Instability of mass transfer in a planet-star system

TL;DR

This paper investigates the instability of mass transfer in planet-star systems, revealing that angular momentum exchange differs from binary stars and analyzing conditions for dynamical mass transfer across various planet types.

Contribution

It demonstrates that the angular momentum transfer mechanism in binary stars does not apply to Roche-lobe filling planets and explores the conditions leading to dynamical mass transfer.

Findings

Mass transfer in planets differs from binary stars due to angular momentum absorption by the host star.

Gas giant planets can undergo dynamical mass transfer depending on their mass and host star properties.

Rocky planets experience sensitive dependence on the equation of state, with smaller planets undergoing a phase of dynamical mass transfer.

Abstract

We show that the angular momentum exchange mechanism governing the evolution of mass transferring binary stars does not apply to Roche-lobe filling planets, because most of the angular momentum of the mass transferring stream is absorbed by the host star. Apart from a correction for the difference in specific angular momentum of the stream and the centre of mass of the planet, the orbit does not expand much on Roche-lobe overflow. We explore the conditions for dynamically unstable Roche-lobe overflow as a function of planet mass and mass and radius (age) of host star and equation of state of planet. For a Sun-like host, gas giant planets in a range of mass and entropy can undergo dynamical mass transfer. Examples of the evolution of the mass transfer process are given. Dynamic mass transfer of rocky planets depend somewhat sensitively on equation of state used. Silicate planets in the range $1 M_{\oplus} <M_{\mathrm{p}} < 10 M_{\oplus} $ typically go through a phase of dynamical mass transfer before settling to slow overflow when their mass drops to less than $1 M_{\oplus}$.