Forming Gas Giants Around a Range of Protostellar M-dwarfs by Gas Disk Gravitational Instability

TL;DR

This study demonstrates that gravitational instability in gaseous disks can form gas giant planets around M-dwarfs, especially in disks with masses above 0.02 solar masses, challenging core-accretion explanations.

Contribution

It provides a comprehensive set of 162 models showing gas giant formation via gravitational instability around M-dwarfs, highlighting the critical disk mass needed.

Findings

Gas giants can form around M-dwarfs via gravitational instability.

Disks with mass ≥ 0.02 solar masses are capable of forming gas giants.

The formation depends on disk mass, temperature, and stellar mass.

Abstract

Recent discoveries of gas giant exoplanets around M-dwarfs (GEMS) from transiting and radial velocity (RV) surveys are difficult to explain with core-accretion models. We present here a homogeneous suite of 162 models of gravitationally unstable gaseous disks. These models represent an existence proof for gas giants more massive than 0.1 Jupiter masses to form by the gas disk gravitational instability (GDGI) mechanism around M-dwarfs for comparison with observed exoplanet demographics and protoplanetary disk mass estimates for M-dwarf stars. We use the Enzo 2.6 adaptive mesh refinement (AMR) 3D hydrodynamics code to follow the formation and initial orbital evolution of gas giant protoplanets in gravitationally unstable gaseous disks in orbit around M-dwarfs with stellar masses ranging from 0.1 $M_\odot$ to 0.5 $M_\odot$. The gas disk masses are varied over a range from disks that are too low in mass to form gas giants rapidly to those where numerous gas giants are formed, therefore revealing the critical disk mass necessary for gas giants to form by the GDGI mechanism around M-dwarfs. The disk masses vary from 0.01 $M_\odot$ to 0.05 $M_\odot$ while the disk to star mass ratios explored range from 0.04 to 0.3. The models have varied initial outer disk temperatures (10 K to 60 K) and varied levels of AMR grid spatial resolution, producing a sample of expected gas giant protoplanets for each star mass. Broadly speaking, disk masses of at least 0.02 $M_\odot$ are needed for the GDGI mechanism to form gas giant protoplanets around M-dwarfs.