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N. Nezhdanova (Columbia U. and Maria Mitchell Obs.), B. Holder (U. of Texas, Austin), V. Strelnitski (Maria Mitchell Obs.)
We perform numerical simulations to determine the regime of amplification (i.e. saturated or unsaturated) for water masers in regions of star formation. Both the velocity distribution and the spatial distribution of masers in these regions are characterized by power law scaling. We compare VLBI maps of the masers with numerical simulations of maser amplification in a medium characterized by these distributions. First, we create a homogeneous, isotropic, Kolmogorov-type random velocity field in a box with 5123 grid points. The power law scaling of the spatial distribution is then imposed by canceling a part of the active cells, so that the remaining part occupies a random fractal set. Finally, the output maser radiation is calculated along columns parallel to the line of sight with the assumptions of either unsaturated (exponential) or saturated (linear) amplification. All realizations show that the high-gain unsaturated amplification creates only a small number of isolated hot spots at the output of the maser, with no self-similar clustering. Under the assumption of saturated amplification, however, the fractal structure is seen through all of the scales, down to the smallest groups of elementary cells. Additionally, we show that without the initial restriction to a fractal set neither amplification regime produces power law scaling of the output maser's spatial distribution. We argue that the observed VLBI maps of the masers are much better described by the case of saturated amplification. The 3-D fractal structure with low fractal dimension and the random, Kolmogorov-type velocity field used in this successful model are in agreement with the hypothesis (Strelnitski et al., ApJ 581, 1180, 2002) that water maser spots are connected to the intermittent dissipation sites of turbulence. This project was supported by the NSF/REU grant AST-0354056 and the Nantucket Maria Mitchell Association.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.