Planet formation around stars of various masses: Hot super-Earths

TL;DR

This paper investigates how different formation mechanisms, such as scattering and migration, influence the occurrence and characteristics of short-period super-Earths around stars of various masses, highlighting the role of stellar mass and metallicity.

Contribution

It presents a comprehensive model comparing planet-planet scattering and migration, revealing how stellar mass and disk properties affect super-Earth formation and distribution.

Findings

Short-period super-Earths are more common around low-mass stars due to migration.

There is an upper stellar mass limit (~1 Solar mass) for super-Earths formed by migration.

Planet frequency varies with metallicity depending on disk mass distribution.

Abstract

We consider trends resulting from two formation mechanisms for short-period super-Earths: planet-planet scattering and migration. We model scenarios where these planets originate near the snow line in ``cold finger'' circumstellar disks. Low-mass planet-planet scattering excites planets to low periastron orbits only for lower mass stars. With long circularisation times, these planets reside on long-period eccentric orbits. Closer formation regions mean planets that reach short-period orbits by migration are most common around low-mass stars. Above ~1 Solar mass, planets massive enough to migrate to close-in orbits before the gas disk dissipates are above the critical mass for gas giant formation. Thus, there is an upper stellar mass limit for short-period super-Earths that form by migration. If disk masses are distributed as a power law, planet frequency increases with metallicity because most disks have low masses. For disk masses distributed around a relatively high mass, planet frequency decreases with increasing metallicity. As icy planets migrate, they shepherd interior objects toward the star, which grow to ~1 Earth mass. In contrast to icy migrators, surviving shepherded planets are rocky. Upon reaching short-period orbits, planets are subject to evaporation processes. The closest planets may be reduced to rocky or icy cores. Low-mass stars have lower EUV luminosities, so the level of evaporation decreases with decreasing stellar mass.