6,383 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
Azimuthal velocity profiles in Rayleigh-stable Taylor-Couette flow and implied axial angular momentum transport
We present azimuthal velocity profiles measured in a Taylor-Couette
apparatus, which has been used as a model of stellar and planetary accretion
disks. The apparatus has a cylinder radius ratio of , an
aspect-ratio of , and the plates closing the cylinders in the
axial direction are attached to the outer cylinder. We investigate angular
momentum transport and Ekman pumping in the Rayleigh-stable regime. The regime
is linearly stable and is characterized by radially increasing specific angular
momentum. We present several Rayleigh-stable profiles for shear Reynolds
numbers , both for
(quasi-Keplerian regime) and (sub-rotating regime)
where is the inner/outer cylinder rotation rate. None of the
velocity profiles matches the non-vortical laminar Taylor-Couette profile. The
deviation from that profile increased as solid-body rotation is approached at
fixed . Flow super-rotation, an angular velocity greater than that of
both cylinders, is observed in the sub-rotating regime. The velocity profiles
give lower bounds for the torques required to rotate the inner cylinder that
were larger than the torques for the case of laminar Taylor-Couette flow. The
quasi-Keplerian profiles are composed of a well mixed inner region, having
approximately constant angular momentum, connected to an outer region in
solid-body rotation with the outer cylinder and attached axial boundaries.
These regions suggest that the angular momentum is transported axially to the
axial boundaries. Therefore, Taylor-Couette flow with closing plates attached
to the outer cylinder is an imperfect model for accretion disk flows,
especially with regard to their stability.Comment: 22 pages, 10 figures, 2 tables, under consideration for publication
in Journal of Fluid Mechanics (JFM
Infrared Classification of Galactic Objects
Unbiased analysis shows that IRAS data reliably differentiate between the
early and late stages of stellar evolution because objects at these stages
clearly segregate in infrared color-color diagrams. Structure in these diagrams
is primarily controlled by the density distribution of circumstellar dust. The
density profile around older objects is the steepest, declining as ,
while young objects have profiles that vary as and flatter. The
different density profiles reflect the different dynamics that govern the
different environments. Our analysis also shows that high mass star formation
is strongly concentrated within \about 5 kpc around the Galactic center, in
support of other studies.Comment: 11 pages, 3 Postscript figures (included), uses aaspp4.sty. To appear
in Astrophysical Journal Letter
Experimental and numerical study of SiON microresonators with air and polymer cladding
A systematic experimental and numerical study of the device performance of waveguide-coupled SiON microresonators with air and polymer cladding is presented. Values of device parameters like propagation losses of the microresonator modes, the off-resonance insertion losses, and the straight waveguide to microresonator coupling are determined by applying a detailed fitting procedure to the experimental results and compared to results of detailed numerical simulations. By comparing the propagation losses of the fundamental TE polarized microresonator mode obtained by fitting to the measured spectra to the also experimentally determined propagation losses in the adjacent straight waveguide and the materials losses, it is possible to identify the loss mechanisms in the microresonator. By comparing experimental results for microresonators with air and polymethylmethacrylate cladding and a detailed numerical study, the influence of the cladding index on the bend losses is evaluated. It is demonstrated that the presence of an upper cladding can, under the right conditions, actually be beneficial for loss reduction
Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow
We investigate the existence of multiple turbulent states in highly turbulent
Taylor-Couette flow in the range of to ,
by measuring the global torques and the local velocities while probing the
phase space spanned by the rotation rates of the inner and outer cylinder. The
multiple states are found to be very robust and are expected to persist beyond
. The rotation ratio is the parameter that most strongly
controls the transitions between the flow states; the transitional values only
weakly depend on the Taylor number. However, complex paths in the phase space
are necessary to unlock the full region of multiple states. Lastly, by mapping
the flow structures for various rotation ratios in a Taylor-Couette setup with
an equal radius ratio but a larger aspect ratio than before, multiple states
were again observed. Here, they are characterized by even richer roll structure
phenomena, including, for the first time observed in highly turbulent TC flow,
an antisymmetrical roll state.Comment: 9 pages, 7 figure
Taylor-Couette turbulence at radius ratio : scaling, flow structures and plumes
Using high-resolution particle image velocimetry we measure velocity
profiles, the wind Reynolds number and characteristics of turbulent plumes in
Taylor-Couette flow for a radius ratio of 0.5 and Taylor number of up to
. The extracted angular velocity profiles follow a log-law more
closely than the azimuthal velocity profiles due to the strong curvature of
this setup. The scaling of the wind Reynolds number with the Taylor
number agrees with the theoretically predicted 3/7-scaling for the classical
turbulent regime, which is much more pronounced than for the well-explored
case, for which the ultimate regime sets in at much lower Ta. By
measuring at varying axial positions, roll structures are found for
counter-rotation while no clear coherent structures are seen for pure inner
cylinder rotation. In addition, turbulent plumes coming from the inner and
outer cylinder are investigated. For pure inner cylinder rotation, the plumes
in the radial velocity move away from the inner cylinder, while the plumes in
the azimuthal velocity mainly move away from the outer cylinder. For
counter-rotation, the mean radial flow in the roll structures strongly affects
the direction and intensity of the turbulent plumes. Furthermore, it is
experimentally confirmed that in regions where plumes are emitted, boundary
layer profiles with a logarithmic signature are created
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