9,880 research outputs found
On Newtonian singularities in higher derivative gravity models
We consider the problem of Newtonian singularity in the wide class of higher
derivative gravity models, including the ones which are renormalizable and
super-renormalizable at the quantum level. The simplest version of the
singularity-free theory has four derivatives and is pretty well-known. We argue
that in all cases of local higher-derivative theories, when the poles of the
propagator are real and simple, the singularities disappear due to the
cancellation of contributions from scalar and tensor massive modes.Comment: Added comment about the black hole solutions in Conclusions section.
Fits published version, plus some misprints corrected in formulas at the end
of Sect.
Macro- and microstructural descriptions of the zebrafish (Danio rerio) liver
The liver is an essential metabolically active organ, responsible for many vital life functions such as metabolism, synthesis, and storage. This study describes the macroscopic and microscopic zebrafish liver and its temporal changes during a period of 13 weeks. Forty-six adult zebrafish were used (46), of which 27 were females and were 19 males. Seven fish were euthanized initially for macroscopic evaluation and 3 fish were euthanized weekly for 13 weeks for microscopic evaluation. Routine histological technique was performed; the slides were stained with haematoxylin/eosin and periodic acid-Schiff. The volumetric (Vv(h)) and numerical (Nv(h)) densities of hepatocytes and vacuolation intensity were determined weekly. The liver was located in the coelomic cavity ventrally to the swim bladder, slightly brownish and divided into 3 hepatic lobes. Microscopically, it consisted of hepatocytes forming a muralium duplex around the sinusoids. The staining of the cytoplasm of hepatocytes was more eosinophilic and more basophilic in males and females, respectively. There was no formation of portal triads and phagocytic cells. The gallbladder had transitional epithelium, and the bile ducts had simple cuboidal epithelium. The volume and numerical density of hepatocytes during the 13 weeks was not statistically different, but these parameters displayed a positive correlation. The vacuolation was due to the presence of glycogen. The zebrafish liver has macro and micro structural particular features while, quantitatively, the volume and number of hepatocytes showed great capacity to respond to the physiological needs of the animal at the time
Optimal Control of Underactuated Mechanical Systems: A Geometric Approach
In this paper, we consider a geometric formalism for optimal control of
underactuated mechanical systems. Our techniques are an adaptation of the
classical Skinner and Rusk approach for the case of Lagrangian dynamics with
higher-order constraints. We study a regular case where it is possible to
establish a symplectic framework and, as a consequence, to obtain a unique
vector field determining the dynamics of the optimal control problem. These
developments will allow us to develop a new class of geometric integrators
based on discrete variational calculus.Comment: 20 pages, 2 figure
Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust
We present CO(1-0) and CO(2-1) observations of a sample of 20
star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with
oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is
observed in ten galaxies and marginally detected in another one. CO fluxes
correlate with the FIR 250 m emission, and the dwarfs follow the same
linear relation that holds for more massive spiral galaxies extended to a wider
dynamical range. We compare different methods to estimate H2 molecular masses,
namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on
H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant
at stellar masses <~ 10 M, contrary to the atomic hydrogen
fraction, M/M, which increases inversely with M. The flattening
of the M/M ratio at low stellar masses does not seem to be related
to the effects of the cluster environment because it occurs for both
HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more
tightly correlated with stellar surface density than metallicity, confirming
that the interstellar gas pressure plays a key role in determining the balance
between the two gaseous components of the interstellar medium. Virgo dwarfs
follow the same linear trend between molecular gas mass and star formation rate
as more massive spirals, but gas depletion timescales, , are not
constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo
cluster environment is removing the atomic gas and dust components of the
dwarfs, but the molecular gas appears to be less affected at the current stage
of evolution within the cluster. However, the correlation between HI deficiency
and the molecular gas depletion time suggests that the lack of gas
replenishment from the outer regions of the disc is lowering the star formation
activity.Comment: 19 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Regular multi-horizon Lee-Wick black holes
In this paper we carry out a detailed analysis of the static spherically
symmetric solutions of a sixth-derivative Lee--Wick gravity model in the
effective delta source approximation. Previous studies of these solutions have
only considered the particular case in which the real and the imaginary part of
the Lee--Wick mass are equal. However, as we show here, the
solutions exhibit an interesting structure when the full parameter space is
considered, owing to the oscillations of the metric that depend on the ratio
. Such oscillations can generate a rich structure of horizons, a sequence
of mass gaps and the existence of multiple regimes for black hole sizes
(horizon position gaps). In what concerns the thermodynamics of these objects,
the oscillation of the Hawking temperature determines the presence of multiple
mass scales for the remnants of the evaporation process and may permit the
existence of cold black holes with zero Hawking temperature~ and
quasi-stable intermediate configurations with and a long
evaporation lifetime. For the sake of generality, we consider two families of
solutions, one with a trivial shift function and the other with a non-trivial
one (dirty black hole). The latter solution has the advantage of reproducing
the modified Newtonian-limit metric of Lee--Wick gravity for small and large
values of~.Comment: 23 pages, 10 figures, 4 table
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