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

Abstract

Aims. Ambipolar diffusion can cause a velocity drift between ions and neutrals. This is one of the non-ideal magnetohydrodynamcis (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 H 13 CO + (3-2) and C 18 O (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 H 13 CO + and C 18 O 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 H 13 CO + and C 18 O 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 H 13 CO + is estimated to be ≫1 on a 100 au scale in B335, so that H 13 CO + 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.

Collisional polarization of molecular ions: a signpost of ambipolar diffusion

“…They furthermore estimated the gas kinetic temperature around the protostar to vary as T ∼ 38 (r/100 au) −0.4 K (see also Evans et al 2015) and the hydrogen number density as n H 2 ∼ 9 × 10 6 (r/100 au) −2.1 cm −3 . As shown in Yen et al (2018), optically thin H 13 CO + transitions probe the midplane of the infalling envelope. Here we expect the magnetic field and ambipolar drift to be strongest.…”

“…ALMA observations by Yen et al (2018) were unable to show such a velocity drift between H 13 CO + and 18 CO in their line profiles toward B335. Instead, Yen et al (2018) constrained the ambipolar drift to be smaller than 0.3 km s −1 at scales of 100 au.…”

“…As a specific example, we considered the nearby starforming region B335 and estimated the viability of collisional polarization detection in the signal of HCO +1 . Yen et al (2018) recently constrained the ambipolar diffusion drift in this source to be <0.3 km s −1 at scales of 100 au. They furthermore estimated the gas kinetic temperature around the protostar to vary as T ∼ 38 (r/100 au) −0.4 K (see also Evans et al 2015) and the hydrogen number density as n H 2 ∼ 9 × 10 6 (r/100 au) −2.1 cm −3 .…”

“…Ambipolar diffusion is generally thought to be a prime regulatory agent in the process of star formation (Mouschovias & Ciolek 1999). However, ambipolar diffusion has yet to be confirmed through direct observations (Yen et al 2018).…”

“…Numerical magnetohydrodynamic (MHD) simulations predict the ambipolar drift to be ∼0.1−1 km s −1 at hundreds of au from the central protostar (Ciolek & Königl 1998;Li et al 2011;Zhao et al 2018). ALMA observations by Yen et al (2018) were unable to show such a velocity drift between H 13 CO + and 18 CO in their line profiles toward B335. Instead, Yen et al (2018) constrained the ambipolar drift to be smaller than 0.3 km s −1 at scales of 100 au.…”