TURBULENT ENTRAINMENT IN MIRA'S COMETARY TAIL

Mira is a red giant star which passes through the Galactic ISM, leaving behind a cometary tail extending over similar to 2 degrees (seen in the spectacular UV image of Martin and coworkers). H I 21 cm emission from this cometary tail has now also been detected. The 21 cm emission shows a velocity that decreases monotonically with increasing distances from Mira (along the axis of the cometary tail). We present a turbulent wake model which gives a simple, analytic prediction for the velocity of the material in the wake as a function of distance from the stellar wind source. We fit this predicted velocity versus position law to the velocities deduced from the 21 cm observations, and find that a wake with a half-opening angle of approximate to 11.8 degrees (and a stellar motion and wind velocity/mass-loss rate corresponding to the values determined for Mira) reproduces the observations.