Lankhaar, BoyBoyLankhaarVlemmings, WouterWouterVlemmingsSurcis, GabrieleGabrieleSurcisvan Langevelde, Huib JanHuib Janvan LangeveldeGroenenboom, Gerrit C.Gerrit C.Groenenboomvan der Avoird, AdAdvan der Avoird2020-10-122020-10-1220182397-3366http://hdl.handle.net/20.500.12386/27742Magnetic fields play an important role during star formation<SUP>1</SUP>. Direct magnetic field strength observations have proven particularly challenging in the extremely dynamic protostellar phase<SUP>2-4</SUP>. Because of their occurrence in the densest parts of star-forming regions, masers, through polarization observations, are the main source of magnetic field strength and morphology measurements around protostars<SUP>2</SUP>. Of all maser species, methanol is one of the strongest and most abundant tracers of gas around high-mass protostellar disks and in outflows. However, as experimental determination of the magnetic characteristics of methanol has remained largely unsuccessful<SUP>5</SUP>, a robust magnetic field strength analysis of these regions could hitherto not be performed. Here, we report a quantitative theoretical model of the magnetic properties of methanol, including the complicated hyperfine structure that results from its internal rotation<SUP>6</SUP>. We show that the large range in values of the Landé g factors of the hyperfine components of each maser line lead to conclusions that differ substantially from the current interpretation based on a single effective g factor. These conclusions are more consistent with other observations<SUP>7,8</SUP> and confirm the presence of dynamically important magnetic fields around protostars. Additionally, our calculations show that (nonlinear) Zeeman effects must be taken into account to further enhance the accuracy of cosmological electron-to-proton mass ratio determinations using methanol 9-12ELETTRONICOenArticle10.1038/s41550-017-0341-82-s2.0-85041358575000423482100015https://www.nature.com/articles/s41550-017-0341-82018NatAs...2..145LFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation