Collisional Pumping of OH Masers near Supernova Remnants
  The collisional pumping of OH masers in non-dissociative C-type shocks near
supernova remnants is considered. The emergence of maser emission in OH lines
is investigated for various shock parameters - the shock speed, the preshock
gas density, the cosmic-ray ionization rate, and the magnetic field strength.
The largest optical depth in the 1720 MHz line is reached at high gas
ionization rates $\zeta \geq 10^{-15}$ s$^{-1}$, an initial density $n_{\rm
H,0} \leq 2 \times 10^4$ cm$^{-3}$, and a shock speed $u_{\rm s} \geq 20$ km
s$^{-1}$. According to our calculations, there is also a level population
inversion for the 6049 and 4765 MHz transitions of excited OH rotational
states. However, the optical depth in these lines is small for all of the
investigated shock parameters, which explains the non-detection of maser
emission in these lines in supernova remnants.
