Episodic accretion in high-mass star formation: An analysis of thermal instability for axially symmetric disks

Context. Similar to their low-mass counterparts, high-mass young stellar objects (HMYSOs) exhibit episodic accretion bursts. Understanding the physical mechanisms behind these bursts is crucial for elucidating the early stages of massive star formation and the evolution of disks around high-mass protostars. Aims. This study aims to investigate the role of thermal instability in triggering accretion outbursts by developing a two-dimensional hydrodynamical model that fully resolves the vertical structure of the inner disk. Our goal is to provide a more realistic depiction of axially symmetric disk dynamics during these events and to assess the observable signatures of such bursts. Methods. We performed simulations of the inner 10 astronomical units of a circumstellar disk surrounding a high-mass protostar. The model we used incorporates heating from viscous dissipation and radiative transport in both the radial and vertical directions. Unlike previous one-dimensional studies, our two-dimensional axially symmetric study resolves the time-dependent vertical disk structure, capturing the complex interplay between radial and vertical dynamics within the disk. Results. Our simulations reveal that thermal instability leads to significant changes in the disk structure. In the inner regions, steep temperature gradients and vigorous convective motions develop at the onset of outbursts, with gas flows differing between the midplane and the upper disk layers rather than following a purely one-dimensional pattern. The energy released during the burst is distributed gradually throughout the disk, producing outbursts with durations of 15–30 years and peak mass accretion rates in the range of 2−3 × 10−4 M⊙ yr−1. Although these bursts are observable, they are insufficiently bright, and their rise times and overall profiles differ from some of the more rapid events seen in observations. Notably, our models also do not produce the weaker “reflares” that sometimes occur atop stronger outbursts in one-dimensional thermal instability calculations. Conclusions. Resolving the full vertical structure of the disk is essential for accurately modeling thermal instability outbursts in high-mass young stellar objects. While thermal instability significantly influences episodic accretion, our results suggest that it appears insufficient on its own to explain the full range of observed outburst phenomena in HMYSOs. Additional mechanisms seem to be required to fully explain the diversity of observed burst phenomena. Future studies incorporating further physical processes are needed to develop a comprehensive understanding of episodic accretion in massive star formation.<p></p>