sess13 - 13. Heating and cooling balance in the...

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13. Heating and cooling balance in the interstellar medium 13.1 Heating by shock waves The gas between stars, commonly referred to as the interstellar medium, and the gas between galaxies, called the intergalactic medium, is subject to a number of heating processes, which must be balanced by cooling, otherwise the temperature of the medium of perpetually increase. Some heating processes operate continuously, like heating by absorption of radiation. If the absorption is done by free particles, like free electrons, all the energy of the absorbed photon is available as heat. If the absorption is doneby exciting or ionizing an atom, much of the photon energy is stored as potential energy in the atom. In case of ionization only the kinetic energy of the freed electron can be thermalized, so the photon energy minus the binding energy of the atomic electron is available as heat. Gas can also be collisionally heated by energetic particles called cosmic rays. Other heating processes like shock heating operate intermittendly. Let us estimate the heating contributed by supernova remnant shocks. Measurements indicate that typically every t SN = 50 years a supernova explodes in the Galaxy and the typical explosion energy is E = 10 51 ergs. The Galaxy can for our purposes be approximated as a disk with radius 10.000 pc (parallax second=parsec, 1 pc = 3 Light-years) and thickness 500 pc, so its volume is V gal 1 . 5 · 10 11 pc 3 . From the jump conditions at strong ( M → ∞ ) hydrodynamical shocks we derived the post- shock temperature as a function of the shock velocity as kT 2 = 2 ( γ - 1) (1 + γ ) 2 m V 2 s T 2 (3 · 10 5 K) V s 100 km / s ! 2 (13 . 1) From the Taylor-Sedov blast-wave solution we know that the shock velocity and radius scale as V s ( t ) = dr s dt = 2 5 x 0 E ρ u t 3 ! 1
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sess13 - 13. Heating and cooling balance in the...

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