The High Angular Resolution Measurement of Ion and Neutral Spectra as a Probe of the Magnetic Field Structure in DR 21(OH)

Abstract

It has been suggested that under average interstellar field strengths, the cyclotron interaction between ions and magnetic fields is strong enough to narrow the line width and suppress the line wings in the ion spectra. We present evidence of the cyclotron interaction effect at arcsec scales on the velocity dispersions in the spectra of ion/neutral molecular species in DR 21(OH) observed with the Owens Valley Radio Observatory Millimeter Array. Using a spatial resolution ~3 times higher than previous Caltech Submillimeter Observatory observations by Houde et al., we show that H13CO+ and H13CN are coexistent at the scale of our observations (6''). In the eastern parts of the DR 21(OH) core where the dynamics is simple, the ion line widths are indeed narrower than the neutral line widths with an average ion-to-neutral line width ratio of 0.82 ± 0.04. We use our results, along with the existing Zeeman and dust/CO polarization data on small scales, to derive the three-dimensional magnetic field structure. We obtain a field strength of 0.44 ± 0.12 mG with an inclination of 36° to the line of sight, directed toward the observer, and a position angle of -75° in the plane of the sky. With the full magnetic field strength derived here, we are able to conclude that the MM1 core of DR 21(OH) is magnetically supercritical, although turbulence provides the dominant support.