Metallicity of the Massive Protoplanets Around HR 8799 If Formed by Gravitational Instability

TL;DR

This study investigates the potential metallicity of massive protoplanets around HR 8799 formed by gravitational instability, focusing on their limited solid material accretion during their short pre-collapse phase.

Contribution

It quantifies the accretion of heavy elements in massive protoplanets based on their mass, location, and formation timescale, revealing they likely have near-stellar compositions.

Findings

Massive protoplanets have short pre-collapse timescales.

Limited solid accretion results in near-stellar compositions.

Accreted heavy elements are small for typical HR 8799 planets.

Abstract

The final composition of giant planets formed as a result of gravitational instability in the disk gas depends on their ability to capture solid material (planetesimals) during their 'pre-collapse' stage, when they are extended and cold, and contracting quasi-statically. The duration of the pre-collapse stage is inversely proportional roughly to the square of the planetary mass, so massive protoplanets have shorter pre-collapse timescales and therefore limited opportunity for planetesimal capture. The available accretion time for protoplanets with masses of 3, 5, 7, and 10 Jupiter masses is found to be 7.82E4, 2.62E4, 1.17E4 and 5.67E3 years, respectively. The total mass that can be captured by the protoplanets depends on the planetary mass, planetesimal size, the radial distance of the protoplanet from the parent star, and the local solid surface density. We consider three radial distances, 24, 38, and 68 AU, similar to the radial distances of the planets in the system HR 8799, and estimate the mass of heavy elements that can be accreted. We find that for the planetary masses usually adopted for the HR 8799 system, the amount of heavy elements accreted by the planets is small, leaving them with nearly stellar compositions.