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

TL;DR

This paper investigates how dust growth via accretion and coagulation influences ion–neutral drift in star-forming environments, with ambipolar diffusion linking dust microphysics to magnetic field evolution. Using self-consistent dust evolution and ionization chemistry in one-zone molecular cloud and dense-core models, the authors compute ambipolar resistivity $\eta_A$ and the resulting drift velocity $v_{\rm drift}$, finding that core-scale $v_{\rm drift} \sim 100$ m s$^{-1}$ is achievable only if grains grow substantially and the magnetic field is $\sim 2\times 10^2$ μG. They further show that cloud-scale drift is more degenerate with dust growth and $B$, limiting its diagnostic power at those scales. The results support using ion–neutral drift measurements as a diagnostic to constrain the dust size distribution and magnetic field strength in cores, consistent with observations like coreshine and the L1544 core, while highlighting the need for multi-dimensional, time-dependent modeling to capture realistic contraction and coagulation histories.

Abstract

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.

Figures (6)

• Figure 1: Relative velocity model of two colliding grains introduced in this study. The plot shows the case where the two grains have the same radius ($a_1 = a_2 = a$) and collide in a gas at $n_{\rm gas}=10^6\,\rm{cm^{-3}}$ with a Mach number of $\mathcal{M}=2$. $\Delta V_{\rm BM}$ represents the contribution from the Brownian motion, whereas $\Delta V_{\rm turb}$ represents the contribution from turbulent motion. $\Delta V = \sqrt{(\Delta V_{\rm BM})^2 + (\Delta V_{\rm turb})^2}$.
• Figure 2: (Left)The dust size distribution at the epoch of $1\times t_{\rm ff}$(light green), $3\times t_{\rm ff}$(middle green), $10\times t_{\rm ff}$(deep green). The initial size distribution is shown in gray. (Right)Dependence of the time evolution of ion-neutral drift velocity in core model on magnetic field strength. In addition to the case of $B=100\ \rm\mu G$(light purple) and $B=200\ \rm\mu G$(purple), we plot the case of $B=1000\ \rm\mu G$(gray) and $B=2000\ \rm\mu G$(red) for reference. The lower and upper horizontal axis show the evolution time in unit of $\rm yr$ and $t_{\rm ff}$, respectively. The observed $v_{\rm drift}$ is indicated by yellow-filled region. The pink line shows $v_{\rm drift}=50\ \rm ms^{-1}$. Three thin gray vertical lines correspond to $t=1,\ 3$ and $10\times\ t_{\rm ff}$.
• Figure 3: Dependency of estimated $v_{\rm drift}$ on (a)the cosmic ray ionization rate; $\zeta_{\rm CR}$ (b)the dominant ion species; $s_{\rm ion}$ in gas phase (c)the intrinsic grain material density; $\rho_{\rm mat}$ (d)the gas velocity; $v_{\rm gas}$, in cloud core model. The difference in each parameter is indicated by the line styles.
• Figure 4: Time evolution of dust size distribution(left) and ion-neutral drift velocity(right) for cloud model, shown in the same format as Figure \ref{['fig:core_vdrift_Bmag']}. The light-blue and blue lines represent magnetic field strengths of $20\ \rm \mu G$ and $50\ \rm \mu G$, respectively.
• Figure 5: The dependence of the time evolution of $v_{\rm drift}$ in the cloud model on (a) $\zeta_{\rm CR}$ (b) $s_{\rm ion}$ (c) $\rho_{\rm mat}$ (d) $v_{\rm gas}$, as in Figure \ref{['fig:core_vdrift_zeta_ion_MatDens_vg']}.