HD100453 is a notable protoplanetary disc where a binary companion drives a pair of prominent spiral arms, detectable using a number of dust and gas tracers. In Rosotti et al 2020 1 we demonstrated how the different pitch angles of the spirals traced in the submillimetre continuum and in scattered light can be used to probe the vertical temperature structure of the disc. The idea is simply that the pitch angle (angle between the spiral arm and the local tangential direction) measured using any particular diagnostic depends on the temperature of the region that produces this emission and is expected to be larger in warmer regions of the disc. Scattered light emission derives from small dust grains several pressure scale heights above the disc mid-plane while sub-millimetre continuum emission derives from large grains settled towards the disc mid- plane. Thus it is possible to use forward modeling to simulate discs with a variety of vertical temperature structures in order to reproduce the observed spiral morphologies.

The lefthand panel shows the spirals in HD100453 at 0.88 mm (blue contours) and in scattered light at 0.6 μm, demonstrating that the latter spirals are more open and thus likely to be associated with warmer conditions (the pitch angles are respectively 6° and 19°). We performed 3D hydrodynamical simulations with the FARGO3D code of a disc subject to the dynamical effects of the companion star (shown as asterisk in figure) and then post-processed these simulations to generate simulated observations using the Monte Carlo radiative transfer simulation, RADMC3D. The polar emission plots (right panel) clearly showed that it is impossible to reproduce the spiral morphologies in both tracers using a disc that is vertically isothermal and that instead the surface layers (from which the scattered light emission originates) has to be around four times hotter than the mid-plane regions.

This result is plausible given that the upper layers are exposed to direct irradiation from the central star. However, our simulations provide the first hydrodynamical demonstration for vertical temperature stratification in protoplanetary discs.