3,269 research outputs found
OH-selected AGB and post-AGB stellar objects II.Blue versus red evolution off the AGB
Using objects found in a systematic survey of the galactic Plane in the
1612-MHz OH line, we discuss in detail two ``sequences'' of post-AGB evolution,
a red and a blue. We argue that the red and the blue groups separate by initial
mass at 4Msun, based on evolutionary-sequence turn-off colours, spectral energy
distributions, outflow velocities and scaleheight. The higher-mass (blue)
objects may have earlier AGB termination. The lower-mass (red) objects undergo
very sudden reddening for IRAS colour R21\sim1.2; these sources must all
undergo a very similar process at AGB termination. The transition colour
corresponds to average initial masses of 1.7Msun. A combined IRAS-MSX colour
proves a very sensitive tool to distinguish lower-mass, early post-AGB objects
from sources still on the AGB and also to distinguish more evolved post-AGB
objects from star-forming regions. The high-mass blue objects are the likely
precursors of bipolar planetary nebulae, whereas the low-mass red objects will
evolve into elliptical planetary nebulae.Comment: 12 pages, LaTex, 7 figures (1 colour), AJ (accepted
Self-similar decay of high Reynolds number Taylor-Couette turbulence
We study the decay of high-Reynolds number Taylor-Couette turbulence, i.e.
the turbulent flow between two coaxial rotating cylinders. To do so, the
rotation of the inner cylinder (Re, the outer cylinder is at
rest) is stopped within 12 s, thus fully removing the energy input to the
system. Using a combination of laser Doppler anemometry and particle image
velocimetry measurements, six decay decades of the kinetic energy could be
captured. First, in the absence of cylinder rotation, the flow-velocity during
the decay does not develop any height dependence in contrast to the well-known
Taylor vortex state. Second, the radial profile of the azimuthal velocity is
found to be self-similar. Nonetheless, the decay of this wall-bounded
inhomogeneous turbulent flow does not follow a strict power law as for decaying
turbulent homogeneous isotropic flows, but it is faster, due to the strong
viscous drag applied by the bounding walls. We theoretically describe the decay
in a quantitative way by taking the effects of additional friction at the walls
into account.Comment: 7 pages, 6 figure
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