Molecular line polarisation from the circumstellar envelopes of asymptotic giant branch stars
In one sentence
ALMA observations of polarized molecular line emission in the circumstellar envelopes of two AGB stars (CW Leo and R Leo), modelled in 3-D, reveal the morphology of the magnetic field that is carried outward by the stellar wind.
What’s the question?
Asymptotic giant branch (AGB) stars are dying low-mass stars whose dusty winds enrich the Galaxy with the elements out of which planets and people are made. Magnetic fields are suspected to play a major role in launching and shaping these winds, but observations of the field on the relevant scales are sparse and hard. Does the polarization of molecular lines, properly modelled, give us the field?
What did we do?
We obtained ALMA observations of several molecular lines toward CW Leo (the carbon-rich workhorse of AGB studies) and R Leo, and detected linear polarization at the few-percent level in multiple lines. We then modelled the data using PORTAL, accounting jointly for the Goldreich-Kylafis effect — magnetic alignment, in which the magnetic field sets the polarization direction — and its competitor, radiative alignment, in which the radiation field does. The two regimes hand over to one another at a critical magnetic-field strength, and seeing the spatial transition between them across the envelope let us constrain the field strength in an entirely novel way. In CW Leo, the recovered field varies with distance from the star; in R Leo, the outer field appears to be carried by the wind itself.
Why does it matter?
Mass-loss from AGB stars is a major Galactic recycling channel. Mapping the magnetic field across an entire envelope — not just at the inner regions where masers light up — is a substantial methodological step toward understanding how that wind is launched and shaped.
My role
Co-author. Contributed the radiative-transfer machinery (PORTAL) used to interpret the polarization, and the underlying alignment theory.