Possible Signatures of Magnetospheric Accretion onto Young Giant Planets

TL;DR

This paper explores how magnetospheric accretion could influence the formation of young gas giant planets during their isolation phase, predicting observable non-thermal signatures like gyrosynchrotron radiation.

Contribution

It extends stellar magnetospheric accretion models to planetary regimes, highlighting its potential role in planetary formation during the isolation phase.

Findings

Magnetospheric accretion may govern gas giant formation during the isolation phase.

Accretion disk truncation occurs at about 2.7 times Jupiter's radius.

Non-thermal emissions like gyrosynchrotron radiation are promising observational signatures.

Abstract

Magnetospheric accretion is an important process for a wide range of astrophysical systems, and may play a role in the formation of gas giant planets. Extending the formalism describing stellar magnetospheric accretion into the planetary regime, we demonstrate that magnetospheric processes may govern accretion onto young gas giants in the isolation phase of their development. Planets in the isolation phase have cleared out large gaps in their surrounding circumstellar disks, and settled into a quasi-static equilibrium with radii only modestly larger than their final sizes (i.e., $ r \sim 1.4 r_{\rm final}$). Magnetospheric accretion is less likely to play a role in a young gas giant's main accretion phase, when the planet's envelope is predicted to be much larger than the planet's Alfv\'en radius. For a fiducial 1 M$_J$ gas giant planet with a remnant isolation phase accretion rate of $\dot{M}_{\odot} =$ 10$^{-10} M_{\odot}{\rm yr}^{-1}=10^{-7}M_{J}{\rm yr}^{-1}$, the disk accretion will be truncated at $\sim 2.7r_J$ (with $r_J$ is Jupiter's radius) and drive the planet to rotate with a period of $\sim$7 hours. Thermal emission from planetary magnetospheric accretion will be difficult to observe; the most promising observational signatures may be non-thermal, such as gyrosynchrotron radiation that is clearly modulated at a period much shorter than the rotation period of the host star.