Long-term Evolution of Protostellar and Protoplanetary Disks. I. Outbursts

TL;DR

This study uses simplified one-dimensional disk models to investigate the conditions leading to large accretion outbursts in protostellar disks, confirming the role of MRI activation and thermal instability in these phenomena.

Contribution

It demonstrates that one-dimensional models can effectively explore long-term disk evolution and outbursts, providing insights into the MRI activation conditions and dust sublimation effects.

Findings

Low-mass protostars can experience AU-scale outbursts due to infall rates.

High infall rates can lead to quasi-steady accretion with thermal instability variations.

MRI activation may be triggered by dust sublimation, influencing outburst behavior.

Abstract

As an initial investigation into the long-term evolution of protostellar disks, we explore the conditions required to explain the large outbursts of disk accretion seen in some young stellar objects. We use one-dimensional time-dependent disk models with a phenomenological treatment of the magnetorotational instability (MRI) and gravitational torques to follow disk evolution over long timescales. Comparison with our previous two-dimensional disk model calculations (Zhu et al. 2009b, Z2009b) indicates that the neglect of radial effects and two-dimensional disk structure in the one-dimensional case makes only modest differences in the results; this allows us to use the simpler models to explore parameter space efficiently. We find that the mass infall rates typically estimated for low-mass protostars generally result in AU-scale disk accretion outbursts, as predicted by our previous analysis (Zhu et al. 2009a,Z2009a). We also confirm quasi-steady accretion behavior for high mass infall rates if the values of $\alpha$-parameter for the magnetorotational instability is small, while at this high accretion rate convection from the thermal instability may lead to some variations. We further constrain the combinations of the $\alpha$-parameter and the MRI critical temperature, which can reproduce observed outburst behavior. Our results suggest that dust sublimation may be connected with full activation of the MRI. This is consistent with the idea that small dust captures ions and electrons to suppress the MRI. In a later paper we will explore both long-term outburst and disk evolution with this model, allowing for infall from protostellar envelopes with differing angular momenta.