Supernovae and their remnants play a fundamental role in the production and acceleration of galactic cosmic rays. Supernova remnants provide the necessary power to sustain the observed sea of cosmic rays which are isotropized in galactic magnetic fields. The shocks driven by expanding ejecta from supernovae of all types provide a natural mechanism for acceleration of cosmic rays, as described in the previous chapter. However, the clear identification of individual cosmic ray sources still remains elusive. The study of gamma rays and neutrinos produced in the interaction of cosmic rays in the sources or in the ambient gas has the potential to provide direct insight into the origin of galactic cosmic rays. In this chapter, after a short description of the Milky Way, we describe the supernovae and the evolution of their remnants. We also study binary systems and the role of star-forming regions.

The Milky Way galaxy

As already introduced in Section 9.2, the Milky Way galaxy is a spiral galaxy composed of a thin disk or galactic plane of radius ∼20 kpc and thickness ∼400– 600 pc, a spherical central region with radius ∼2–3 kpc (also known as the “bulge” or “galactic center”), and a halo which extends to more then ∼30 kpc away from the center (Figure 13.1). The majority of standard matter (to be distinguished from dark matter) is concentrated in the thin disk composed of stars and interstellar medium (ISM). The ISM is filled by gas, dust and cosmic rays and it accounts for 10–15% of the total mass of the galactic plane. The gas is very inhomogeneously distributed at small scales, and it is mostly confined to discrete clouds. Only a few per cent of the interstellar volume is occupied by dense accumulation of ISM. The turbulent, ionized component of the ISM is threaded with a magnetic field that plays an important intermediate role connecting cosmic rays with the ISM. To understand the origin of galactic cosmic rays and the energy involved, we review here the ISM and star-forming regions. Of particular interest is the feedback exercised by supernova explosions and their remnants on the ISM and the related triggering of star formation. We also describe the galactic center region and, for completeness, the dark matter halo.