Long-Lived Eccentricities in Accretion Disks

Abstract

Accretion disks can be eccentric: they support $m=1$ modes that are global and slowly precessing. But whether the modes remain trapped in the disk---and hence are long-lived---depends on conditions at the outer edge of the disk. Here we show that in disks with realistic boundaries, in which the surface density drops rapidly beyond a given radius, eccentric modes are trapped and hence long-lived. We focus on pressure-only disks around a central mass, and show how this result can be understood with the help of a simple second-order WKB theory. We show that the longest lived mode is the zero-node mode in which all of the disk's elliptical streamlines are aligned, and that this mode decays coherently on the viscous timescale of the disk. Hence such a mode, once excited, will live for the lifetime of the disk. It may be responsible for asymmetries seen in recent images of protoplanetary disks.

Submitted to ApJ. Comments are welcomed

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• Probing the eccentricity in protostellar discs -- Modeling kinematics and morphologies
• Ring Formation in Protoplanetary Disks Driven by an Eccentric Instability
• Spiral Arms in Broad-line Regions of Active Galactic Nuclei. I. Reverberation and Differential Interferometric Signals of Tightly Wound Cases
• Discs are born eccentric
• An eccentric wave in the circumstellar disc of the Be/X-ray binary X Persei
• Insights from Analytical Theory of Eccentric Circumbinary Disks