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

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 Rp ≳ 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.

Episodic accretion and mergers during growth of massive protostars

“…This is due to TI of the inner disc. These are relatively long duration (tens of years) periodic bursts, quite distinct in their characteristics from those of object disruption bursts (Elbakyan et al 2021).…”

“…At low enough mass accretion rates in the disc, this opacity/ionisation transition results in the well known hydrogen ionisation instability of discs, including cataclysmic variables and dwarf novae systems (Lin et al 1985). The inner region of the disc keeps switching between the low and the high stable branches, resulting in the significant accretion variability onto the star (e.g., Lodato & Clarke 2004;Elbakyan et al 2021). At higher accretion rates, the innermost region of the disc is permanently on the hot stable solution branch.…”

The burst mode of accretion in massive star formation with stellar inertia

“…The white hexagons are the burst measures from the lightcurves calculated on the basis of the accretion rate histories of our simulated protostars and the coloured symbols stand for real bursts of high-mass stars. The red pentagon represents the burst of NGC 6334I MM1, which duration has been constrained to t bst ≈ 40−130 yr (Hunter et al 2021) and the correspondingly accreted mass estimated to M bst ≈ 0.1−0.3 M (Hunter et al 2021;Elbakyan et al 2021). Similarly, the yellow diamond marks the burst of G358.93-0.003 MM1, which properties are t bst ≈ 0.75 yr and M bst ≈ 0.566 MJ (Stecklum et al 2021;Elbakyan et al 2021).…”

“…The red pentagon represents the burst of NGC 6334I MM1, which duration has been constrained to t bst ≈ 40−130 yr (Hunter et al 2021) and the correspondingly accreted mass estimated to M bst ≈ 0.1−0.3 M (Hunter et al 2021;Elbakyan et al 2021). Similarly, the yellow diamond marks the burst of G358.93-0.003 MM1, which properties are t bst ≈ 0.75 yr and M bst ≈ 0.566 MJ (Stecklum et al 2021;Elbakyan et al 2021). The blue circle and the green triangle stand for the bursts monitored from the young massive stellar object S255 NIRS 3 (t bst ≈ 2.25 yr, M bst ≈ 2 MJ), see Caratti o Garatti et al (2017); Elbakyan et al (2021), and that of the recently-discovered outbursts of M17 MIR (t bst ≈ 15 yr, M bst ≈ 31.43 MJ), see Chen et al (2021); Elbakyan et al (2021) and references therein.…”

“…During the burst itself, the accretion rate sharply increases, generating a swelling of the protostellar radius, which, in its turn, provokes rapid excursions to the cold part of the Herzsprung-Russell diagram and triggers the intermittency of the irradiation fluxes filling the H II region of a young high-mass star . Lastly, a parameter study exploring the properties and burst characteristics of a large collection of models for the formation of massive young stellar objects concluded that no massive protostars should form without bursts, which span over a bimodal distribution of short (∼ 1 − 10 yr) and long (∼ 10 3 − 10 4 yr) flares (Meyer et al 2021), see also Elbakyan et al (2021).…”