Oral Presentation
Accretion bursts in high-mass protostars
Presenter: Vardan Elbakyan (University of Leicester)
Recent observations of high-mass young stellar objects (HMYSOs) with masses M∗≥10M⊙ uncovered outbursts with accretion rates exceeding ～10−3M⊙ yr−1. We utilise 1D time-dependent models of protoplanetary discs around HMYSOs to study burst properties. 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 (MRI) activation instabilities. The outbursts triggered by these instabilities, however, always have too low accretion rates 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.
Outflows and radiation feedback is a well-known challenge to formation of young massive stars. Disc mediated accretion is one way which may reduce these feedback effects substantially. 3D simulations show that these discs fragment onto multiple self-gravitating objects that migrate towards the central massive star rapidly, possibly merging with it. However, numerical limitations so far precluded resolving the inner tens of AU in such simulations. We model the disc and migrating object dynamics in this innermost region, aiming to determine their fate. We post-process results of a previous 3D simulation of a high-mass young stellar object (HMYSO) disc growth with a 1D code that couples migrating objects to the disc evolution self-consistently. We find that the fate of migrating objects depends strongly on the physical size of the HMYSO. For compact HMYSOs, with radii smaller than ～10 AU, migrating objects are tidally disrupted, producing powerful disc mediated bursts similar in nature, but much brighter than the FU Ori outbursts of low-mass YSOs. These bursts may be so bright as to exceed the HMYSO Eddington limit, ensuring that a good fraction of the disrupted object mass is launched into powerful outflows. On the other hand, migrating objects end up merging with HMYSOs that are bloated to size R≥100R⊙.
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