Martin HoudeWestern Science

Probing the Turbulent Dissipation Range and Magnetic Field Strengths in Molecular Clouds

I introduced a new approach and technique for probing magnetic fields in star forming regions through the comparison of spectral lines from coexistent ion and neutral molecular species (Houde et al. 2000, ApJ, 536, 857; 2000 ApJ, 537, 245). A simple model and observational evidence were then put forth to explain the systematically detected ion line narrowing (compared to the neutral) as a signature of the magnetic field. In a recent collaboration with Dr. H. Li, we revisited my previous work and produced a refinement of the model that improved on the power and fruitfulness of this technique (Li & Houde 2008, ApJ, 677, 1151). We convincingly established that the aforementioned ion line narrowing effect arises through the so-called ambipolar diffusion of the turbulent component of the magnetic field. Ambipolar diffusion is the fundamental process by which, for example, magnetic field support against gravity is weakened with time in star-forming regions; a process that had been predicted more than 50 years ago in a landmark paper by L. Mestel and L. Spitzer (1956, MNRAS, 116, 503). We showed that ambipolar diffusion was responsible for the systematic downward shift of the ion turbulent power spectrum relative to that of the neutral species. Since ambipolar diffusion is characterized by the decoupling of ion and neutral motions at (small) dissipation scales, we also showed how this technique allows for the estimation of the ambipolar diffusion dissipation scale and the strength of the plane-of-the-sky component of the magnetic field.

Ambipolar Diffusion and Turbulent Magnetic Fields in Molecular Clouds

Fig. 1. Comparison of (left) HCN and HCO+, and (right) H13CN and H13CO+ (J = 4 → 3) spectral lines detected in M17. The relative narrowness of the molecular ions is clearly seen.


Ions vs. Neutrals

Fig. 2. Graph showing the line widths of HCO+ vs. HCN and H13CO+ vs. H13CN spectral lines detected in several turbulent molecular clouds. The straight line corresponds to a slope of unity for cases where the ions and neutrals would have the same line width. The narrowness of molecular ion lines is clearly seen from the absence of data points to the left of this straight line.