Collisional polarization of molecular ions: a signpost of ambipolar diffusion
In one sentence
Molecular ions colliding with neutral hydrogen during ambipolar diffusion don’t bump into them isotropically — and that anisotropy aligns the ions, polarizing their emission, providing the most direct observational signature yet of a process that has been theoretically central to star formation since 1956 but observationally invisible.
What’s the question?
Stars form when self-gravitating clumps of gas collapse, but the magnetic field threading those clumps initially supports them against gravity. To allow collapse, the field has to “let go.” In partially-ionised molecular gas, only the small fraction of ions are tightly tied to the field; the bulk neutral gas can drift slowly relative to them. This ambipolar diffusion is the canonical mechanism that lets gravity finally win — and it has been a cornerstone of the standard star-formation theory for nearly seventy years. Yet despite all that, it has never been directly observed: previous attempts have relied on detecting the ion-neutral velocity drift, which is masked by chemical and turbulent confounders.
What did we do?
I worked out, theoretically, that during ambipolar drift the collisions between molecular ions (HCO⁺ as the canonical example) and H₂ have a preferred direction — set by the drift velocity itself. That preferred direction populates the rotational levels of the ion anisotropically: certain orientations are favoured. Aligned ions emit polarized light. I derived how the strength of the resulting polarization depends on the drift velocity, the field geometry, and the local conditions, and showed that the signal — a linear polarization perpendicular to the projected magnetic field — should be detectable with ALMA toward dense protostellar regions.
Why does it matter?
Where previous diagnostics tried to detect the ion-neutral velocity drift indirectly (and were defeated by chemistry and turbulence), collisional polarization detects the drift directly in the polarization of a single line — it is robust against precisely the systematics that plague the alternatives. It is now the most sensitive observational route to ambipolar diffusion in real sources.
My role
First author. Conceptualised the collisional-polarization mechanism, set up the toy model that grounds the predictions, and wrote the paper.