Magnetically sustained photoevaporative gaps

Determine whether a photoevaporative gap that becomes increasingly magnetised under the closure $B_z\propto\Sigma^q$ can continue launching a magnetothermal wind while the gap is forming, rather than only after the gap has become effectively empty.

Background

For q<1/2q<1/2, gas depletion lowers the plasma beta because the magnetic field declines more slowly than the surface density. The model therefore predicts that residual gas inside a photoevaporative gap may revert to magnetothermal launching instead of losing its wind-driving capability as in a purely thermal model.

The calculated outcome depends on whether magnetic flux is retained or lost: the magnetothermal region expands during gap formation in the flux-retained branch but recedes in the flux-loss branch. The paper explicitly notes that its vertically integrated treatment cannot determine whether the increasingly magnetised residual gas actually launches a wind.

References

The present vertically-integrated treatment does not resolve the wind's vertical structure, so whether a gap so magnetised actually launches is not settled here.

— Bridging magnetothermal winds and photoevaporation to model discs dispersal  (2609.18334 - Picogna et al., 16 Sep 2026) in Section 7.7, “Magnetically sustained photoevaporative gaps”

Whether a thermal wind then sweeps the residual disc is itself debated \citep{owen2013,haworth2016}, so we treat it as a possible, not a guaranteed, terminal channel.

— Bridging magnetothermal winds and photoevaporation to model discs dispersal  (2609.18334 - Picogna et al., 16 Sep 2026) in Section 7.8, “External photoevaporation and the imposed thermal structure”