English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We present unstratified 3D MHD simulations of an accretion disk with aboundary layer (BL) that have a duration $\sim 1000$ orbital periods at theinner radius of the accretion disk. We find the surprising result that angularmomentum piles up in the boundary layer, which results in a rapidly rotatingbelt of accreted material at the surface of the star. The angular momentumstored in this belt increases monotonically in time, which implies that angularmomentum transport mechanisms in the BL are inefficient and do not couple theaccretion disk to the star. This is in spite of the fact that magnetic fieldsare advected into the BL from the disk and supersonic shear instabilities inthe BL excite acoustic waves. In our simulations, these waves only carry asmall fraction ($\sim 10 \%$) of the angular momentum required for steady stateaccretion. Using analytical theory and 2D viscous simulations in the $R-\phi$plane, we derive an analytical criterion for belt formation to occur in the BLin terms of the ratio of the viscosity in the accretion disk to the viscosityin the BL. Our MHD simulations have a dimensionless viscosity ($\alpha$) in theBL that is at least a factor of $\sim 100$ smaller than that in the disk. Wediscuss the implications of these results for BL dynamics and emission.

Inefficient Angular Momentum Transport in Accretion Disk Boundary Layers: Angular Momentum Belt in the Boundary Layer