8,297 research outputs found
Field of Dreams
To the new executive director of the UNLV Research Foundation, the university’s planned research park is 115 acres of opportunity
Magnetic field gradient effects on the magnetorotational instability
The magnetorotational instability (MRI), also known as the Balbus -- Hawley
instability, is thought to have an important role on the initiation of
turbulence and angular momentum transport in accretion discs. In this work, we
investigate the effect of the magnetic field gradient in the azimuthal
direction on MRI. We solve the magnetohydrodynamic equations by including the
azimuthal component of the field gradient. We find the dispersion relation and
calculate the growth rates of the instability numerically. The inclusion of the
azimuthal magnetic field gradient produces a new unstable region on wavenumber
space. It also modifies the growth rate and the wavelength range of the
unstable mode: the higher the magnitude of the field gradient, the greater the
growth rate and the wider the unstable wavenumber range. Such a gradient in the
magnetic field may be important in T Tauri discs where the stellar magnetic
field has an axis which is misaligned with respect to the rotation axis of the
disc.Comment: 8 pages, 3 figures, accepted for publication in A
Revisiting the Value of Research
Sure, they’re interesting research stories. But what does research really mean to UNLV, to the community, and to you
End Notes
Several early faculty were committed to research at a relatively unlikely time in UNLV’s history. What can we learn from them
Instability of non-Keplerian warped discs
Many accretion discs are thought to be warped. Recent hydrodynamical
simulations show that (i) discs can break into distinct planes when the
amplitude of an imposed warp is sufficiently high and the viscosity
sufficiently low, and that (ii) discs can tear up into discrete rings when an
initially planar disc is subject to a forced precession. Previously, we
investigated the local stability of isolated, Keplerian, warped discs in order
to understand the physics causing an accretion disc to break into distinct
planes, finding that anti-diffusion of the warp amplitude is the underlying
cause. Here, we explore the behaviour of this instability in disc regions where
the rotation profile deviates from Keplerian. We find that at small warp
amplitudes non-Keplerian rotation can stabilize the disc by increasing the
critical warp amplitude for instability, while at large warp amplitudes
non-Keplerian rotation can lead to an increased growth rate for discs that are
unstable. Tidal effects on discs in binary systems are typically weak enough
such that the disc remains close to Keplerian rotation. However, the inner
regions of discs around black holes are strongly affected, with the smallest
radius at which the disc can break into discrete planes being a function of the
black hole spin. We suggest that interpreting observed frequencies in the power
spectra of light curves from accreting compact objects as nodal and apsidal
precession of discrete orbits requires an instability that can break the disc
into discrete rings such as the one explored here.Comment: 12 pages, 8 figures, accepted for publication in MNRA
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