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

TL;DR

This study uses a two-dimensional hydrodynamical model to analyze thermal instability in high-mass protostellar disks, revealing its role in episodic accretion bursts and emphasizing the importance of vertical structure resolution for accurate modeling.

Contribution

It introduces a 2D model that fully resolves vertical disk structure, providing new insights into thermal instability's impact on episodic accretion in high-mass star formation.

Findings

Thermal instability causes 15-30 year outbursts with high accretion rates.

Vertical structure resolution reveals complex radial-vertical dynamics during outbursts.

Observed bursts' brightness and timescales are inconsistent with thermal instability alone.

Abstract

High-mass young stellar objects exhibit episodic accretion bursts similar to their low-mass counterparts. Understanding these outbursts is crucial for elucidating massive star formation and disk evolution around high-mass protostars. We investigate thermal instability's role in triggering accretion outbursts using a two-dimensional hydrodynamical model that fully resolves the vertical structure of the inner disk. This approach provides a more realistic depiction of axially symmetric disk dynamics and assesses observable burst signatures. We simulate the inner 10 astronomical units of a circumstellar disk around a high-mass protostar, incorporating viscous heating and radiative transport in radial and vertical directions. Unlike previous one-dimensional studies, our two-dimensional model resolves time-dependent vertical disk structure, capturing complex radial-vertical dynamics. Our simulations show thermal instability causes significant structural changes. Steep temperature gradients and vigorous convection develop at outburst onset, with gas flows differing between midplane and upper layers. Energy release produces 15-30 year outbursts with peak accretion rates of $2-3\times10^{-4}~\rm M_{\odot}~\rm{yr}^{-1}$. While observable, these bursts are insufficiently bright with rise times differing from rapid observed events. Our models lack the "reflares" seen in one-dimensional calculations. Resolving full vertical disk structure is essential for accurate thermal instability modeling. While thermal instability significantly influences episodic accretion, it appears insufficient alone to explain observed HMYSO outburst diversity. Additional mechanisms are required for comprehensive understanding.