H2O Masers and Supersonic Turbulence

Abstract

We use unpublished and published VLBI results to investigate the geometry and the statistical properties of the velocity field traced by H2O masers in five galactic regions of star formation — Sgr B2(M), W49N, W51(MAIN), W51N, and W3(OH). In all sources the angular distribution of the H2O hot spots demonstrates approximate self-similarity (fractality) over almost four orders of magnitude in scale, with the calculated fractal dimension d between ≈ 0.2 and 1.0. In all sources, the lower order structure functions for the line-of-sight component of the velocity field are satisfactorily approximated by power laws, with the exponents near their classic Kolmogorov values for the high-Reynolds-number incompressible turbulence. These two facts, as well as the observed significant excess of large deviations of the two-point velocity increments from their mean values, strongly suggest that the H2O masers in regions of star formation trace turbulence. We propose a new conceptual model of these masers in which maser hot spots originate at the sites of ultimate dissipation of highly supersonic turbulence produced in the ambient gas by the intensive gas outflow from a newly-born star. Due to the high brightness and small angular sizes of masing hot spots and the possibility of measuring their positions and velocities with high precision, they become a unique probe of supersonic turbulence.