High-Speed Interstellar Gas Dynamics: Shocks Moderated by Cosmic Rays
Abstract
Cosmic rays interact with the thermal interstellar gas through a cosmic-ray sound speed" of the order of (cosmic-ray energy density/thermal gas density)1'2, typically km . Gas motions faster than the thermal sound speed may he moderated by the cosmic rays. The cosmic-ray sound speed and the jump conditions across cosmic-ray-dominated shocks are derived here on the assumption that the cosmic rays and the interstellar gas interact via resonant hydromagnetic waves. Cosmic-ray shocks occur where gas is driven through the cosmic rays by gravity, for instance at the "high-velocity" clouds if these are gas falling into our Galaxy, or where cosmic rays experience a density gradient, for instance around supernovae. In all cases, the cosmic rays lose energy to the gas, affecting our understanding of both the cosmic rays (is there any need to store them in a galactic "halo"?) and interstellar gas dynamics (are cosmic rays the missing source of kinetic energy for interstellar clouds?). In case the high-velocity clouds are subsonic, however, the cosmic rays may cushion their infall so that no shock occurs at all.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1971
- DOI:
- 10.1086/151187
- Bibcode:
- 1971ApJ...170...53W