Probing the ion-neutral drift velocity toward the L1544 prestellar core

Context . 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 matter may decouple, leading to differences in the infalling velocities of ions and neutrals known as ambipolar diffusion. Aims . 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. Methods . We observed the deuterated N 2 D + ion and the neutral para-NH 2 D species toward 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. Results . We find a mean ion-neutral velocity difference of ∼0.05 km/s toward the core. We interpret the observed ion-neutral velocity difference in L1544 as a signature of ambipolar diffusion by comparing it with predictions from self-consistent calculations of ambipolar resistivity, including dust grain growth. 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. Conclusions . 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 toward 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.