5.2. The Three-Phase Model
This paved the way for a true theoretical tour de force, the McKee-Ostriker (MO) model of the supernova-dominated, three-phase ISM. In their model, supernovae produce the `hot ionized medium' (HIM), the ~ 106 K component of the ISM in their bubble interiors, as well as enhancing the formation of the `cold neutral' (CNM), `warm neutral' (WNM) and `warm ionized' (WIM) media along the compressed edges of remnants. There are still only two fundamental phases (CNM, HIM) in the MO theory. The WNM and WIM are restricted to the interface regions of the neutral clouds, and the WIM in direct contact with the HIM and photoionized by thermal emission from it.
The model attempts to balance the thermal and mass exchange between the different phases. The energy input from supernovae is offset by radiation cooling from the four media. The mass lost by cloud evaporation into the HIM is balanced by dense shell formation of swept up interstellar material. The model is known to be incorrect in many details. There are numerous observations of local, dense clouds that are highly overpressured with respect to their environment. The model does not treat the influence of magnetic fields which are known to thread throughout the ISM and are likely to suppress thermal evaporation. The magnetic fields are of the right strength to play an important role in equilibrating the pressure balance. But in the absence of a better working model, the MO theory remains the dominant conceptual framework.
It remains unclear whether thermal pressure balance between the phases is an essential feature of the MO picture. Pressure equilibrium should exist between the cosmic rays, the magnetic field and the kinetic interstellar component. But the latter need not be the thermal pressure of the gas. Random, turbulent `bulk' motions of the gas have a high enough energy density to provide the required effective pressure. The minimum interstellar pressure, P = n T, is probably about ~ 25,000 cm-3 K and may even be higher.