Stringent limits on the magnetic field strength in the disc of TW Hya
In one sentence
Deep ALMA observations of CN polarization toward TW Hya yielded the first real upper limits on the magnetic field strength inside a planet-forming disk, putting genuine observational pressure on theoretical models of how disks accrete onto their stars.
What’s the question?
Theory has long required protoplanetary disks to be magnetised: magnetic fields drive accretion onto the central star, launch winds, and influence where planets form. Yet there were essentially no direct measurements of the magnetic field inside a real disk. CN, a paramagnetic radical, was the strongest theoretical candidate for a polarized magnetic-field tracer in disks — but until this paper, no observation had been deep enough to set a meaningful constraint.
What did we do?
We obtained very sensitive ALMA observations of CN N=2-1 hyperfine transitions toward TW Hya, the closest and best-studied protoplanetary disk, and searched for the circular polarization that the Zeeman effect should produce. We found none — and the resulting upper limit on the line-of-sight field, |Bz| < 0.8 mG, is genuinely tight. We additionally detected marginal continuum dust polarization consistent with radiatively aligned grains.
Why does it matter?
Even a non-detection can be a strong test. Our limits are too low for some classical magnetised-accretion models that needed strong vertical fields, while remaining consistent with newer, weakly-magnetised wind-driven disk models. The paper set the observational ceiling against which subsequent theoretical and observational work — including the eventual first radially resolved measurement — has had to argue.
My role
Second author. Set up the CN Zeeman models used to derive the upper limits, and was closely involved in the interpretation of the data. Wrote the appendix.