Maser Flares Driven by Isothermal Shock Waves

We use 3D computer modelling to investigate the timescales and radiativeoutput from maser flares generated by the impact of shock-waves on astronomicalunit-scale clouds in interstellar and star-forming regions, and incircumstellar regions in some circumstances. Physical conditions are derivedfrom simple models of isothermal hydrodynamic (single-fluid) and C-type (ionicand neutral fluid) shock-waves, and based on the ortho-H_2O 22-GHztransition. Maser saturation is comprehensively included, and we find that themost saturated maser inversions are found predominantly in the shockedmaterial. We study the effect on the intensity, flux density and duration offlares of the following parameters: the pre-shock level of saturation, theobserver's viewpoint, and the shock speed. Our models are able to reproduceobserved flare rise times of a few times 10 days, specific intensities of up to10^5 times the saturation intensity and flux densities of order100(R/d)^2Jy from a source of radius R astronomical units at a distance ofd kiloparsec. We found that flares from C-type shocks are approximately 5times more likely to be seen by a randomly placed observer than flares fromhydrodynamically shocked clouds of similar dimensions. We computed intrinsicbeaming patterns of the maser emission, finding substantial extension of thepattern parallel to the shock front in the hydrodynamic models. Beaming solidangles for hydrodynamic models can be as small as 1.3× 10^-5sr, butare an order of magnitude larger for C-type models.