The elusive structure of the diffuse molecular gas: shocks or vortices in compressible turbulence?

The cold diffuse interstellar medium must harbor pockets of hot gas to prod uce the large observed abundances of molecular species, the formation of wh ich requires much more energy than available in the bulk of its volume. The se hot spots have so far escaped direct detection but observations and mode ling severely constrain their phase-space structure i.e. they must have a s mall volume filling factor (a few%), surface filling factors larger than un ity with large fluctuations about average and comparable velocity structure in "pencil beams" and "large beams" The dissipation of the non-thermal energy of supersonic turbulence occurs i n bursts, either in shocks or in the regions of large shear at the boundary of coherent vortices. These two processes are susceptible to generate loca lized hot regions in the cold medium. Yet, it is of interest to determine w hich of them, if any, dominates the dissipation of turbulence in the cold i nterstellar medium. In this paper, we analyze the spatial and kinematic properties of two subse ts of hydrodynamical compressible turbulence: the regions of largest negati ve divergence and those of largest vorticity and confront them with the obs ervational constraints. We find that these two subsets fulfill the constrai nts equally well. A similar analysis should be conducted in the future on s imulations of MI-ID turbulence.