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.
Highlights
FU Orionis-type stars, or FUors, are low-mass young stellar objects (LMYSOs) that show accretion outbursts in which the accretion rate onto the young stellar object (YSO) rises from M ∗ ∼ (10−7−10−8) M yr−1 to (10−5−10−4) M yr−1 for a few tens to 100 yr (Audard et al 2014)
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 M ∗ ∼ 10−4 M yr−1
We examine which scenarios proposed in the literature so far to explain accretion bursts of LMYSOs can apply to the episodic accretion in high-mass young stellar objects (HMYSOs)
Summary
FU Orionis-type stars, or FUors, are low-mass young stellar objects (LMYSOs) that show accretion outbursts in which the accretion rate onto the young stellar object (YSO) rises from M ∗ ∼ (10−7−10−8) M yr−1 to (10−5−10−4) M yr−1 for a few tens to 100 yr (Audard et al 2014). The planet itself may be tidally disrupted at these radii, flooding the inner disc with ‘new matter’, simultaneously producing outbursts with requisite properties (Nayakshin & Lodato 2012) while avoiding the ‘too many hot Jupiters’ conundrum. Planets made by the classical core accretion scenario are too dense for this to work, but planets made by the gravitational instability (GI) in the outer massive disc (Rafikov 2005) migrate inwards very rapidly and can power FUor-like flares (Vorobyov & Basu 2005, 2010; Boley et al 2010). When a sufficient amount of material accumulates in the DZ to heat it up, magnetorotational instability (MRI) develops, producing bursts with properties similar to those observed (Armitage et al 2001; Zhu et al 2009, 2010; Kadam et al 2020; Vorobyov et al 2020). Physical conditions in the circumstellar discs of HMYSOs are very much different from those in their low-mass counterparts, providing an exciting opportunity to break the degeneracy between the existing FUor models
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