Core-assisted gas capture instability: a new mode of giant planet formation by gravitationally unstable discs

Giant planet formation in the core accretion plus gas capture (CA) paradigm is predicated by the formation of a core, assembled by the coagulation of grains and later by planetesimals within a protoplanetary disc. As the core mass increases beyond a critical value, the hydrogen-dominated atmosphere around the core becomes self-gravitating and collapses on to the core, triggering rapid gas accretion which can lead to the formation of a gaseous planet. In contrast, in the disc instability paradigm, giant planet formation is believed to be independent of core formation: massive self-gravitating gas fragments cool radiatively and collapse as a whole independently of whether there is a core. In this paper, we show that giant planet formation in the disc instability model may be also enhanced by core formation for reasons physically very similar to the CA paradigm. In the model explored here, efficient grain sedimentation within an initial fragment (rather than the disc) leads to the formation of a core composed of heavy elements. We find that massive atmospheres form around cores and undergo collapse as a critical core mass is exceeded, analogous to CA theory. The critical mass of the core to initiate such a collapse depends on the fragment mass and metallicity, as well as core luminosity, but ranges from less than 1 to as much as similar to 80 Earth masses. We therefore suggest that there are two channels for the collapse of a gaseous fragment to planetary scales within the disc instability model: (i) H-2 dissociative collapse of the entire gaseous clump and (ii) core-assisted gas capture, as presented here. We suggest that the first of these two is favoured in metal-poor environments and for fragments y similar or equal to 5-10 Jupiter masses, whereas the second is favoured in metal-rich environments and fragments of lower mass.