Turbulence in HII regions: New results

Summary

A synopsis of results stemming from the analysis of the radial velocity fluctuation fields of 6 HII regions (Sh 142, M 17, Sh 158, Sh 170, Orion and Sh 212) are presented. In addition new data from the DENSITY fluctuation fields of the HII region Sh 269 will also be shown. The analysis was done using the well known two-point correlation functions. However I innovated by using the higher order structure functions on the Sh 269 data. PDF increment calculations were also done hinting at the presence of intermittency in Sh 269.

Introduction

HII regions were the first interstellar objects where scale dependent brightness and velocity fluctuations were identified and attributed to turbulent motions (von Hoerner 1951; Courtes 1955; Münch 1958). The study of turbulent motions in HII regions was then forgotten for many years until the work of Roy & Joncas (1985) and of O'Dell and collaborators later on. The discovery of such motions in HII regions should not come as a surprise. These objects possess large scale velocity gradients that are explained by ionized gas flows produced by the erosion of the parent molecular cloud. The newly born massive stars produce the necessary UV photon flux. Turbulence becomes a natural companion of the kinematics of HII regions since the ionized gas flows can reach twice the speed of sound enabling the Reynolds number to reach high values (ℜ > 10⁵).