Vertical magnetic field strength (Bz) from accretion disc models, compared with the 1σ limit obtained in our ALMA observations (Bz < 0.8 mG, indicated by the arrow). The solid line is the T Tauri model from Shu et al. (2007). The dotted line is the maximum field strength Bz, max from Okuzumi et al. (2014), taking a magnetic field strength at the outer edge of the disc (Rout = 230 au) of B∞ = 25 μG. The red long dashed line assumes B∞ = 1 mG, which corresponds to the field strength measured in proto-stellar cores and dense parts of molecular clouds, and it includes efficient advection with D = 0.025 (see text). At R < 230 au, the latter two models are identical. The solid circle shows the surface magnetic field measured on TW Hya (e.g., Sokal et al. 2018).
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