Probing the ion-neutral drift velocity towards the L1544 prestellar core: Detection of ambipolar diffusion using N$_2$D$^+$ and para-NH$_2$D

TL;DR

This study detects ion-neutral drift velocities in a prestellar core, providing observational evidence of ambipolar diffusion and highlighting the influence of dust grain growth on core collapse dynamics.

Contribution

First observational detection of ambipolar diffusion in a prestellar core through ion-neutral velocity differences, linking dust grain growth to magnetic decoupling.

Findings

Ion-neutral velocity difference of ~0.05 km/s observed in L1544.

No significant linewidth difference detected between ions and neutrals.

Dust grain growth influences ambipolar resistivity and core evolution.

Abstract

The dynamical role of the magnetic field in the star formation process is tightly linked to the coupling between matter and the field. This coupling is due to the interaction between ions and neutrals in the partially ionized interstellar medium. When the ionization degree drops in the dense environment of prestellar cores, the magnetic field and the matter may decouple, leading to differences in the infalling velocities of ions and neutrals known as ambipolar diffusion. The onset of gravitational collapse resulting from ion-neutral decoupling has never been observed. The aim of this work is to search for signatures of ambipolar diffusion within a prestellar core. We observed the deuterated N$_2$D$^+$ ion and the neutral para-NH$_2$D species towards the prototypical prestellar core L1544. These two species are ideal tracers of prestellar cores sampling the same high densities in the core interior. We compared the velocity centroid and linewidth maps of the ion-neutral pair. We find a mean ion-neutral velocity difference of $\sim$0.05 km/s towards the core. By comparing with predictions from self-consistent calculations of the ambipolar resistivity including dust grain growth, we interpret the observed ion-neutral velocity difference in L1544 as a signature of ambipolar diffusion. We do not detect a significant ion-neutral linewidth difference that may be attributed to the subsonic infall motions of the gas in L1544 and geometrical effects in the presence of inclination. These results emphasize the role of dust grain growth at the prestellar core stage in setting the ambipolar resistivity and regulating the dynamical evolution of dense cores towards their collapse into protostars. We propose that measurements of ion-neutral drift velocities provide new constraints on the total magnetic field strength and the dust size distribution within prestellar cores.