Hypersonic Turbulence of H₂O Masers

Summary

H₂O masers near young stars show turbulent motions of many times sound speed. These motions appear on scales of 1 to 300 AU, much smaller than the 10⁴ AU sizes of H₂O maser clusters. Turbulent velocity differences between the masers are typically 10 to 100 km s⁻¹, much larger than typical sound and Alfven speeds of ∼ 0.8 km s⁻¹. These velocity differences show the powerlaw correlation functions characteristic of fluid turbulence, over several orders of magnitude in separation. The index is close to that predicted by the Kolmogorov theory. Maser features also show internal turbulence, on scales of < 1 AU, consistent with Alfvenic turbulence.

Introduction

H₂O masers are among the most spectacular astrophysical masers; they are found near late-type stars, around galactic nuclei, and near young stars. Those near young stars are among the brightest and most numerous. As Strelnitski & Sunyaev (1973) first proposed, the strong winds from these stars accelerate and power the masers. The population inversion required for maser action arises at shocks, in the outflowing wind and where it meets ambient material (Litvak 1969, Strelnitski 1984, Elitzur, Hollenbach, & McKee 1989, Kaufman & Neufeld 1996). Each masing region contains between one and several hundred individual masing cloudlets, known as features.

The kinematics of clusters of H₂O masers have been studied in detail with very-long baseline interferometry (VLBI), in part because comparison of proper motions with Doppler shifts can yield trigonometric distances to these objects (Genzel et al. 1981, Reid et al. 1988, Gwinn, Moran, & Reid 1992).