Testing planet formation from the ultraviolet to the millimeter

Abstract

Gaps imaged in protoplanetary discs are suspected to be opened by planets. We compute the present-day mass accretion rates 𝑀 p of seven hypothesized gap-embedded planets, plus the two confirmed planets in PDS 70, by combining disc gas surface densities Σ gas from C 18 O observations with planet masses 𝑀 p from simulations fitted to observed gaps. Assuming accretion is Bondi-like, we find in eight out of nine cases that 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 − 10 2 𝑀 ⊕ for the non-PDS 70 planets, and 𝑀 p ∼ 1 − 10 𝑀 J for the PDS 70 planets. For another fifteen gaps without C 18 O data, we predict Σ gas by assuming their planets are accreting at their time-averaged 𝑀 p . Bondi accretion rates for PDS 70b and c are orders of magnitude higher than accretion rates implied by measured U-band and H𝛼 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. Key words: planets and satellites: formation -planets and satellites: general -planets and satellites: fundamental parametersprotoplanetary discs -planet-disc interactions 1 The gaps simulated by Zhang et al. (2018) and similar works are opened because of the repulsive Lindblad torques exerted by planets. These planetdisk simulations neglect accretion onto the planet (cf. Rosenthal et al. 2020 and references therein).