The Endgame of Gas Giant Formation: Accretion Luminosity and Contraction Post-Runaway

Abstract

Giant planets are thought to form by runaway gas accretion onto solid cores. Growth must eventually stop running away, ostensibly because planets open gaps (annular cavities) in their surrounding discs. Typical models stop runaway by artificially capping the accretion rate and lowering it to zero over an arbitrarily short time-scale. In reality, post-runaway accretion persists as long as the disc remains. During this final and possibly longest phase of formation, when the planet is still emerging from the disc, its mass can more than double, and its radius contracts by orders of magnitude. By drawing from the theory of how gaps clear, we find that post-runaway accretion luminosities diverge depending on disc viscosity: luminosities fall in low-viscosity discs but continue to rise past runaway in high-viscosity discs. This divergence amounts to a factor of $10^2$ by the time the disc disperses. Irrespective of the specifics of how planets interact with discs, the observed luminosity and age of an accreting planet can be used to calculate its instantaneous mass, radius, and accretion rate. We perform this exercise for the planet candidates embedded within the discs orbiting PDS 70, HD 163296, and MWC 758, inferring masses of $1-10M_{\mathrm{J}}$, accretion rates of $0.1-10M_{\mathrm{J}}$/Myr, and radii of $1-10R_{\mathrm{J}}$. Our radii are computed self-consistently from the planet's concurrent contraction and accretion and do not necessarily equal the value of $2R_{\mathrm{J}}$ commonly assumed; in particular, the radius depends on the envelope opacity as $R\propto {\kappa }^{0.5}$.

Accepted to MNRAS

References in corpus (26)

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Cited by in corpus (15)

• Constraining the Nature of the PDS 70 Protoplanets with VLTI/GRAVITY
• Atmospheric Regimes and Trends on Exoplanets and Brown Dwarfs
• Keck/NIRC2 $L$'-Band Imaging of Jovian-Mass Accreting Protoplanets around PDS 70
• Heavy-metal Jupiters by major mergers: metallicity vs. mass for giant planets
• How Consumption and Repulsion Set Planetary Gap Depths and the Final Masses of Gas Giants
• The maximum accretion rate of a protoplanet: how fast can runaway be?
• Breaking the centrifugal barrier to giant planet contraction by magnetic disc braking
• ISPY-NACO Imaging Survey for Planets around Young stars. The demographics of forming planets embedded in protoplanetary disks
• Self-consistent modelling of the dust component in protoplanetary and circumplanetary disks: the case of PDS 70
• Inflated Eccentric Migration of Evolving Gas-Giants I: Accelerated Formation and Destruction of Hot and Warm Jupiters
• MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4-5 $\mu$m. II. Constraints on the mass and radius of the enshrouded planet PDS 70 b
• Testing planet formation from the ultraviolet to the millimeter
• Searching for Protoplanets around MWC 758 and MWC 480 in Br-$\gamma$ using Kernel Phase and SCExAO/CHARIS
• Dynamical stability of giant planets: the critical adiabatic index in the presence of a solid core
• Inflated Eccentric Migration of evolving gas giants II: Numerical methodology and basic concepts