2019MNRAS.489.3879B

A small subset of young stellar objects exhibit 'see-saw' temporal variations in their mid-infrared spectral energy distribution (SED); as the flux shortwards of a fulcrum wavelength (λ_f) increases, the flux longwards of this wavelength decreases (and vice versa) over time-scales of weeks to years. While previous studies have shown that an opaque, axisymmetric occulter of variable height can cause this behaviour in the SED of these objects, the conditions under which a single λ_f occurs have not previously been determined, nor the factors determining its value. Using radiative transfer modelling, we conduct a parametric study of the exemplar of this class, LRLL 31 to explore this phenomenon, and confirm that the cause of this flux variation is likely due to the change in height of the optically thick inner rim of the accretion disc at the dust sublimation radius, or some other phenomenon that results in a similar appearance. We also determine that a fulcrum wavelength only occurs for high inclinations, where the line of sight intersects the accretion disc. Accepting that the disc of LRLL 31 is highly inclined, the inner rim radius, radial and vertical density profiles are independently varied to gauge what effect this had on λ_f and its position relative to the silicate feature near 10 µm. While λ_f is a function of each of these parameters, it is found to be most strongly dependent on the vertical density exponent β. All other factors being held constant, only for flatter discs (β < 1.2) did we find a λ_f beyond the silicate feature.