Testing planet formation from the ultraviolet to the millimeter

TL;DR

This study estimates the current mass accretion rates of planets in protoplanetary discs, suggesting many are nearing their final masses and highlighting discrepancies with observed accretion signatures, implying hidden luminosity or dusty environments.

Contribution

It provides the first comprehensive analysis of accretion rates for multiple gap-embedded planets using observational data and models, offering insights into their growth and circumplanetary environments.

Findings

Most planets' accretion rates are consistent with their current mass and age.

PDS 70 planets may be nearing their final masses.

Discrepancies between accretion rates and observed luminosities suggest hidden or obscured accretion processes.

Abstract

Gaps imaged in protoplanetary discs are suspected to be opened by planets. We compute the present-day mass accretion rates $\dot{M}_{\rm p}$ 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 $\Sigma_{\rm gas}$ from C$^{18}$O observations, and planet masses $M_{\rm p}$ from simulations fitted to observed gaps. Assuming accretion is Bondi-like, we find in eight out of nine cases that $\dot{M}_{\rm p}$ is consistent with the time-averaged value given by the current planet mass and system age, $M_{\rm p}/t_{\rm age}$. As system ages are comparable to circumstellar disc lifetimes, these gap-opening planets may be undergoing their last mass doublings, reaching final masses of $M_{\rm p} \sim 10-10^2 \, M_\oplus$ for the non-PDS 70 planets, and $M_{\rm p} \sim 1-10 \, M_{\rm J}$ for the PDS 70 planets. For another fifteen gaps without C$^{18}$O data, we predict $\Sigma_{\rm gas}$ by assuming their planets are accreting at their time-averaged $\dot{M}_{\rm p}$. Bondi accretion rates for PDS 70b and c are orders of magnitude higher than accretion rates implied by measured U-band and H$\alpha$ 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.