Hydrogen-losing planets in transition discs around young protostars

We point out that protoplanets created in the framework of the tidal downsizing (TD) theory for planet formation play a very important role for the evolution of accretion discs hosting them. Since all TD protoplanets are initially as massive as ∼10MJ, they are able to open very deep gaps in their discs, and even completely isolate the inner disc flows from the outer ones. Furthermore, in contrast to other planet formation theories, TD protoplanets are mass donors for their protostars. One potentially observable signature of planets being devoured by their protostars are FU Ori like outbursts, and episodic protostar accretion more generally, as discussed by a number of authors recently. Here, we explore another observational implication of the TD hypothesis: dust poor inner accretion flows, which we believe may be relevant to some of the observed millimetre-bright transitional discs around protostars. In our model, a massive protoplanet interrupts the flow of the outer dust-rich disc on its protostar, and at the same time loses a part of its dust-poor envelope into the inner disc. This then powers the observed gas-but-no-dust accretion on to the star. Upon a more detailed investigation, we find that this scenario is quite natural for young massive discs but is less so for older discs, e.g., those whose self-gravitating phase has terminated a fraction of a Myr or more ago. This stems from the fact that TD protoplanets of such an age should have contracted significantly, and so are unlikely to lose much mass. Therefore, we conclude that either (i) the population of ‘transition discs’ with large holes and dust-poor accretion is much younger than generally believed, or (ii) there is a poorly understood stage for late removal of dust-poor envelopes from TD planets; (iii) another explanation for the observations is correct.