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

James Cadman,Ken Rice,Cassandra Hall

doi:10.1093/mnras/stab905

Abstract

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 time-scales are typically Myr whilst formation through GI will occur within the first ≈104–105 yr of disc evolution. We focus our analysis on the 4–13MJup planet observed at R ≈ 30 au. We find CA formation time-scales 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 Md,crit = 0.3 M⊙. Viscous evolution models of the disc’s mass history indicate that it was likely massive enough to exceed Md,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 1.6–13.3MJup, consistent with the planets that have been observed. We therefore propose that the inferred planets in the disc surrounding AB Aurigae may be evidence of planet formation through GI.

Highlights

Most of the known exoplanets are believed to have formed in discs of gas and dust around young stars
In this paper we focus on the formation history of planet P1 (CA vs. gravitational instability (GI)), and whether it is possible that the AB Aurigae system could be evidence of planet formation through GI
Planet formation begins with a phase of core growth, which may either be slow or rapid depending on the local planetesimal surface density

Summary

Introduction

Most of the known exoplanets are believed to have formed in discs of gas and dust around young stars. Owing to recent advances in high resolution infrared (IR) imaging we are capable of observing the planet formation process taking place Observations of these discs have revealed substructures indicative of the presence of planetary companions, such as rings (ALMA Partnership et al 2015; Andrews et al 2016; Avenhaus et al 2018; Bertrang et al 2018; Dipierro et al 2018; Huang et al 2018a), gaps (Andrews et al 2011; Perez et al 2015; Ginski et al 2016; van Boekel et al 2017) and spirals (Garufi et al 2013; Grady et al 2013; Benisty et al 2015; Pérez et al 2016; Tang et al 2017; Huang et al 2018b; Dong et al 2018), and recently it has even become possible to directly image giant protoplanets forming (Keppler et al 2018; Müller et al 2018; Haffert et al 2019; Boccaletti et al 2020).

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