9,801 research outputs found
Enhancement of photoacoustic detection of inhomogeneities in polymers
We report a series of experiments on laser pulsed photoacoustic excitationin
turbid polymer samples addressed to evaluate the sound speed in the samples and
the presence of inhomogeneities in the bulk. We describe a system which allows
the direct measurement of the speed of the detected waves by engraving the
surface of the piece under study with a fiduciary pattern of black lines. We
also describe how this pattern helps to enhance the sensitivity for the
detection of an inhomogeneity in the bulk. These two facts are useful for
studies in soft matter systems including, perhaps, biological samples. We have
performed an experimental analysis on Grilon(R) samples in different situations
and we show the limitations of the method.Comment: 8 pages, 7 figure
The Role of Gut Adaptation in the Potent Effects of Multiple Bariatric Surgeries on Obesity and Diabetes
Bariatric surgical procedures such as vertical sleeve gastrectomy (VSG) and Roux-en-Y gastric bypass (RYGB) are the most potent treatments available to produce sustained reductions in body weight and improvements in glucose regulation. While traditionally these effects are attributed to mechanical aspects of these procedures, such as restriction and malabsorption, a growing body of evidence from mouse models of these procedures points to physiological changes that mediate the potent effects of these surgeries. In particular, there are similar changes in gut hormone secretion, bile acid levels, and composition after both of these procedures. Moreover, loss of function of the nuclear bile acid receptor (FXR) greatly diminishes the effects of VSG. Both VSG and RYGB are linked to profound changes in the gut microbiome that also mediate at least some of these surgical effects. We hypothesize that surgical rearrangement of the gastrointestinal tract results in enteroplasticity caused by the high rate of nutrient presentation and altered pH in the small intestine that contribute to these physiological effects. Identifying the molecular underpinnings of these procedures provides new opportunities to understand the relationship of the gastrointestinal tract to obesity and diabetes as well as new therapeutic strategies to harness the effectiveness of surgery with less-invasive approaches
The Rayleigh-Brillouin Spectrum in Special Relativistic Hydrodynamics
In this paper we calculate the Rayleigh-Brillouin spectrum for a relativistic
simple fluid according to three different versions available for a relativistic
approach to non-equilibrium thermodynamics. An outcome of these calculations is
that Eckart's version predicts that such spectrum does not exist. This provides
an argument to question its validity. The remaining two results, which differ
one from another, do provide a finite form for such spectrum. This raises the
rather intriguing question as to which of the two theories is a better
candidate to be taken as a possible version of relativistic non-equilibrium
thermodynamics. The answer will clearly require deeper examination of this
problem.Comment: 13 pages, no figures. Accepted for publication in Phys. Rev.
New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae
We present 22 new (+3 confirmed) cataclysmic variables (CVs) in the non
core-collapsed globular cluster 47 Tucanae (47 Tuc). The total number of CVs in
the cluster is now 43, the largest sample in any globular cluster so far. For
the identifications we used near-ultraviolet (NUV) and optical images from the
Hubble Space Telescope, in combination with X-ray results from the Chandra
X-ray Observatory. This allowed us to build the deepest NUV CV luminosity
function of the cluster to date. We found that the CVs in 47 Tuc are more
concentrated towards the cluster center than the main sequence turnoff stars.
We compared our results to the CV populations of the core-collapsed globular
clusters NGC 6397 and NGC 6752. We found that 47 Tuc has fewer bright CVs per
unit mass than those two other clusters. That suggests that dynamical
interactions in core-collapsed clusters play a major role creating new CVs. In
47 Tuc, the CV population is probably dominated by primordial and old
dynamically formed systems. We estimated that the CVs in 47 Tuc have total
masses of approx. 1.4 M_sun. We also found that the X-ray luminosity function
of the CVs in the three clusters is bimodal. Additionally, we discuss a
possible double degenerate system and an intriguing/unclassified object.
Finally, we present four systems that could be millisecond pulsar companions
given their X-ray and NUV/optical colors. For one of them we present very
strong evidence for being an ablated companion. The other three could be CO- or
He-WDs.Comment: Published on MNRAS. 31 Pages, 23 Figures, 5 Tables. Minor changes
with respect to previous arXiv versio
Interaction energy functional for lattice density functional theory: Applications to one-, two- and three-dimensional Hubbard models
The Hubbard model is investigated in the framework of lattice density
functional theory (LDFT). The single-particle density matrix with
respect the lattice sites is considered as the basic variable of the many-body
problem. A new approximation to the interaction-energy functional
is proposed which is based on its scaling properties and which recovers exactly
the limit of strong electron correlations at half-band filling. In this way, a
more accurate description of is obtained throughout the domain of
representability of , including the crossover from weak to strong
correlations. As examples of applications results are given for the
ground-state energy, charge-excitation gap, and charge susceptibility of the
Hubbard model in one-, two-, and three-dimensional lattices. The performance of
the method is demonstrated by comparison with available exact solutions, with
numerical calculations, and with LDFT using a simpler dimer ansatz for .
Goals and limitations of the different approximations are discussed.Comment: 25 pages and 8 figures, submitted to Phys. Rev.
Discovery of near-ultraviolet counterparts to millisecond pulsars in the globular cluster 47 Tucanae
We report the discovery of the likely white dwarf companions to radio
millisecond pulsars 47 Tuc Q and 47 Tuc S in the globular cluster 47 Tucanae.
These blue stars were found in near-ultraviolet images from the Hubble Space
Telescope for which we derived accurate absolute astrometry, and are located at
positions consistent with the radio coordinates to within 0.016 arcsec
(0.2sigma). We present near-ultraviolet and optical colours for the previously
identified companion to millisecond pulsar 47 Tuc U, and we unambiguously
confirm the tentative prior identifications of the optical counterparts to 47
Tuc T and 47 Tuc Y. For the latter, we present its radio-timing solution for
the first time. We find that all five near-ultraviolet counterparts have
U300-B390 colours that are consistent with He white dwarf cooling models for
masses ~0.16-0.3 Msun and cooling ages within ~0.1-6 Gyr. The Ha-R625 colours
of 47 Tuc U and 47 Tuc T indicate the presence of a strong Ha absorption line,
as expected for white dwarfs with an H envelope.Comment: Accepted for publication on MNRAS. 12 pages, 5 figures, 3 table
Hypothalamic Vitamin D Improves Glucose Homeostasis and Reduces Weight
Despite clear associations between vitamin D deficiency and obesity and/or type 2 diabetes, a causal relationship is not established. Vitamin D receptors (VDRs) are found within multiple tissues, including the brain. Given the importance of the brain in controlling both glucose levels and body weight, we hypothesized that activation of central VDR links vitamin D to the regulation of glucose and energy homeostasis. Indeed, we found that small doses of active vitamin D, 1α,25-dihydroxyvitamin D3 (1,25D3) (calcitriol), into the third ventricle of the brain improved glucose tolerance and markedly increased hepatic insulin sensitivity, an effect that is dependent upon VDR within the paraventricular nucleus of the hypothalamus. In addition, chronic central administration of 1,25D3 dramatically decreased body weight by lowering food intake in obese rodents. Our data indicate that 1,25D3-mediated changes in food intake occur through action within the arcuate nucleus. We found that VDR colocalized with and activated key appetite-regulating neurons in the arcuate, namely proopiomelanocortin neurons. Together, these findings define a novel pathway for vitamin D regulation of metabolism with unique and divergent roles for central nervous system VDR signaling. Specifically, our data suggest that vitamin D regulates glucose homeostasis via the paraventricular nuclei and energy homeostasis via the arcuate nuclei
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