISM), we derive the timescales as a function of the cloud size and of pressure of the ambient warm medium. We find that the evaporation timescale of the clouds of 0.01 pc is about 1Myr with standard ISM pressure, $p/k_B\sim 10^{3.5}$ K cm${}^{-3}$, and for clouds larger than about 0.1 pc it depends strongly on the pressure. In high pressure cases, there exists a critical radius for clouds growing as a function of pressure, but the minimum critical size is $\sim$ 0.03 pc for a standard environment. If tiny HI clouds exist ubiquitously, our analysis suggests two implications: tiny HI clouds are formed continuously with the timescale of 1Myr, or the ambient pressure around the clouds is much higher than the standard ISM pressure. We also find that the results agree well with those obtained by assuming quasi-steady state evolution. The cloud-size dependence of the timescale is well explained by an analytic approximate formula derived by Nagashima, Koyama & Inutsuka. We also compare it with the evaporation rate given by McKee & Cowie.

4 pages, 3 figures

References in corpus (12)

• The Neutral Atomic Phases of the ISM in the Galaxy
• An Origin of Supersonic Motions in Interstellar Clouds
• Thermal condensation in a turbulent atomic hydrogen flow
• Molecular Cloud Formation in Shock-Compressed Layers
• On the Hydrodynamic Interaction of Shock Waves with Interstellar Clouds. II. The Effect of Smooth Cloud Boundaries on Cloud Destruction and Cloud Turbulence
• Thermal Instability-Induced Interstellar Turbulence
• The Field Condition: A New Constraint of Spatial Resolution in Simulations of the nonlinear Development of Thermal Instability
• The thinnest cold HI clouds in the diffuse interstellar medium?
• Tiny HI Clouds in the Local ISM
• Dynamics of Conductive/Cooling Fronts: Cloud Implosion and Thermal Solitons
• Interstellar Phase Transitions Stimulated by Time-dependent Heating
• Evaporation and Condensation of HI clouds in thermally bistable interstellar media: semi-analytic description of isobaric dynamics of curved interfaces

Cited by in corpus (16)

• Thermal-Instability-Driven Turbulent Mixing in Galactic Disks: I. Effective Mixing of Metals
• HI Clouds in the Lower Halo: I. The Galactic All-Sky Survey Pilot Region
• GBT Detection of Polarization-dependent HI Absorption and HI Outflows in Local ULIRGs and Quasars
• Formation of Interstellar Clouds: Parker Instability with Phase Transitions
• The Effect of Anisotropic Conduction on the Thermal Instability in the Interstellar Medium
• Arecibo Multi-Epoch HI Absorption Measurements Against Pulsars: Tiny-Scale Atomic Structure
• Turbulence measurements from HI absorption spectra
• Tiny scale opacity fluctuations from VLBA, MERLIN and VLA observations of HI absorption toward 3C 138
• Thermal Instability and Multiphase Gas in the Simulated Interstellar Medium with Conduction, Viscosity and Magnetic Fields
• Self-Sustained Turbulence without Dynamical Forcing: A Two-Dimensional Study of a Bistable Interstellar Medium
• The structure function of Galactic \HI opacity fluctuations on AU scales based on MERLIN, VLA and VLBA data
• Turbulent power spectrum in warm and cold neutral medium using the Galactic HI 21 cm emission
• Extraordinary increase of lifetime of localized cold clouds by the viscous effect in thermally-unstable two-phase interstellar media
• Two-component model of the interaction of an interstellar cloud with surrounding hot plasma
• A photo-ionized canopy for the shock-excited Criss-Cross Nebula
• Modeling the Accretion of High-Velocity Clouds from a Rotating Halo