Letter to the Editor

Accretion bursts in high-mass protostars: A new test bed for models of episodic accretion

¹ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
e-mail: vardan.elbakyan@leicester.ac.uk
² Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia
³ University of Vienna, Department of Astrophysics, Vienna 1180, Austria
⁴ Ural Federal University, 51 Lenin Str., 620051 Ekaterinburg, Russia
⁵ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

Received: 24 March 2021
Accepted: 16 June 2021

Abstract

Aims. It is well known that low-mass young stellar objects (LMYSOs) gain a significant portion of their final mass through episodes of very rapid accretion, with mass accretion rates up to Ṁ_∗ ∼ 10⁻⁴ M_⊙ yr⁻¹. Recent observations of high-mass young stellar objects (HMYSOs) with masses M_∗ ≳ 10 M_⊙ uncovered outbursts with accretion rates exceeding Ṁ_∗ ∼ 10⁻³ M_⊙ yr⁻¹. Here, we examine which scenarios proposed in the literature so far to explain accretion bursts of LMYSOs can also apply to the episodic accretion in HMYSOs.

Methods. We utilise 1D time-dependent models of protoplanetary discs around HMYSOs to study burst properties.

Results. We find that discs around HMYSOs are much hotter than those around their low-mass cousins. As a result, a much more extended region of the disc is prone to the thermal hydrogen ionisation and magnetorotational activation instabilities. The former, in particular, is found to be ubiquitous in a very wide range of accretion rates and disc viscosity parameters. The outbursts triggered by these instabilities, however, always have too low of an Ṁ_∗ and are one to several orders of magnitude too long compared to those observed from HMYSOs to date. On the other hand, bursts generated by tidal disruptions of gaseous giant planets formed by the gravitational instability of the protoplanetary discs yield properties commensurate with observations, provided that the clumps are in the post-collapse configuration with planet radius R_p ≳ 10 Jupiter radii. Furthermore, if observed bursts are caused by disc ionisation instabilities, then they should be periodic phenomena with the duration of the quiescent phase comparable to that of the bursts. This may yield potentially observable burst periodicity signatures in the jets, the outer disc, or the surrounding diffuse material of massive HMYSOs. Bursts produced by disruptions of planets or more massive objects are not expected to be periodic phenomena, although multiple bursts per protostar are possible.

Conclusions. Observations and modelling of episodic accretion bursts across a wide range of young stellar object (YSO) masses is a new promising avenue to break the degeneracy between models of episodic accretion in YSOs.