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In the cold clouds of gas and dust where stars will one day ignite, some atoms have lost electrons and become charged. Charged particles feel the magnetic field, while neutral ones do not, so the charged ones slip through the neutral ones like water through a sieve. Astronomers have measured this drift speed: about a hundred meters per second. That's no small feat — faster than a race car.

But why exactly that speed? It's all about the dust grains. Tiny specks scattered through the cloud latch onto charged particles and slow them down, like seaweed slows a boat. The larger the grains, the stronger the friction. And the dust in clouds isn't idle; it grows. Atoms from the gas stick to it, and grains collide and merge, like a snowball rolling down a hill.

The scientists built a computer model and tested how dust growth affects the drift. It turned out that growth alone isn't enough. To get the measured hundred meters per second, you also need a strong magnetic field — about ten times stronger than previously thought for such clouds. Only when the model included both grown dust and a stronger field did the speed match the observations.

This means that a simple measurement of the drift speed can tell you what's hidden in the cloud: how big the dust is and how strong the magnetic field is. Like how the sound of an engine can tell you a car is climbing a hill with a heavy trailer.

Original abstract

as written by the authors arXiv:2602.01333

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.