Can Warm Neutral Medium Survive inside Molecular Clouds?

Abstract

Recent high-resolution numerical simulations have suggested that the interstellar atomic hydrogen clouds have a complex two-phase structure. Since molecular clouds form through the contraction of H i gas, the question arises as to whether this structure is maintained in the molecular phase or not. Here we investigate whether the warm neutral atomic hydrogen can exist in molecular clouds. We calculate how far a piece of warm neutral medium (WNM) that is not heated by the UV photons could penetrate into the cloud and find that in the absence of any heating it is unlikely that a large fraction of the WNM would survive inside high-pressure molecular clouds. We then consider two possible heating mechanisms, namely, dissipation of turbulent energy and dissipation of MHD waves propagating in the WNM inside the cloud. We find that the second possibility is sufficient to allow the existence of WNM inside a molecular cloud of size '1 pc having pressure equal to '10P ISM. This result suggests the possibility that channels of magnetized WNM may provide efficient energy injection for sustaining internal turbulence, which otherwise decays in a crossing time.