The role of thermal instability in accretion outbursts in high-mass stars

Abstract

High-mass young stellar objects (HMYSOs) can exhibit episodic bursts of accretion, accompanied by intense outflows and luminosity variations. Understanding the underlying mechanisms driving these phenomena is crucial for elucidating the early evolution of massive stars and their feedback on star formation processes. Thermal instability (TI) due to hydrogen ionisation is among the most promising mechanisms of episodic accretion in low-mass ($M_* protostars. Its role in HMYSOs has not yet been determined. Here we investigate the properties of TI outbursts in young massive ($M_* 5$ msun ) stars, and compare them to those that have been observed to date. We employed a 1D numerical model to simulate TI outbursts in HMYSO accretion discs. We varied the key model parameters, such as stellar mass, mass accretion rate onto the disc, and disc viscosity, to assess the TI outburst properties. Our simulations show that modelled TI bursts can replicate the durations and peak accretion rates of long outbursts (a few years to decades) observed in HMYSOs with similar mass characteristics. However, they struggle with short-duration bursts (less than a year) with short rise times (a few weeks or months), suggesting the need for alternative mechanisms. Moreover, while our models match the durations of longer bursts, they fail to reproduce the multiple outbursts seen in some HMYSOs, regardless of model parameters. We also emphasise the significance of not just evaluating model accretion rates and durations, but also performing photometric analysis to thoroughly evaluate the consistency between model predictions and observational data. Our findings suggest that some other plausible mechanisms, such as gravitational instabilities and disc fragmentation, can be responsible for generating the observed outburst phenomena in HMYSOs, and we underscore the need for further investigation into alternative mechanisms driving short outbursts. However, the physics of TI is crucial in sculpting the inner disc physics in the early bright epoch of massive star formation, and comprehensive parameter space exploration; the use of 2D modelling is essential to obtaining a more detailed understanding of the underlying physical processes. By bridging theoretical predictions with observational constraints, this study contributes to advancing our knowledge of HMYSO accretion physics and the early evolution of massive stars.