smaller radius that corresponds to its new specific angular momentum. This process causes inward migration of material that belongs to the disc and leads to contraction of the disc. This derivation does not take into account interaction between adjacent rings, which is determined by viscous processes.

### Viscous redistribution

Although the nature of the viscous processes that occur in accretion discs are still debated [see e.g. @armitage11], we do know that they are responsible for transporting material inwards through the disc. When this happens, some material has to move outward to conserve the total angular momentum of the disc, $J_{\rm disc}$. Both viscous evolution and accretion of ISM cause material to drift inwards where it is eventually accreted onto the star and lost from the disc together with the angular momentum it carried. The outward spreading of material at the outer edge of the disc could make it more vulnerable to ram pressure stripping, which in turn also robs the disc of its angular momentum.

Dynamical encounters
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In dense stellar systems, disc radii have been shown to be truncated due to close stellar encounters [@breslau14; @rosotti14; @vincke15]. The last study shows that in dense clusters ($\bar{\rho}_{\rm cluster} \approx 500\,\mathrm{pc^{-3}}$), 