2,418 research outputs found
Binary planetary nebulae nuclei towards the Galactic bulge. II. A penchant for bipolarity and low-ionisation structures
Considerable effort has been applied towards understanding the precise
shaping mechanisms responsible for the diverse range of morphologies exhibited
by planetary nebulae (PNe). A binary companion is increasingly gaining support
as a dominant shaping mechanism, however morphological studies of the few PNe
that we know for certain were shaped by binary evolution are scarce or biased.
Newly discovered binary central stars (CSPN) from the OGLE-III photometric
variability survey have significantly increased the sample of post
common-envelope (CE) nebulae available for morphological analysis. We present
Gemini South narrow-band images for most of the new sample to complement
existing data in a qualitative morphological study of 30 post-CE nebulae.
Nearly 30% of nebulae have canonical bipolar morphologies, however this rises
to 60% once inclination effects are incorporated with the aid of geometric
models. This is the strongest observational evidence yet linking CE evolution
to bipolar morphologies. A higher than average proportion of the sample shows
low-ionisation knots, filaments or jets suggestive of a binary origin. These
features are also common around emission-line nuclei which may be explained by
speculative binary formation scenarios for H-deficient CSPN.Comment: Accepted for publication in A&
Testing modified gravity with globular cluster velocity dispersions
Globular clusters (GCs) in the Milky Way have characteristic velocity
dispersions that are consistent with the predictions of Newtonian gravity, and
may be at odds with Modified Newtonian Dynamics (MOND). We discuss a modified
gravity (MOG) theory that successfully predicts galaxy rotation curves, galaxy
cluster masses and velocity dispersions, lensing, and cosmological
observations, yet produces predictions consistent with Newtonian theory for
smaller systems, such as GCs. MOG produces velocity dispersion predictions for
GCs that are independent of the distance from the galactic center, which may
not be the case for MOND. New observations of distant GCs may produce strong
criteria that can be used to distinguish between competing gravitational
theories.Comment: 4 pages, 2 figures; accepted for publication in Ap
The turbulent boundary layer on a porous plate: An experimental study of the heat transfer behavior with adverse pressure gradients
An experimental investigation of the heat transfer behavior of the near equilibrium transpired turbulent boundary layer with adverse pressure gradient has been carried out. Stanton numbers were measured by an energy balance on electrically heated plates that form the bottom wall of the wind tunnel. Two adverse pressure gradients were studied. Two types of transpiration boundary conditions were investigated. The concept of an equilibrium thermal boundary layer was introduced. It was found that Stanton number as a function of enthalpy thickness Reynolds number is essentially unaffected by adverse pressure gradient with no transpiration. Shear stress, heat flux, and turbulent Prandtl number profiles were computed from mean temperature and velocity profiles. It was concluded that the turbulent Prandtl number is greater than unity in near the wall and decreases continuously to approximately 0.5 at the free stream
Field Equations and Conservation Laws in the Nonsymmetric Gravitational Theory
The field equations in the nonsymmetric gravitational theory are derived from
a Lagrangian density using a first-order formalism. Using the general
covariance of the Lagrangian density, conservation laws and tensor identities
are derived. Among these are the generalized Bianchi identities and the law of
energy-momentum conservation. The Lagrangian density is expanded to
second-order, and treated as an ``Einstein plus fields'' theory. From this, it
is deduced that the energy is positive in the radiation zone.Comment: 16 pages, RevTeX. Additional equations supplie
Fundamental parameter-free solutions in Modified Gravity
Modified Gravity (MOG) has been used successfully to explain the rotation
curves of galaxies, the motion of galaxy clusters, the Bullet Cluster, and
cosmological observations without the use of dark matter or Einstein's
cosmological constant. We now have the ability to demonstrate how these
solutions can be obtained directly from the action principle, without resorting
to the use of fitted parameters or empirical formulae. We obtain numerical
solutions to the theory's field equations that are exact in the sense that no
terms are omitted, in two important cases: the spherically symmetric, static
vacuum solution and the cosmological case of an homogeneous, isotropic
universe. We compare these results to selected astrophysical and cosmological
observations.Comment: 11 pages, 8 figures; accepted for publication in CQ
NGC 3603 - a Local Template for Massive Young Clusters
We present a study of the star cluster associated with the massive Galactic
HII region NGC3603 based on near-IR broad-- and narrowband observations taken
with ISAAC/VLT under excellent seeing conditions (<0.4''). We discuss
color-color diagrams and address the impact of the high UV flux on the disk
evolution of the low-mass stars.Comment: 3 pages, 3 figures. To appear in the Proceedings of IAU Symposium 207
"Extragalactic Star Clusters", eds. E. Grebel, D. Geisler and D. Minitt
Nonexistence theorems for traversable wormholes
Gauss-Bonnet formula is used to derive a new and simple theorem of
nonexistence of vacuum static nonsingular lorentzian wormholes. We also derive
simple proofs for the nonexistence of lorentzian wormhole solutions for some
classes of static matter such as, for instance, real scalar fields with a
generic potential obeying and massless fermions fields
Can the Copernican principle be tested by cosmic neutrino background?
The Copernican principle, stating that we do not occupy any special place in
our universe, is usually taken for granted in modern cosmology. However recent
observational data of supernova indicate that we may live in the under-dense
center of our universe, which makes the Copernican principle challenged. It
thus becomes urgent and important to test the Copernican principle via
cosmological observations. Taking into account that unlike the cosmic photons,
the cosmic neutrinos of different energies come from the different places to us
along the different worldlines, we here propose cosmic neutrino background as a
test of the Copernican principle. It is shown that from the theoretical
perspective cosmic neutrino background can allow one to determine whether the
Copernican principle is valid or not, but to implement such an observation the
larger neutrino detectors are called for.Comment: JHEP style, 10 pages, 4 figures, version to appear in JCA
Linearisation Instabilities of the Massive Nonsymmetric Gravitational Theory
The massive nonsymmetric gravitational theory is shown to posses a
linearisation instability at purely GR field configurations, disallowing the
use of the linear approximation in these situations. It is also shown that
arbitrarily small antisymmetric sector Cauchy data leads to singular evolution
unless an ad hoc condition is imposed on the initial data hypersurface.Comment: 14 pages, IOP style for submission to CQG. Minor changes and
additional background material adde
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