EXor outbursts from disk amplification of stellar magnetic cycles

TL;DR

This paper proposes a model where stellar magnetic cycles influence disk amplification, triggering EXor outbursts in protostellar disks through magnetic flux diffusion and the magnetorotational instability.

Contribution

It introduces a novel mechanism linking stellar dynamo cycles to disk outbursts via magnetic flux diffusion and dead zone dynamics.

Findings

Magnetic flux diffusion can trigger disk outbursts near the dead zone boundary.

Stellar magnetic activity correlates with observed EXor outbursts.

Changes in magnetic polarity may enhance outburst strength.

Abstract

EXor outbursts - moderate-amplitude disk accretion events observed in Class I and Class II protostellar sources - have time scales and amplitudes that are consistent with the viscous accumulation and release of gas in the inner disk near the dead zone boundary. We suggest that outbursts are indirectly triggered by stellar dynamo cycles, via poloidal magnetic flux that diffuses radially outward through the disk. Interior to the dead zone the strength of the net field modulates the efficiency of angular momentum transport by the magnetorotational instability. In the dead zone changes in the polarity of the net field may lead to stronger outbursts because of the dominant role of the Hall effect in this region of the disk. At the level of simple estimates we show that changes to kG-strength stellar fields could stimulate disk outbursts on 0.1 AU scales, though this optimistic conclusion depends upon the uncertain efficiency of net flux transport through the inner disk. The model predicts a close association between observational tracers of stellar magnetic activity and EXor events.