2022MNRAS.510.1657C

Gaps imaged in protoplanetary discs are suspected to be opened by planets. We compute the present-day mass accretion rates {dot}M_p of seven hypothesized gap-embedded planets, plus the two confirmed planets in the PDS 70 disc. The accretion rates are based on disc gas surface densities Σ_gas from C¹⁸O observations, and planet masses M_p from simulations fitted to observed gaps. Assuming accretion is Bondi-like, we find in eight out of nine cases that {dot}M_p is consistent with the time-averaged value given by the current planet mass and system age, M_p/t_age. As system ages are comparable to circumstellar disc lifetimes, these gap-opening planets may be undergoing their last mass doublings, reaching final masses of M_p ∼10-10²M_⊕ for the non-PDS 70 planets, and M_p ∼1-10 M_J for the PDS 70 planets. For another 15 gaps without C¹⁸O data, we predict Σ_gas by assuming their planets are accreting at their time-averaged {dot}M_p. Bondi accretion rates for PDS 70b and c are orders of magnitude higher than accretion rates implied by measured U-band and H α fluxes, suggesting most of the accretion shock luminosity emerges in as yet unobserved wavebands, or that the planets are surrounded by dusty, highly extincting, quasi-spherical circumplanetary envelopes. Thermal emission from such envelopes or from circumplanetary discs, on Hill sphere scales, peaks at wavelengths in the mid-to-far-infrared and can reproduce observed mm-wave excesses.