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We use unpublished and published VLBI results to investigate the geometry and the statistical properties of the velocity field traced by H2O masers in five galactic regions of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 4 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-ofsight component of the velocity field are satisfactorily approximated by power laws, with the exponents near their classic Kolmogorov values for 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. Because of 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. Subject headings: ISM: jets and outflows — masers — turbulence
Highlights
The idea of an energy cascade through a hierarchy of scales (Richardson 1922; Kolmogorov 1941a; Obukhov 1941), the phenomenological theory of turbulent energy dissipation (Kolmogorov 1941a,b, 1942), and the experimental and theoretical results related to the intermittency of turbulent velocity fields are the cornerstones of the present understanding of incompressible turbulence
This paper summarizes a series of our studies of H2O masers as tracers of supersonic turbulence in regions of star formation
We investigated two statistical properties of the velocity field traced by H2O masers in the same five sources: (1) the low-order two-point velocity structure functions, and (2) the probability distribution for the deviations of the two-point velocity increment from its mean value at different spatial scales
Summary
The idea of an energy cascade through a hierarchy of scales (Richardson 1922; Kolmogorov 1941a; Obukhov 1941), the phenomenological theory of turbulent energy dissipation (Kolmogorov 1941a,b, 1942), and the experimental and theoretical results related to the intermittency of turbulent velocity fields (see Frisch 1995 for references) are the cornerstones of the present understanding of incompressible turbulence. VLBI studies of the proper motions of several bright H2O maser sources associated with newly-born stars have revealed expansion of the clusters of maser spots — participation in gas outflows from these stars (see Anderson and Genzel 1993 for a review). Approximating the proper motion vectors by a simple model of expanding and rotating gas leaves a residual dispersion of ≈ 15 km s−1 per axis, which is considerably larg√er than the errors of these observations (Reid et al 1988) This value corresponds to ≈ 3 × 15 ≈ 26 km s−1 for the total velocity vector and can be considered the characteristic turbulent velocity dispersion at the largest spatial scale covered by the maser cluster.
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