Kinematics of neutral and ionzied gas in the candidate protostar with efficient magnetic braking B335

Abstract

Ambipolar diffusion can cause a velocity drift between ions and neutrals. This is one of the non-ideal MHD effects proposed to enable the formation of large Keplerian disks with sizes of tens of au (Zhao et al. 2018). To observationally study ambipolar diffusion in collapsing protostellar envelopes, we analyzed the ALMA H¹³CO⁺ (3–2) and C¹⁸O (2–1) data of the protostar B335, which is a candidate source with efficient magnetic braking (Yen et al. 2015). We constructed kinematical models to fit the velocity structures observed in H¹³CO⁺ and C¹⁸O. With our kinematical models, the infalling velocities in H¹³CO⁺ and C¹⁸O are both measured to be 0.85 ± 0.2 km s⁻¹ at a radius of 100 au, suggesting that the velocity drift between the ionized and neutral gas is at most 0.3 km s⁻¹ at a radius of 100 au in B335. The Hall parameter for H¹³CO⁺ is estimated to be ≫1 on a 100 au scale in B335, so that H¹³CO⁺ is expected to be attached to the magnetic field. Our non-detection or upper limit of the velocity drift between the ionized and neutral gas could suggest that the magnetic field remains rather well coupled to the bulk neutral material on a 100 au scale in B335, and that any significant field-matter decoupling, if present, likely occurs only on a smaller scale, leading to an accumulation of magnetic flux and thus efficient magnetic braking in the inner envelope in B335.