Episodic accretion and mergers during growth of massive protostars

TL;DR

This study uses 3D and 1D simulations to explore how episodic accretion bursts in massive protostars are triggered by the dynamics of circumstellar disc fragments and their interactions, influenced by magnetic winds and stellar conditions.

Contribution

It provides new insights into the physical mechanisms behind accretion bursts, highlighting the roles of disc physics, magnetic winds, and object interactions in massive protostar growth.

Findings

High-density clumps crossing the inner boundary can cause observable bursts.

Magnetised winds enable migrating objects to produce bursts by crossing migration traps.

Object interactions can alter disc instabilities, affecting burst duration and luminosity.

Abstract

3D simulations of high mass young stellar object (HMYSO) growth show that their circumstellar discs fragment onto multiple self-gravitating objects. Accretion of these by HMYSO may explain episodic accretion bursts discovered recently. We post-process results of a previous 3D simulation of a HMYSO disc with a 1D code that resolves the disc and object dynamics down to the stellar surface. We find that burst-like deposition of material into the inner disc seen in 3D simulations by itself does not always signify powerful accretion bursts. Only high density post-collapse clumps crossing the inner computational boundary may result in observable bursts. The rich physics of the inner disc has a significant impact on the expected accretion bursts: (1) In the standard turbulent viscosity discs, migrating objects can stall at a migration trap at the distance of a few au from the star. However, in discs powered by magnetised winds, the objects are able to cross the trap and produce bursts akin to those observed so far. (2) Migrating objects may interact with and modify the thermal (hydrogen ionisation) instability of the inner disc, which can be responsible for longer duration and lower luminosity bursts in HMYSOs. (3) If the central star is bloated to a fraction of an au by a previous episode of high accretion rate, or if the migrating object is particularly dense, a merger rather than a disc-mediated accretion burst results; (4) Object disruption bursts may be super-Eddington, leading to episodic feedback on HMYSO surroundings via powerful outflows.