Ion-Neutral Drift Velocity as a Diagnostic of Dust Growth and Magnetic Field in Star-Forming Environments

Recent observations have revealed that the ion-neutral drift velocity in star-forming molecular clouds and dense cores is on the order of 100 m s^-1. Theoretical studies have shown that, in ambipolar diffusion, the process responsible for the differential motion between ions and neutrals, the dust size distribution has a significant impact on the magnetic resistivities. In this study, we perform simulations to investigate how dust growth through accretion and coagulation affects the ion-neutral drift velocity in molecular clouds and cores. We find that, on core scales, both dust growth and a magnetic field strength of 200 microgauss are required to reproduce the observed drift velocity. We suggest that measurements of ion-neutral drift velocity, particularly on core scales, may serve as a new diagnostic to constrain the dust size distribution and magnetic field strength in such environments.

💡 Research Summary

This paper investigates how dust growth—through accretion of gas‑phase species and grain‑grain coagulation—affects the ion‑neutral drift velocity (v_drift) that arises from ambipolar diffusion in star‑forming molecular clouds and dense cores. Recent observations have measured drift speeds of order 100 m s⁻¹, yet theoretical work has shown that the magnetic resistivities, especially the ambipolar resistivity η_A, are highly sensitive to the abundance of small dust grains because they provide surfaces for ion and electron recombination. The authors therefore set out to quantify the impact of an evolving dust size distribution on η_A and on the resulting drift speed, and to assess whether measurements of v_drift can serve as a diagnostic of both the dust population and the magnetic field strength.

Methodology
Two representative environments are modeled: (1) a typical molecular cloud with gas number density n_gas = 10⁴ cm⁻³ and magnetic field strengths B = 20 µG and 50 µG, and (2) a dense core with n_gas = 10⁶ cm⁻³ and B = 100 µG and 200 µG. The initial dust size distribution follows the classic MRN power law (a⁻³·⁵) from 5 nm to 250 nm. Dust evolution is computed using the continuum equation of Hirashita & Aoyama (2019). Accretion of atomic oxygen (which quickly becomes H₂O on grain surfaces) is treated with a temperature‑dependent timescale τ(a) that scales linearly with grain radius. Coagulation is modeled by integrating over the grain mass spectrum with a collision kernel α(m₁,m₂) = σ₁,₂ ΔV, where σ₁,₂ = π(a₁ + a₂)² and ΔV combines Brownian motion and turbulence‑induced relative velocities (ΔV = √(ΔV_BM² + ΔV_turb²)). Turbulent Mach numbers M = 0.5, 1.0, 2.0 are explored; the authors adopt M = 2.0 as the fiducial case for the dense core.

Ionization chemistry follows the analytical framework of Tsukamoto & Okuzumi (2022) with modifications for cold, low‑density gas. The equilibrium equations include cosmic‑ray ionization (ζ_CR = 10⁻¹⁷ s⁻¹ or 10⁻¹⁶ s⁻¹), adsorption of ions and electrons onto grains, and gas‑phase recombination. Because the grain charge parameter τ ≪ 1, only charge states Z = −1, 0, +1 are retained. The authors solve the charge‑neutrality condition for the ratio ε = (n_i s_i u_i)/(n_e s_e u_e) using a Newton‑Raphson scheme, yielding ion, electron, and grain number densities as well as the mean grain charge ⟨Z⟩. Conductivities σ_O (Ohmic), σ_H (Hall), and σ_P (Pedersen) are then computed from the species’ cyclotron frequencies, collision rates with neutrals, and charge states. The ambipolar resistivity follows η_A = c²/(4π)