tokens: ‘truncation’ of the disc could have shrunk the dust disc to its tiny radius. On the other hand, the SED is also compatible with some amount of large dust outside of 0.6 au. For this scenario, radial ‘migration’ coupled with fragmentation interior to the H 2 O snowline (located at 0.6 au) result in a cliff in Σ dust of two orders of magnitude and in a sudden increase of grain sizes. Our modelling shows that this scenario matches the observed SED for outer dust disc radii of R dust, 2 ≲ 45 au. In both scenarios, a dust mass inside of 0.6 au bigger than ∼1 M ⊕ is required to match the observed SED, however due to the optically thick nature of the dust, we do not obtain an upper limit on the dust mass. In the ‘migration’ scenario, the dust mass needs to be fine-tuned to ∼1 M ⊕ inside the H 2 O water snowline in order to match the SED as the dust masses inside and outside of 0.6 au are dependent on each other. In order to distinguish further between these scenarios, the gas radius of the disc can offer important information. If it is truncated at very small radii as well, a dynamical truncation of the disc seems likely. If it is more extended, radial drift in combination with dust physics at the water snowline offer a plausible explanation. We present predictions for CO J = 2–1 fluxes, invoking various M gas – R gas combinations, that will enable future observations to potentially distinguish between these two scenarios. Finally, we note that regardless of the underlying mechanism for the small-scale dust disc, its optically thick nature suggests that several M ⊕ of dust can be located within a small radial extent from the central star. Such a configuration suggests that V410 X-ray 1 may be a potential precursor to a system of tightly packed terrestrial planets, such as the recently discovered TRAPPIST-1 system. APPENDIX A: DUST PROCESSING AT THE WATER SNOWLINE In Section 4.2 , we study whether the truncated dust disc could be related to physical processes happening at the water snowline. The key property that changes across the snowline is frag , the maximum collision velocity of grains for which coagulation, rather than fragmentation, is to be expected. Following Blum Wurm ( 2000 ) and Gundlach Blum ( 2015 ), frag is reduced by an order of magnitude interior to the snowline. If grain growth is limited by fragmentation, the local value of frag is linked to the maximum expected grain size through equating the relative collision velocity Δ for the largest grains at any location with frag . Δ for grains that are coupled to a turbulent flow via drag forces is given by (Voelk et al. 1980 ; Ormel Cuzzi 2007 ): where α is the Shakura–Sunyaev viscosity parameter (Shakura Sunyaev 1973 ), c s the sound speed, and St is the local Stokes number (ratio of drag time to dynamical time) for the largest grains. In the Epstein drag regime, the midplane St for grains of size a max is given bywhere we have assumed a Gaussian vertical structure. When growth is
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