Boy Lankhaar

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Characterization of methanol as a magnetic field tracer in star-forming regions

B. Lankhaar, W. Vlemmings, G. Surcis, H. J. van Langevelde, G. C. Groenenboom, A. van der Avoird · Nature Astronomy (2018)

In one sentence

We worked out, from first principles, how methanol responds to a magnetic field, and used the result to convert ten years of brilliant-but-loosely-interpreted maser observations into proper magnetic-field measurements at the cradles of massive stars.

What’s the question?

Massive stars are born deep inside dust-shrouded gas clouds, opaque to optical telescopes but lit up at radio wavelengths by masers — natural amplifiers in molecules like methanol that flag the immediate surroundings of forming stars. The circular polarization of methanol masers had been observed for years and was believed to encode the local magnetic-field strength via the Zeeman effect. But converting “fraction of polarization” into “milligauss” requires knowing how the molecule actually responds to a field — and methanol, with its complicated internal rotation and tangled hyperfine structure, had never been characterised quantitatively.

What did we do?

I built a quantum-chemical model of methanol’s Zeeman effect from first principles, treating the internal-rotation problem properly, and tabulated a complete set of Zeeman coefficients for the maser transitions astronomers actually use. I then went back to a decade of published circular-polarization observations toward high-mass star-forming regions and reanalysed them with the new coefficients in hand.

Why does it matter?

Methanol masers are now a quantitative probe of magnetic fields around forming massive stars, not just a qualitative tracer. The reanalysis tightened the field strengths reported in the literature — in many cases substantially — and put the use of methanol as a magnetic-field tracer on a firm footing for the first time.

My role

First author. Performed the quantum-chemical modelling, did the astronomical reanalysis, and wrote the paper.

In the press