Evidence for Ambipolar Diffusion from Ion-Neutral Velocity Drift in HH 212

Abstract

We present the Atacama Large Millimeter/submillimeter Array Band 7 observations of the hyperfine structure of C${}^{17}$O (3-2) and the H${}^{13}$CO${}^{+}$ (4-3) molecular line emissions of the Class 0 protostellar system HH 212 at $\sim$ 44 au spatial resolution. Radiative transfer models interpreted in the framework of non-ideal MHD of the envelope allow us to estimate an envelope mass of $M_{env}\sim 0.12M_{\odot }$ and a mass infall rate of ${\dot{M}}_{env}\sim 4.4-5.7\times 10^{-6}{M}_{\odot }{\mathrm{yr}}^{-1}$, consistent with values reported in the literature. The models also reveal a systematic radial velocity drift between the neutral tracer (C${}^{17}$O (3-2)) and the ionized species (H${}^{13}$CO${}^{+}$ (4-3)) of $0.1-0.3$ km s${}^{-1}$, exceeding the spectral resolution of 0.105 km s${}^{-1}$ and consistent with possible ambipolar diffusion in the envelope. The ambipolar diffusion fraction, $f_{AD}\sim 0.1-1$, and the Hall parameter, ${\beta }_{i,H_2}\gg 1$, indicate partial ion-neutral decoupling, with non-ideal effects becoming more significant toward the inner regions. In the envelope, our analysis shows that rotational energy decreases from $\sim 50\%$ of the gravitational energy at $R_{cen}$ to $\sim 25\%$ at $2R_{cen}$ and $\sim 4\%$ at 1100 au. In contrast, for $B_0=1$ mG, magnetic energy increases from $\sim 0.4\%$ of the gravitational energy at $R_{cen}$ to $\sim 2.6\%$ at $2R_{cen}$ and $\sim 20\%$ at 1100 au. The drift velocity expected from radial force balance is consistent with the modeled drift velocity within $\sim 300-400$ au of the central source but is lower at larger radii. These results are consistent with a scenario in which ambipolar diffusion leads to partial ion-neutral decoupling, allowing neutrals to move faster than ions within the envelope.

arXiv comment15 pages, 6 figures, accepted for publication in The Astrophysical Journals