Photoionizing Shocks

Abstract

Up to this point, we have considered only stars as the sources of photoionizing photons. However, the cooling zones of shocks, and more especially, fast shocks, produce copious amounts of EUV or soft X-ray photons. These escape both upstream and downstream to be absorbed in either a precursor photoionized zone or in a narrow photoionized zone adjacent to the recombination region of the shock. In very fast shocks, the photon spectrum may be quite hard, so that the characteristics of the precursor photoionized region become quite difficult to distinguish from regions photoionized by power-law spectra, such as found close to active galactic nuclei (AGN). A necessary condition for the development of these photoionized zones is that the shock is fully radiative. For shocks with a velocities below 600 km s-1, in plasma of solar composition, the recombination timescale is given approximately by (8.77). Here instead we take the timescale for the flow to cool to 104 K from the immediate hot postshock region. By the time the plasma has dropped to this temperature, all the photionizing radiation can be generated by the cooling plasma has been radiated, upstream and downstream. This is at a point in the flow just prior to the downstream postshock photoionization and recombination regions. A fit to a series of detailed models gives:

$$ {\tau _{cool}} \sim 232.9{\left( {\frac{n}{{c{m^{ - 3}}}}} \right)^{ - 1}}{\left( {\frac{{{v_s}}}{{100km{s^{ - 1}}}}} \right)^{4.205}}yr, $$

((11.1))

for υ _s = 100–600 km s^-1, to within about 10% of the detailed models.

“He saw, but blasted by excess of light, Closed his eyes in endless night”

— Thomas Gray