Can Warm Neutral Medium Survive inside Molecular Clouds?

Recent high resolution numerical simulations have suggested that theinterstellar atomic hydrogen clouds have a complex two-phase structure. Sincemolecular clouds form through the contraction of HI gas, the question arises asto whether this structure is maintained in the molecular phase or not. Here weinvestigate whether the warm neutral atomic hydrogen (WNM) can exist inmolecular clouds. We calculate how far a piece of WNM which is not heated bythe UV photons could penetrate into the cloud, and find that in the absence ofany heating it is unlikely that large fraction of WNM survives inside highpressure molecular clouds. We then consider two possible heating mechanisms,namely dissipation of turbulent energy and dissipation of MHD waves propagatingin the WNM inside the cloud. We find that the second one is sufficient to allowthe existence of WNM inside a molecular cloud of size $\simeq$ 1 pc havingpressure equal to $\simeq 10 \times P_{\rm ISM}$. This result suggests thepossibility that channels of magnetised WMN may provide efficient energyinjection for sustaining internal turbulence which otherwise decays in acrossing time.Comment: accepted for publication in Ap