382 research outputs found
Stability of radiation-pressure dominated disks. I. The dispersion relation for a delayed heating alpha-viscosity prescription
We derive and investigate the dispersion relation for accretion disks with
retarded or advanced heating. We follow the alpha-prescription but allow for a
time offset (\tau) between heating and pressure perturbations, as well as for a
diminished response of heating to pressure variations. We study in detail
solutions of the dispersion relation for disks with radiation-pressure fraction
1 - \beta . For \tau <0 (delayed heating) the number and sign of real solutions
for the growth rate depend on the values of the time lag and the ratio of
heating response to pressure perturbations, \xi . If the delay is larger than a
critical value (e.g., if \Omega \tau <-125 for \alpha =0.1, \beta =0 and \xi
=1) two real solutions exist, which are both negative. These results imply that
retarded heating may stabilize radiation-pressure dominated accretion disks.Comment: 11 pages, 10 figures, to be submitted to A&
A melanin-independent interaction between Mc1r and Met signalling pathways is required for HGF-dependent melanoma
Melanocortin 1 receptor (MC1R) signaling stimulates black eumelanin production through a cAMP-dependent pathway. MC1R polymorphisms can impair this process, resulting in a predominance of red phaeomelanin. The red hair, fair skin and UV sensitive phenotype is a well-described melanoma risk factor. MC1R polymorphisms also confer melanoma risk independent of pigment. We investigated the effect of Mc1r deficiency in a mouse model of UV-induced melanoma. C57BL/6-Mc1r+/+-HGF transgenic mice have a characteristic hyperpigmented black phenotype with extra-follicular dermal melanocytes located at the dermal/epidermal junction. UVB induces melanoma, independent of melanin pigmentation, but UVA-induced and spontaneous melanomas are dependent on black eumelanin. We crossed these mice with yellow C57BL/6-Mc1re/e animals which have a non-functional Mc1r and produce predominantly yellow phaeomelanin. Yellow C57BL/6-Mc1re/e-HGF mice produced no melanoma in response to UVR or spontaneously even though the HGF transgene and its receptor Met were expressed. Total melanin was less than in C57BL/6-Mc1r+/+-HGF mice, hyperpigmentation was not observed and there were few extra-follicular melanocytes. Thus, functional Mc1r was required for expression of the transgenic HGF phenotype. Heterozygous C57BL/6-Mc1re/+-HGF mice were black and hyperpigmented and, although extra-follicular melanocytes and skin melanin content were similar to C57BL/6-Mc1r+/+-HGF animals, they developed UV-induced and spontaneous melanomas with significantly less efficiency by all criteria. Thus, heterozygosity for Mc1r was sufficient to restore the transgenic HGF phenotype but insufficient to fully restore melanoma. We conclude that a previously unsuspected melanin-independent interaction between Mc1r and Met signaling pathways is required for HGF-dependent melanoma and postulate that this pathway is involved in human melanoma
Nonlinear absorption and nonlinear refraction: Maximizing the merit factors
Both nonlinear absorption and nonlinear refraction are effects that are potentially useful for a plethora of applications in photonics, nanophotonics and biophotonics. Despite substantial attention given to these phenomena by researchers studying the merits of disparate systems such as organic materials, hybrid materials, metal-containing molecules and nanostructures, it is virtually impossible to compare the results obtained on different materials when varying parameters of the light beams and different techniques are employed. We have attempted to address the problem by studying the properties of various systems in a systematic way, within a wide range of wavelengths, and including the regions of onephoton, two-photon and three-photon absorption. The objects of our studies have been typical nonlinear chromophores, such as π-conjugated molecules, oligomers and polymers, organometallics and coordination complexes containing transition metals, organometallic dendrimers, small metal-containing clusters, and nanoparticles of various kinds, including semiconductor quantum dots, plasmonic particles and rare-earth doped nanocrystals. We discuss herein procedures to quantify the nonlinear response of all of these systems, by defining and comparing the merit factors relevant for various applications
Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz
Observations of the Galactic Center supermassive black hole Sagittarius A*
(Sgr A*) with very long baseline interferometry (VLBI) are affected by
interstellar scattering along our line of sight. At long radio observing
wavelengths (cm), the scattering heavily dominates image
morphology. At 3.5 mm (86 GHz), the intrinsic source structure is no longer
sub-dominant to scattering, and thus the intrinsic emission from Sgr A* is
resolvable with the Global Millimeter VLBI Array (GMVA). Long-baseline
detections to the phased Atacama Large Millimeter/submillimeter Array (ALMA) in
2017 provided new constraints on the intrinsic and scattering properties of Sgr
A*, but the stochastic nature of the scattering requires multiple observing
epochs to reliably estimate its statistical properties. We present new
observations with the GMVA+ALMA, taken in 2018, which confirm non-Gaussian
structure in the scattered image seen in 2017. In particular, the ALMA-GBT
baseline shows more flux density than expected for an anistropic Gaussian
model, providing a tight constraint on the source size and an upper limit on
the dissipation scale of interstellar turbulence. We find an intrinsic source
extent along the minor axis of as both via extrapolation of
longer wavelength scattering constraints and direct modeling of the 3.5 mm
observations. Simultaneously fitting for the scattering parameters, we find an
at-most modestly asymmetrical (major-to-minor axis ratio of )
intrinsic source morphology for Sgr A*.Comment: 18 pages, 10 figures, submitted to Ap
Search for Neutrinoless Double-Beta Decay of Te with CUORE-0
We report the results of a search for neutrinoless double-beta decay in a
9.8~kgyr exposure of Te using a bolometric detector array,
CUORE-0. The characteristic detector energy resolution and background level in
the region of interest are FWHM and ~counts/(keVkgyr), respectively. The
median 90%~C.L. lower-limit sensitivity of the experiment is and surpasses the sensitivity of previous searches. We find
no evidence for neutrinoless double-beta decay of Te and place a
Bayesian lower bound on the decay half-life, ~ at 90%~C.L. Combining CUORE-0 data with the 19.75~kgyr
exposure of Te from the Cuoricino experiment we obtain at 90%~C.L.~(Bayesian), the most stringent
limit to date on this half-life. Using a range of nuclear matrix element
estimates we interpret this as a limit on the effective Majorana neutrino mass,
-- .Comment: 6 pages, 5 figures, updated version as published in PR
CUORE-0 results and prospects for the CUORE experiment
With 741 kg of TeO2 crystals and an excellent energy resolution of 5 keV
(0.2%) at the region of interest, the CUORE (Cryogenic Underground Observatory
for Rare Events) experiment aims at searching for neutrinoless double beta
decay of 130Te with unprecedented sensitivity. Expected to start data taking in
2015, CUORE is currently in an advanced construction phase at LNGS. CUORE
projected neutrinoless double beta decay half-life sensitivity is 1.6E26 y at 1
sigma (9.5E25 y at the 90% confidence level), in five years of live time,
corresponding to an upper limit on the effective Majorana mass in the range
40-100 meV (50-130 meV). Further background rejection with auxiliary bolometric
detectors could improve CUORE sensitivity and competitiveness of bolometric
detectors towards a full analysis of the inverted neutrino mass hierarchy.
CUORE-0 was built to test and demonstrate the performance of the upcoming CUORE
experiment. It consists of a single CUORE tower (52 TeO2 bolometers of 750 g
each, arranged in a 13 floor structure) constructed strictly following CUORE
recipes both for materials and assembly procedures. An experiment its own,
CUORE-0 is expected to reach a sensitivity to the neutrinoless double beta
decay half-life of 130Te around 3E24 y in one year of live time. We present an
update of the data, corresponding to an exposure of 18.1 kg y. An analysis of
the background indicates that the CUORE performance goal is satisfied while the
sensitivity goal is within reach.Comment: 10 pages, 3 figures, to appear in the proceedings of NEUTRINO 2014,
26th International Conference on Neutrino Physics and Astrophysics, 2-7 June
2014, held at Boston, Massachusetts, US
Studying black holes on horizon scales with space-VLBI
The Event Horizon Telescope (EHT) recently produced the first horizon-scale image of a supermassive black hole. Expanding the array to include a 3-meter space telescope operating at >200 GHz enables mass measurements of many black holes, movies of black hole accretion flows, and new tests of general relativity that are impossible from the ground
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