Collisional Pumping of H$${}_{2}$$O and СH$${}_{3}$$OH Masers in C-Type Shock Waves

Abstract

The collisional pumping of H $${}_{2}$$ O and СH $${}_{3}$$ OH masers in magnetohydrodynamic nondissociative C-type shocks is considered. A grid of C-type shock models with speeds in the range 5–70 km s $${}^{-1}$$ and preshock gas densities $$n_{\textrm{H}_{2},0}=10^{4}$$ – $$10^{7}$$ cm $${}^{-3}$$ is constructed. The large velocity gradient approximation is used to solve the radiative transfer equation in molecular lines. The para-H $${}_{2}$$ O 183.3 GHz and ortho-H $${}_{2}$$ O 380.1 and 448.0 GHz transitions are shown to be inverted and to have an optical depth along the shock velocity $$|\tau|\sim 1$$ at relatively low gas densities in the maser zone, $$n_{\textrm{H}_{2}}\gtrsim 10^{5}$$ – $$10^{6}$$ cm $${}^{-3}$$ . Higher gas densities, $$n_{\textrm{H}_{2}}\gtrsim 10^{7}$$ cm $${}^{-3}$$ , are needed for efficient pumping of the remaining H $${}_{2}$$ O masers. Simultaneous generation of H $${}_{2}$$ O and class I СH $${}_{3}$$ OH maser emission in a shock is possible at preshock gas densities $$n_{\textrm{H}_{2},0}\approx 10^{5}$$ cm $${}^{-3}$$ and shock speeds in the range $$u_{\textrm{s}}\approx 17.5{-}22.5$$ km s $${}^{-1}$$ . The possibility of detecting class I СH $${}_{3}$$ OH and para-H $${}_{2}$$ O 183.3 GHz masers in star-forming regions and near supernova remnants is investigated.