Accreting planets as dust dams in `transition' discs

Abstract: We investigate under what circumstances an embedded planet in a protoplanetary disc may sculpt the dust distribution such that it observationally presents as a transition' disc. We concern ourselves withtransition' discs that have large holes ($\gtrsim 10$ AU) and high accretion rates ($\sim 10^{-9}-10^{-8}$ M$_\odot$ yr$^{-1}$). Particularly, those discs which photoevaporative models struggle to explain. Assuming the standard picture for how massive planets sculpt their parent discs, along with the observed accretion rates in transition' discs, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small ($s\lesssim 1\mu$m) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disc with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of atransition' disc (NIR dip in the SED, mm cavity and high accretion rate). At suitably high planet masses ($\gtrsim 3-4$ M$_J$), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disc that is optically thin (vertically and radially) to Infra-Red radiation. We use our models to calculate synthetic observations and present a observational evolutionary scenario for a forming planet, sculpting its parent disc. The planet-disc system will present as a `transition' disc with a dip in the SED, only when the planet mass and planetary accretion rate is high enough. At other times it will present as a disc with a primordial SED, but with a cavity in the mm, as observed in a handful of protoplanetary discs.