How Long Can Tiny H I Clouds Survive?

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 1pc, which corresponds to tiny H I clouds recently discovered by Braun & Kanekar and Stanimirović & 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/kB ∼ 10 K cm, 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 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. Subject headings: hydrodynamics – ISM: clouds – ISM: kinematics and dynamics