184 research outputs found
Ab initio simulations of accretion disks instability
We show that accretion disks, both in the subcritical and supercritical
accretion rate regime, may exhibit significant amplitude luminosity
oscillations. The luminosity time behavior has been obtained by performing a
set of time-dependent 2D SPH simulations of accretion disks with different
values of alpha and accretion rate. In this study, to avoid any influence of
the initial disk configuration, we produced the disks injecting matter from an
outer edge far from the central object. The period of oscillations is 2 - 50 s
respectively for the two cases, and the variation amplitude of the disc
luminosity is 10^38 - 10^39 erg/s. An explanation of this luminosity behavior
is proposed in terms of limit cycle instability: the disk oscillates between a
radiation pressure dominated configuration (with a high luminosity value) and a
gas pressure dominated one (with a low luminosity value). The origin of this
instability is the difference between the heat produced by viscosity and the
energy emitted as radiation from the disk surface (the well-known thermal
instability mechanism). We support this hypothesis showing that the limit cycle
behavior produces a sequence of collapsing and refilling states of the
innermost disk region.Comment: 11 pages, 15 Postscript figures, uses natbib.sty, accepted for
publication in MNRA
Shock oscillation model for quasi-periodic oscillations in stellar mass and supermassive black holes
We numerically examine centrifugally supported shock waves in 2D rotating accretion flows
around a stellar mass (10 M) and a supermassive (106 M) black holes over a wide range of
input accretion rates of 107 M\u2d9 /M\u2d9 E 10 124. The resultant 2D shocks are unstable with time
and the luminosities show quasi-periodic oscillations (QPOs) with modulations of a factor of
2\u20133 and with periods of a tenth of a second to several hours, depending on the black hole
masses. The shock oscillation model may explain the intermediate frequency QPOs with 1\u2013
10 Hz observed in the stellar mass black hole candidates and also suggest the existence of
QPOs with the period of hours in active galactic nuclei. When the accretion rate M\u2d9 is low, the
luminosity increases in proportion to the accretion rate. However, when M\u2d9 greatly exceeds
the Eddington critical rate M\u2d9 E, the luminosity is insensitive to the accretion rate and is kept
constantly around 3c3LE. On the other hand, the mass-outflow rate M\u2d9 loss increases in proportion
to M\u2d9 and it amounts to about a few per cent of the input mass-flow rat
3D SPH Simulations of Shocks in Accretion Flows around black holes
We present the simulation of 3D time dependent flow of rotating ideal gas
falling into a Schwarzschild black hole. It is shown that also in the 3D case
steady shocks are formed in a wide range of parameters (initial angular
momentum and thermal energy). We therefore highlight the stability of the
phenomenon of shock formation in sub keplerian flows onto black holes, and
reenforce the role of the shocks in the high luminosity emission from black
hole candidates. The simulations have been performed using a parallelized code
based on the Smoothed Particles Hydrodynamics method (SPH). We also discuss
some properties of the shock problem that allow its use as a quantitative test
of the accuracy of the used numerical method. This shows that the accuracy of
SPH is acceptable although not excellent.Comment: 9 pages, 22 figure
Radiative Shocks in Rotating Accretion Flows around Black Holes
It is well known that the rotating inviscid accretion flows with adequate
injection parameters around black holes could form shock waves close to the
black holes, after the flow passes through the outer sonic point and can be
virtually stopped by the centrifugal force.
We examine numerically such shock waves in 1D and 2D accretion flows, taking
account of cooling and heating of the gas and radiation transport.
The numerical results show that the shock location shifts outward compared
with that in the adiabatic solutions and that the more rarefied ambient density
leads to the more outward shock location.
In the 2D-flow, we find an intermediate frequency QPO behavior of the shock
location as is observed in the black hole candidate
GRS 1915+105.Comment: 11pages, 5 Postscript figures, to appear in PASJ, Vol.56, No.3, 200
Use of biochar as filler for biocomposite blown films: Structure-processing-properties relationships
In this work, biocomposite blown films based on poly(butylene adipate-co-terephthalate) (PBAT) as biopolymeric matrix and biochar (BC) as filler were successfully fabricated. The materials were subjected to a film-blowing process after being compounded in a twin-screw extruder. The preliminary investigations conducted on melt-mixed PBAT/BC composites allowed PBAT/BC 5% and PBAT/BC 10% to be identified as the most appropriate formulations to be processed via film blowing. The blown films exhibited mechanical performances adequate for possible application as film for packaging, agricultural, and compost bags. The addition of BC led to an improvement of the elastic modulus, still maintaining high values of deformation. Water contact angle measurements revealed an increase in the hydrophobic behavior of the biocomposite films compared to PBAT. Additionally, accelerated degradative tests monitored by tensile tests and spectroscopic analysis revealed that the filler induced a photo-oxidative resistance on PBAT by delaying the degradation phenomena
SPH simulations of Shakura-Sunyaev instability at intermediate accretion rates
We show that a standard Shakura-Sunyaev accretion disc around a black hole
with an accretion rate lower than the critical Eddington limit does show the
instability in the radiation pressure dominated zone. We obtain this result
performing time-dependent simulations of accretion disks for a set of values of
the viscosity parameter and accretion rate. In particular we always find the
occurrence of the collapse of the disc: the instability develops always towards
a collapsed gas pressure dominated disc and not towards the expansion. This
result is valid for all initial configurations we tested. We find significant
convective heat flux that increases the instability development time, but is
not strong enough to inhibit the disc collapse. A physical explanation of the
lack of the expansion phase is proposed considering the role of the radial heat
advection. Our finding is relevant since it excludes the formation of the hot
comptonizing corona -often suggested to be present- around the central object
by the mechanism of the Shakura-Sunyaev instability. We also show that, in the
parameters range we simulated, accretion disks are crossed by significant
amplitude acoustic waves.Comment: 8 pages, 12 Postscript figures, uses natbib.sty, accepted for
publication in MNRA
Steady shocks around black holes produced by sub-keplerian flows with negative energy
We discuss a special case of formation of axisymmetric shocks in the
accretion flow of ideal gas onto a Schwarzschild black hole: when the total
energy of the flow is negative. The result of our analysis enlarges the
parameter space for which these steady shocks are exhibited in the accretion of
gas rotating around relativistic stellar objects. Since keplerian disks have
negative total energy, we guess that, in this energy range, the production of
the shock phenomenon might be easier than in the case of positive energy. So
our outcome reinforces the view that sub-keplerian flows of matter may
significantly affect the physics of the high energy radiation emission from
black hole candidates. We give a simple procedure to obtain analytically the
position of the shocks. The comparison of the analytical results with the data
of 1D and 2D axisymmetric numerical simulations confirms that the shocks form
and are stable.Comment: 5 pages, 5 figures, accepted by MNRAS on 10 November 200
Left atrial trajectory impairment in hypertrophic cardiomyopathy disclosed by geometric morphometrics and parallel transport
The analysis of full Left Atrium (LA) deformation and whole LA deformational trajectory in time has been poorly investigated and, to the best of our knowledge, seldom discussed in patients with Hypertrophic Cardiomyopathy. Therefore, we considered 22 patients with Hypertrophic Cardiomyopathy (HCM) and 46 healthy subjects, investigated them by three-dimensional Speckle Tracking Echocardiography, and studied the derived landmark clouds via Geometric Morphometrics with Parallel Transport. Trajectory shape and trajectory size were different in Controls versus HCM and their classification powers had high AUC (Area Under the Receiving Operator Characteristic Curve) and accuracy. The two trajectories were much different at the transition between LA conduit and booster pump functions. Full shape and deformation analyses with trajectory analysis enabled a straightforward perception of pathophysiological consequences of HCM condition on LA functioning. It might be worthwhile to apply these techniques to look for novel pathophysiological approaches that may better define atrio-ventricular interaction
A simple model of radiative emission in M87
We present a simple physical model of the central source emission in the M87
galaxy. It is well known that the observed X-ray luminosity from this galactic
nucleus is much lower than the predicted one, if a standard radiative
efficiency is assumed. Up to now the main model invoked to explain such a
luminosity is the ADAF (Advection-Dominated-Accretion-Flow) model. Our approach
supposes only a simple axis-symmetric adiabatic accretion with a low angular
momentum together with the bremsstrahlung emission process in the accreting
gas. With no other special hypothesis on the dynamics of the system, this model
agrees well enough with the luminosity value measured by Chandra.Comment: 11 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Quality of life assessment in amyloid transthyretin (ATTR) amyloidosis
Background: Amyloid transthyretin (ATTR) amyloidosis is caused by the systemic deposition of transthyretin molecules, either normal (wild-type ATTR, ATTRwt) or mutated (variant ATTR, ATTRv). ATTR amyloidosis is a disease with a severe impact on patients’ quality of life (QoL). Nonetheless, limited attention has been paid to QoL so far, and no specific tools for QoL assessment in ATTR amyloidosis currently exist. QoL can be evaluated through patient-reported outcome measures (PROMs), which are completed by patients, or through scales, which are compiled by clinicians. The scales investigate QoL either directly or indirectly, i.e., by assessing the degree of functional impairment and limitations imposed by the disease. Design: Search for the measures of QoL evaluated in phase 2 and phase 3 clinical trials on ATTR amyloidosis. Results: Clinical trials on ATTR amyloidosis have used measures of general health status, such as the Short Form 36 Health Survey (SF-36), or tools developed in other disease settings such as the Kansas City Cardiomyopathy Questionnaire (KCCQ) or adaptations of other scales such as the modified Neuropathy Impairment Score +7 (mNIS+7). Conclusions: Scales or PROMs for ATTR amyloidosis would be useful to better characterize newly diagnosed patients and to assess disease progression and response to treatment. The ongoing ITALY (Impact of Transthyretin Amyloidosis on Life qualitY) study aims to develop and validate 2 PROMs encompassing the whole phenotypic spectrum of ATTRwt and ATTRv amyloidosis, that might be helpful for patient management and may serve as surrogate endpoints for clinical trials
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