The diverse lives of massive protoplanets in self-gravitating discs

Abstract

Gas giant planets may form early-on during the evolution of protostellar discs, while these are relatively massive. We study how Jupiter-mass planet-seeds (termed protoplanets) evolve in massive, but gravitationally stable (Q > ∼ 1.5), discs using radiative hydrodynamic simulations. We find that the protoplanet initially migrates inwards rapidly, until it opens up a gap in the disc. Thereafter, it either continues to migrate inwards on a much longer timescale or starts migrating outwards. Outward migration occurs when the protoplanet resides within a gap with gravitationally unstable edges, as a high fraction of the accreted gas is high angular momentum gas from outside the protoplanet's orbit. The effect of radiative heating from the protoplanet is critical in determining the direction of the migration and the eccentricity of the protoplanet. Gap opening is facilitated by efficient cooling that may not be captured by the commonly used β-cooling approximation. The protoplanet initially accretes at a high rate (∼ 10 −3 M J yr −1 ), and its accretion luminosity could be a few tenths of the host star's luminosity, making the protoplanet easily observable (albeit only for a short time). Due to the high gas accretion rate, the protoplanet generally grows above the deuterium-burning mass-limit. Protoplanet radiative feedback reduces its mass growth so that its final mass is near the brown dwarf-planet boundary. The fate of a young planet-seed is diverse and could vary from a gas giant planet on a circular orbit at a few AU from the central star to a brown dwarf on an eccentric, wide orbit.

Forming Pop III binaries in self-gravitating discs: how to keep the orbital angular momentum

“…Such an "outside-dominant" accretion widens the binary orbit. Similar phenomena have been reported in recent studies on both the planetary and BH migrations (e.g., Stamatellos & Inutsuka 2018;Muñoz et al 2019).…”

“…The above results suggest that the outward migration is only expected with a massive gas disk, or in an early evolutionary stage of the protostellar accretion where the gas supply from the cloud envelope still continues. This is also consistent with previous studies on the planetary migration, for which a gravitationally stable disk is normally assumed (e.g., Stamatellos & Inutsuka 2018). The similar outward migration has been reported but looks modest in comparison to our fiducial cases presented in the main part.…”

“…The binary separation turns to increase, and becomes almost constant of 10-20 AU in ∼ 100 years. Such an outward migration has been also reported in previous studies on the star/planet formation (e.g., Lin & Papaloizou 2012;Zhu et al 2012;Nayakshin 2017b;Stamatellos & Inutsuka 2018). We investigate this phenomenon later in Section 3.3.…”

The Planetary Accretion Shock. II. Grid of Postshock Entropies and Radiative Shock Efficiencies for Nonequilibrium Radiation Transport

“…The other main outcome is the amount of radiation reaching the Hill sphere. This should be useful input for studies of the thermo-chemical feedback of planets on the local protoplanetary disc, for instance as in a number of recent papers (Cleeves et al 2015;Cridland et al 2017;Stamatellos & Inutsuka 2018;Rab et al 2019). Within the simplification of a spherical accretion geometry, our results show that essentially all of the accretion shock luminosity is expected to reach the local nebula, and that a high Rosseland optical depth, at least up to ∆τ R ∼ 10, does not lead to significant extinction of the bolometric shock luminosity in the accretion flow.…”

Constraining the initial planetary population in the gravitational instability model

“…This would act in a degenerate way with steepening the initial mass function for fragmentation. Gravitational instability has often been invoked as a mechanism for forming brown dwarfs (Kratter et al 2010;Stamatellos & Inutsuka 2018;Moe et al 2018) though this has not yet been implemented in a universal population synthesis model. These planets would need to undergo a runaway accretion phase since we know the occurrence rate of 2-75M J objects is rare at wide separations (Vigan et al 2017).…”