Talk detail

Participant	Nath, Sujit Kumar
Institution	Indian Institute of Science - C. V. Raman Avenue - Bangalore - Karnataka - India

Session	AC2	Accepted	Yes	Order	8	Time	17:10	20'
Talk	Oral abstract	Title	Origin of instability in astrophysical accretion disks: faster growth rate than magnetorotational instability
Coauthors	Nath, Sujit Kumar; Mukhopadhyay, Banibrata
Abstract	In this talk I shall address a very famous problem regarding the infall of matter in astrophysical accretion disks which are ubiquitous in astrophysics. The particular emphasis will be on the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, which are extensively seen in astrophysics, are Rayleigh stable, but must be turbulent in order to explain observed data. Since molecular viscosity is negligible in these systems, for very large astrophysical length scale, people have argued for turbulent viscosity for energy dissipation and hence to explain infall of matter. The primary theme of this talk is how these accretion disks can be made turbulent at the first place to give rise to turbulent viscosity. A very famous concept known as magnetorotational instability (MRI) has already been proposed to be a viable solution of this problem. However there are several drawbacks of MRI. MRI does not work for neutral disks which have very weak or zero magnetic field and also it does not work for the disks with strong magnetic field. I shall discuss the importance of transient growth over MRI to produce turbulence in accretion disks. Transient growth is more generic kind of energy growth which exists in all kind of accretion disks whether it is charged or neutral. It can be shown that for high Reynolds number flows (which are indeed the case for astrophysical accretion disks) transient growth can make the system nonlinear much faster than MRI and can be a plausible primary source of turbulence. We have found transient growth favorable perturbation modes which grows faster than the fastest growing MRI mode. We have also explicitly measured the strength of magnetic field for which accretion disks are stable under linear perturbation. This work provides a generic alternative route to turbulence and a clear picture of the linear instability of accretion disks.
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