Supernova (SN) explosions of massive stars are a dominant source of kinetic energy in the interstellar medium. Strong, supersonic stellar winds are also an important source in the case of the extreme most massive stars ( 40 M). Mechanical feedback structures the ISM, with immediate consequences corresponding to supernova remnants (SNRs), stellar wind-driven bubbles, and superbubbles resulting from combined SNe and winds from multiple stars.
While the SNRs evolve, in the simplest description, according to the Sedov (1959) model, the wind-driven bubbles and superbubbles are thought to evolve according to a similar, Sedov-like adiabatic model for constant energy input (Pikel'ner 1968; Castor et al. 1975): the central supersonic wind drives a shock into the ambient ISM, piling up a radiatively cooled, dense shell; and a reverse shock near the source thermalizes the wind's kinetic energy, thereby generating a hot (106 - 107 K), low-density (n ~ 10-2 - 10-3 cm-3) medium that dominates the bubble volume (Figure 3). This heating process is believed to be the origin of the diffuse hot, ionized medium (HIM) in the interstellar medium. Assuming that the hot bubble interior remains adiabatic, the self-similar shell evolution follows the simple analytic relations,
where R and v are the shell radius and expansion velocity, L is the input mechanical power, and t is the age. Once SNe begin to explode, they quickly dominate L, and the standard treatment is to consider the discrete SNe as a constant energy input (e.g., Mac Low & McCray 1988). Hence, we may write L in terms of the SN parameters:
where N* is the number of SNe, E51 is the SN energy, and te is the total time during which the SNe occur.
Figure 3. The adiabatic wind-driven bubble model (see text). The shell at the outer shock consists of swept-up ISM on the outside and shocked wind material on the inside, separated by a contact discontinuity.
There are several approaches to testing the standard, adiabatic shell evolution, and by extension, our understanding of mechanical feedback. In the first instance, we can examine the properties and kinematics of individual shell systems and carry out rigorous comparisons with the model predictions. Secondly, we can also examine statistical properties of entire shell populations in galaxies, and compare with model predictions. And thirdly, we can carry out spatial correlations of shells with regions of recent star formation, to confirm the existence of putative stellar progenitors. All three of these methods require high spatial resolution, and thus the Local Group offers by far the best, and often the only feasible, laboratory.