Thermal Wave Instability as an Origin of Gap and Ring Structures in Protoplanetary Disks

Abstract: Recent millimeter and infrared observations have shown that gap and ring-like structures are common in both dust thermal emission and scattered-light of protoplanetary disks. We investigate the impact of the so-called Thermal Wave Instability (TWI) on the millimeter and infrared scattered-light images of disks. We perform 1+1D simulations of the TWI and confirm that the TWI operates when the disk is optically thick enough for stellar light, i.e., small-grain-to-gas mass ratio of $\gtrsim0.0001$. The mid-plane temperature varies as the waves propagate and hence gap and ring structures can be seen in both millimeter and infrared emission. The millimeter substructures can be observed even if the disk is fully optically thick since it is induced by the temperature variation, while density-induced substructures would disappear in the optically thick regime. The fractional separation between TWI-induced ring and gap is $\Delta r/r \sim$ 0.2-0.4 at $\sim$ 10-50 au, which is comparable to those found by ALMA. Due to the temperature variation, snow lines of volatile species move radially and multiple snow lines are observed even for a single species. The wave propagation velocity is as fast as $\sim$ 0.6 ${\rm au~yr^{-1}}$, which can be potentially detected with a multi-epoch observation with a time separation of a few years.