Paper:	Evaporation Timescales of HI Clouds
Volume:	365, SINS — Small Ionized and Neutral Structures in the Diffuse Interstellar Medium
Page:	121
Authors:	Nagashima, M.; Inutsuka, S.; Koyama, H.
Abstract:	We estimate the evaporation timescale for spherical H I clouds consisting of the cold neutral medium surrounded by the warm neutral medium, with special attention to the recently discovered tiny H I clouds. Utilizing onedimensional spherically symmetric numerical simulations of the two-phase interstellar medium (ISM), we derive the timescales as a function of the cloud size and pressure of the ambient warm medium. We find that the evaporation timescale of the clouds of 0.01 pc is about 1 Myr with standard ISM pressure, p/kB ~ 103.5 K cm−3; for clouds larger than about 0.1 pc, the lifetime depends strongly on the pressure. In high pressure cases, there is a critical radius for clouds growing as a function of pressure, but the minimum critical size is ~ 0.03 pc for a standard environment. If tiny H I clouds exist ubiquitously, our analysis suggests two implications: tiny H I clouds are formed continuously with the timescale of 1 Myr, or the ambient pressure around the clouds is much higher than the standard ISM pressure.

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