AB Aurigae: Possible evidence of planet formation through the gravitational instability

TL;DR

This study investigates whether the giant planets observed around AB Aurigae formed via gravitational instability, using simulations and disc evolution models to support the hypothesis given the system's young age and observed planet characteristics.

Contribution

The paper provides evidence that the planets around AB Aurigae likely formed through gravitational instability, contrasting with core accretion models, supported by hydrodynamic simulations and disc mass analysis.

Findings

Disc mass likely exceeded critical fragmentation threshold

Fragments could initially form with masses 1.6-13.3 M_Jup

Observed planets' properties are consistent with GI formation

Abstract

Recent observations of the protoplanetary disc surrounding AB Aurigae have revealed the possible presence of two giant planets in the process of forming. The young measured age of $1-4$Myr for this system allows us to place strict time constraints on the formation histories of the observed planets. Hence we may be able to make a crucial distinction between formation through core accretion (CA) or the gravitational instability (GI), as CA formation timescales are typically Myrs whilst formation through GI will occur within the first $\approx10^4-10^5$yrs of disc evolution. We focus our analysis on the $4-13$M$_{\rm Jup}$ planet observed at $R\approx30$AU. We find CA formation timescales for such a massive planet typically exceed the system's age. The planet's high mass and wide orbit may instead be indicative of formation through GI. We use smoothed particle hydrodynamic simulations to determine the system's critical disc mass for fragmentation, finding $M_{\rm d,crit}=0.3$M$_{\odot}$. Viscous evolution models of the disc's mass history indicate that it was likely massive enough to exceed $M_{\rm d,crit}$ in the recent past, thus it is possible that a young AB Aurigae disc may have fragmented to form multiple giant gaseous protoplanets. Calculations of the Jeans mass in an AB Aurigae-like disc find that fragments may initially form with masses $1.6-13.3$M$_{\rm Jup}$, consistent with the planets which have been observed. We therefore propose that the inferred planets in the disc surrounding AB Aurigae may be evidence of planet formation through GI.