How Long Can Tiny H i Clouds Survive?

Abstract

We estimate the evaporation timescale for spherical H I clouds consisting of the cold neutral medium surrounded by the warm neutral medium. We focus on clouds smaller than 1 pc, which corresponds to tiny H I clouds recently discovered by Braun & Kanekar and Stanimirovic & 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 1 Myr with standard ISM pressure, p/kB ~ 103.5 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 ~ 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. 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.