Direct evidence for two-fluid effects in molecular clouds

Abstract

We present a combination of theoretical and simulation-based examinations of the role of two-fluid ambipolar drift on molecular linewidths. The dissipation provided by ion-neutral interactions can produce a significant difference between the widths of neutral molecules and the widths of ionic species, comparable to the sound speed. We demonstrate that Alfvén waves and certain families of magnetosonic waves become strongly damped on scales comparable to the ambipolar diffusion scale. Using the riemann code, we simulate two-fluid turbulence with ionization fractions ranging from 10-2 to 10-6. We show that the wave damping causes the power spectrum of the ion velocity to drop below that of the neutral velocity when measured on a relative basis. Following a set of motivational observations by Li & Houde, we produce synthetic linewidth-size relations that show a difference between the ion and neutral linewidths, illustrating that two-fluid effects can have an observationally detectable role in modifying the MHD turbulence in the clouds. © 2010 The Authors. Journal compilation © 2010 RAS.