Constraint on ion–neutral drift velocity in the Class 0 protostar B335 from ALMA observations

¹ European Southern Observatory (ESO), Karl-chwarzschild-Str. 2, 85748 Garching, Germany
e-mail: hyen@eso.org
² Max-Planck-Institut für extraterrestrische Physik (MPE), 85748 Garching, Germany
³ Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, 10617 Taipei, Taiwan
⁴ Astronomy Department, University of Virginia, Charlottesville, VA 22904, USA
⁵ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720, USA
⁶ Department of Physics and Astronomy, Graduate School of Science and Engineering, Kagoshima University, 1 Chome-21-35 Kōrimoto, Kagoshima-shi, Kagoshima 890-0065, Japan

Received: 29 October 2017
Accepted: 21 March 2018

Abstract

Aims. Ambipolar diffusion can cause a velocity drift between ions and neutrals. This is one of the non-ideal magnetohydrodynamics (MHD) effects proposed to enable the formation of large-scale Keplerian disks with sizes of tens of au. To observationally study ambipolar diffusion in collapsing protostellar envelopes, we compare here gas kinematics traced by ionized and neutral molecular lines and discuss the implication on ambipolar diffusion.

Methods. We analyzed the data of the H¹³CO⁺ (3–2) and C¹⁸O (2–1) emission in the Class 0 protostar B335 obtained with our ALMA observations. We constructed kinematical models to fit the velocity structures observed in the H¹³CO⁺ and C¹⁸O emission and to measure the infalling velocities of the ionized and neutral gas on a 100 au scale in B335.

Results. A central compact (~1′′–2′′) component that is elongated perpendicular to the outflow direction and exhibits a clear velocity gradient along the outflow direction is observed in both lines and most likely traces the infalling flattened envelope. With our kinematical models, the infalling velocities in the H¹³CO⁺ and C¹⁸O emission 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.

Conclusions. 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 this source, 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. This result is consistent with the expectation from the MHD simulations with a typical ambipolar diffusivity and those without ambipolar diffusion. On the other hand, the high ambipolar drift velocity of 0.5–1.0 km s⁻¹ on a 100 au scale predicted in the MHD simulations with an enhanced ambipolar diffusivity by removing small dust grains, where the minimum grain size is 0.1 μm, is not detected in our observations. However, because of our limited angular resolution, we cannot rule out a significant ambipolar drift only in the midplane of the infalling envelope. Future observations with higher angular resolutions (~0. ′′1) are needed to examine this possibility and ambipolar diffusion on a smaller scale.