The generation and transformation of polarization signals in molecular lines through collective anisotropic resonant scattering
In one sentence
When many molecules in a gas cloud act collectively, rather than as independent absorbers, they can convert one type of polarization into another — a quietly powerful mechanism that probably shapes a number of puzzling molecular-line polarization signals in the interstellar medium.
What’s the question?
The textbook picture of light passing through a molecular gas treats each molecule as an independent absorber and emitter. But in a sufficiently dense, sufficiently ordered region, the molecules can act together as a diffraction ensemble, scattering light coherently in the forward direction. This is anisotropic resonant scattering (ARS). What happens to polarization when ARS is active?
What did we do?
We built the formal radiative-transfer theory of ARS in molecular lines and tracked, component by component, what it does to each Stokes parameter. The headline results: ARS can convert linear polarization to circular (a Faraday-like conversion), it can rotate polarization angles (a Faraday-like rotation), and — most strikingly — it can generate polarization from light that arrives unpolarized. We also computed how those signatures depend on the magnetic-field configuration, giving observers a recipe for spotting ARS in real data.
Why does it matter?
If ARS is at work in even a fraction of the molecular regions where unusual polarization signatures have been reported, then attributing those signatures directly to local emission-region polarization — and therefore to local magnetic fields — is wrong. The framework also raises the productive flip-side question: can ARS itself be turned into a diagnostic of dense molecular gas?
My role
Co-author. Helped sharpen the underlying theory of resonance scattering.