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I cannot understand the meaning of "shock cooling" which occurred in supernova. And I saw the sentence like "Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope". Could you explain the mechanism of shock cooling in supernova?

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Shock cooling in a SN is the scenario where kinetic energy carried by the shock transforms into other energy forms: adiabatic expansion, radiation, acceleration. A shock is actually a term meaning discontinuity. For example, a density profile of steady-wind medium is $r^{-2}$ while that of external part of ejecta is $r^{-n}$ where $7 \le n \le 12$ as typical values in core-collapse supernovae, but at the interface between the medium and the external ejecta, where they interacts, its density profile spikes before asymptotically decaying to the characteristic profile of each region. The interface is the shock.

The mechanism of shock cooling in SN involves processes which are not completely understood. What we know is that the shock is formed as a double-shock structure: forward and reverse. The forward shock propagates outward, while the reverse one goes inward in mass coordinate. (Note that in spatial coordinate both shocks propagate outward). Kinetic energy in each shock is transformed by interactions. At a forward shock, typically the energy is transformed into acceleration of relativistic particles, which in turn radiate via synchrotron or inverse Compton processes, and collisionally interacts with the medium. At a reverse shock, the shock energy thermalizes ejecta, which in turn thermally radiates.

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