Interstellar Matter Hydrodynamics and the Dispersal and Mixing of Heavy Elements

It is shown here that the ejecta from type II supernovae follow a long excursion into the galactic environment before they mix with the ISM. We point to the various changes in temperature, density, and pressure experienced by the ejecta along their inevitable ride, indicating the main hydrodynamical events and physical processes taking part in these changes. The long list of possible ways to disrupt the contact discontinuity that separates the thermalized ejecta from the swept up matter, such as cloud crushing, thermal evaporation, hydrodynamical instabilities, as well as the effects caused by explosions inside wind-driven shells and by fragmented ejecta, are evaluated. Diffusion is found to be very effective in the hot coronal phase, causing the ejecta of correlated supernova explosions to mix with the matter evaporated and ablated from clouds and the cool outer shell. Once the supernova activity from a dying OB association comes to an end, the hot matter is able to cool by radiation. However, given the density and temperature fluctuations in the hot medium, cooling acts in a differential way. This is shown to lead to condensation of the metal-rich gas into small droplets able to fall back and settle onto the disk of the galaxy. The droplets are likely to become molecular and thus their diffusion into the cold matter phases (either H I or H₂), where molecules remain bound, is another dispersal agent which together with the motion of clouds and differential galactic rotation lead to a more homogeneous distribution of droplets, but not to their mixing with the ISM. True mixing occurs upon the birth of new generations of massive stars. These dissociate and disrupt, through photoionization, the heavy element droplets favoring their almost immediate diffusion into the H II region volume, finally changing the composition of the ISM.