127 research outputs found
Dust-trapping Rossby vortices in protoplanetary disks
One of the most challenging steps in planet formation theory is the one
leading to the formation of planetesimals of kilometre size. A promising
scenario involves the existence of vortices able to concentrate a large amount
of dust and grains in their centres. Up to now this scenario has been studied
mostly in 2D razor thin disks. A 3D study including, simultaneously, the
formation and resulting dust concentration of the vortices with vertical
settling, was still missing. The Rossby wave instability self-consistently
forms 3D vortices, which have the unique quality of presenting a large scale
vertical velocity in their centre. Here we aim to study how this newly
discovered effect can alter the dynamic evolution of the dust. We perform
global 3D simulations of the RWI in a radially and vertically stratified disk
using the code MPI-AMRVAC. After the growth phase of the instability, the gas
and solid phases are modelled by a bi-fluid approach, where the dust is
considered as a fluid without pressure. Both the drag force of the gas on the
dust and the back-reaction of the dust on the gas are included. Multiple grain
sizes from 1mm to 5cm are used with a constant density distribution. We obtain
in a short timescale a high concentration of the largest grains in the
vortices. Indeed, in 3 rotations the dust-to-gas density ratio grows from 10^-2
to unity leading to a concentration of mass up to that of Mars in one vortex.
The presence of the radial drift is also at the origin of a dust pile-up at the
radius of the vortices. Lastly, the vertical velocity of the gas in the vortex
causes the sedimentation process to be reversed, the mm size dust is lifted and
higher concentrations are obtained in the upper layer than in the mid-plane.Comment: Accepted for publication in Astronomy and Astrophysic
Tsallis non-extensive statistics, intermittent turbulence, SOC and chaos in the solar plasma. Part two: Solar Flares dynamics
In the second part of this study and similarly with part one, the nonlinear
analysis of the solar flares index is embedded in the non-extensive statistical
theory of Tsallis [1]. The triplet of Tsallis, as well as the correlation
dimension and the Lyapunov exponent spectrum were estimated for the SVD
components of the solar flares timeseries. Also the multifractal scaling
exponent spectrum, the generalized Renyi dimension spectrum and the spectrum of
the structure function exponents were estimated experimentally and
theoretically by using the entropy principle included in Tsallis non extensive
statistical theory, following Arimitsu and Arimitsu [2]. Our analysis showed
clearly the following: a) a phase transition process in the solar flare
dynamics from high dimensional non Gaussian SOC state to a low dimensional also
non Gaussian chaotic state, b) strong intermittent solar corona turbulence and
anomalous (multifractal) diffusion solar corona process, which is strengthened
as the solar corona dynamics makes phase transition to low dimensional chaos:
c) faithful agreement of Tsallis non equilibrium statistical theory with the
experimental estimations of i) non-Gaussian probability distribution function,
ii) multifractal scaling exponent spectrum and generalized Renyi dimension
spectrum, iii) exponent spectrum of the structure functions estimated for the
sunspot index and its underlying non equilibrium solar dynamics. e) The solar
flare dynamical profile is revealed similar to the dynamical profile of the
solar convection zone as far as the phase transition process from SOC to chaos
state. However the solar low corona (solar flare) dynamical characteristics can
be clearly discriminated from the dynamical characteristics of the solar
convection zone.Comment: 21 pages, 11 figures, 1 table. arXiv admin note: substantial text
overlap with arXiv:1201.649
On rapid migration and accretion within disks around supermassive black holes
Galactic nuclei should contain a cluster of stars and compact objects in the
vicinity of the central supermassive black hole due to stellar evolution, minor
mergers and gravitational dynamical friction. By analogy with protoplanetary
migration, nuclear cluster objects (NCOs) can migrate in the accretion disks
that power active galactic nuclei by exchanging angular momentum with disk gas.
Here we show that an individual NCO undergoing runaway outward migration
comparable to Type III protoplanetary migration can generate an accretion rate
corresponding to Seyfert AGN or quasar luminosities. Multiple migrating NCOs in
an AGN disk can dominate traditional viscous disk accretion and at large disk
radii, ensemble NCO migration and accretion could provide sufficient heating to
prevent the gravitational instability from consuming disk gas in star
formation. The magnitude and energy of the X-ray soft excess observed at
~0.1-1keV in Seyfert AGN could be explained by a small population of
~10^{2}-10^{3} accreting stellar mass black holes or a few ULXs. NCO migration
and accretion in AGN disks are therefore extremely important mechanisms to add
to realistic models of AGN disks.Comment: 6 pages, 2 figures, MNRAS Letters (accepted
Intermediate mass black holes in AGN disks: I. Production & Growth
Here we propose a mechanism for efficiently growing intermediate mass black
holes (IMBH) in disks around supermassive black holes. Stellar mass objects can
efficiently agglomerate when facilitated by the gas disk. Stars, compact
objects and binaries can migrate, accrete and merge within disks around
supermassive black holes. While dynamical heating by cusp stars excites the
velocity dispersion of nuclear cluster objects (NCOs) in the disk, gas in the
disk damps NCO orbits. If gas damping dominates, NCOs remain in the disk with
circularized orbits and large collision cross-sections. IMBH seeds can grow
extremely rapidly by collisions with disk NCOs at low relative velocities,
allowing for super-Eddington growth rates. Once an IMBH seed has cleared out
its feeding zone of disk NCOs, growth of IMBH seeds can become dominated by gas
accretion from the AGN disk. However, the IMBH can migrate in the disk and
expand its feeding zone, permitting a super-Eddington accretion rate to
continue. Growth of IMBH seeds via NCO collisions is enhanced by a pile-up of
migrators.
We highlight the remarkable parallel between the growth of IMBH in AGN disks
with models of giant planet growth in protoplanetary disks. If an IMBH becomes
massive enough it can open a gap in the AGN disk. IMBH migration in AGN disks
may stall, allowing them to survive the end of the AGN phase and remain in
galactic nuclei. Our proposed mechanisms should be more efficient at growing
IMBH in AGN disks than the standard model of IMBH growth in stellar clusters.
Dynamical heating of disk NCOs by cusp stars is transferred to the gas in a AGN
disk helping to maintain the outer disk against gravitational instability.
Model predictions, observational constraints and implications are discussed in
a companion paper (Paper II).Comment: 11 pages, 4 figures, MNRAS (accepted
Estudo dos efeitos de diferentes doses de surfactante exógeno para o tratamento da síndrome de aspiração de mecônio em coelhos recém-nascidos
OBJECTIVE: To evaluate the effects of 2 different doses of exogenous surfactant on pulmonary mechanics and on the regularity of pulmonary parenchyma inflation in newborn rabbits. METHOD: Newborn rabbits were submitted to tracheostomy and randomized into 4 study groups: the Control group did not receive any material inside the trachea; the MEC group was instilled with meconium, without surfactant treatment; the S100 and S200 groups were instilled with meconium and were treated with 100 and 200 mg/kg of exogenous surfactant (produced by Instituto Butantan) respectively. Animals from the 4 groups were mechanically ventilated during a 25-minute period. Dynamic compliance, ventilatory pressure, tidal volume, and maximum lung volume (P-V curve) were evaluated. Histological analysis was conducted using the mean linear intercept (Lm), and the lung tissue distortion index (SDI) was derived from the standard deviation of the means of the Lm. One-way analysis of variance was used with a = 0.05. RESULTS: After 25 minutes of ventilation, dynamic compliance (mL/cm H2O · kg) was 0.87 ± 0.07 (Control); 0.49 ± 0.04 (MEC*); 0.67 ± 0.06 (S100); and 0.67 ± 0.08 (S200), and ventilatory pressure (cm H2O) was 9.0 ± 0.9 (Control); 16.5 ± 1.7 (MEC*); 12.4 ± 1.1 (S100); and 12.1 ± 1.5 (S200). Both treated groups had lower Lm values and more homogeneity in the lung parenchyma compared to the MEC group: SDI = 7.5 ± 1.9 (Control); 11.3 ± 2.5 (MEC*), 5.8 ± 1.9 (S100); and 6.7 ± 1.7 (S200) (*P < 0.05 versus all the other groups). CONCLUSIONS: Animals treated with surfactant showed significant improvement in pulmonary mechanics and more regularity of the lung parenchyma in comparison to untreated animals. There was no difference in results after treatment with either of the doses used.OBJETIVO: Avaliar os efeitos de duas diferentes doses de surfactante exógeno sobre a mecânica pulmonar e sobre a regularidade da expansão do parênquima pulmonar em coelhos recém-nascidos. MÉTODO: Coelhos recém-nascidos foram traqueostomizados e randomizados em quatro grupos de estudo: grupo-Controle, sem aspiração de mecônio; grupo MEC, com aspiração de mecônio e sem tratamento com surfactante exógeno; grupos S100 e S200, ambos com aspiração de mecônio e tratados respectivamente com 100 e 200 mg/kg de surfactante exógeno (produzido e fornecido pelo Instituto Butantan). Os animais dos 4 grupos foram ventilados por 25 minutos. A mecânica pulmonar foi avaliada a partir dos valores de complacência dinâmica, pressão ventilatória, volume-corrente e volume pulmonar máximo (curva P-V). A análise histológica foi feita calculando-se o diâmetro alveolar médio (Lm) e o índice de distorção através do desvio padrão do Lm. Utilizou-se ANOVA One Way com a = 0,05. RESULTADOS: Após 25 minutos de ventilação, os valores de complacência dinâmica (ml/cm H2O.kg) foram: 0,87± 0,07 (Controle); 0,49±0,04 (MEC*); 0,67±0,06 (S100) e 0,67±0,08 (S200) e de pressão ventilatória (cm H2O): 9,0± 0,9 (Controle); 16,5±1,7 (MEC*); 12,4±1,1 (S100) e 12,1±1,5 (S200). Ambos os grupos tratados tiveram padrão de expansão do parênquima mais homogêneo em relação aos animais não tratados: índice de distorção de 7,5± 1,9 (Controle); 11,3±2,5 (MEC*); 5,8±1,9 (S100) e 6,7±1,7 (S200) (*p < 0,05 vs outros grupos). CONCLUSÕES: Animais tratados com surfactante mostraram melhora significativa da mecânica pulmonar e maior homogeneidade do padrão de expansão pulmonar comparados ao grupo não tratado. Não houve influência das doses de surfactante utilizadas
Rossby Wave Instability in three dimensional discs
The Rossby wave instability (RWI) is a promising mechanism for producing
large-scale vortices in protoplanetary discs. The instability operates around a
density bump in the disc, and the resulting vortices may facilitate
planetesimal formation and angular momentum transfer in the disc dead zone.
Most previous works on the RWI deal with two-dimensional (height-integrated)
discs. However, vortices may have different dynamical behaviours in 3D than in
2D. Recent numerical simulations of the RWI in 3D global discs by Meheut et al.
have revealed intriguing vertical structure of the vortices, including
appreciable vertical velocities. In this paper we present a linear analysis of
the RWI in 3D global models of isothermal discs. We calculate the growth rates
of the Rossby modes (of various azimuthal wave numbers m = 2 - 6) trapped
around the fiducial density bump and carry out 3D numerical simulations to
compare with our linear results. We show that the 3D RWI growth rates are only
slightly smaller than the 2D growth rates, and the velocity structures seen in
the numerical simulations during the linear phase are in agreement with the
velocity eigenfunctions obtained in our linear calculations. This numerical
benchmark shows that numerical simulations can accurately describe the
instability. The angular momentum transfer rate associated with Rossby vortices
is also studied.Comment: 9 pages, 10 figures, accepted for publication in MNRA
Rossby wave instability at dead zone boundaries in 3D resistive magnetohydrodynamical global models of protoplanetary disks
It has been suggested that the transition between magnetorotationally active
and dead zones in protoplanetary disks should be prone to the excitation of
vortices via Rossby wave instability (RWI). However, the only numerical
evidence for this has come from alpha disk models, where the magnetic field
evolution is not followed, and the effect of turbulence is parametrized by
Laplacian viscosity. We aim to establish the phenomenology of the flow in the
transition in 3D resistive-magnetohydrodynamical models. We model the
transition by a sharp jump in resistivity, as expected in the inner dead zone
boundary, using the Pencil Code to simulate the flow. We find that vortices are
readily excited in the dead side of the transition. We measure the mass
accretion rate finding similar levels of Reynolds stress at the dead and active
zones, at the level. The vortex sits in a pressure
maximum and does not migrate, surviving until the end of the simulation. A
pressure maximum in the active zone also triggers the RWI. The magnetized
vortex that results should be disrupted by parasitical magneto-elliptic
instabilities, yet it subsists in high resolution. This suggests that either
the parasitic modes are still numerically damped, or that the RWI supplies
vorticity faster than they can destroy it. We conclude that the resistive
transition between the active and dead zones in the inner regions of
protoplanetary disks, if sharp enough, can indeed excite vortices via RWI. Our
results lend credence to previous works that relied on the alpha-disk
approximation, and caution against the use of overly reduced azimuthal coverage
on modeling this transition.Comment: 9 pages, 7 figures, accepted by Ap
Oleic acid is an endogenous ligand of TLX/NR2E1 that triggers hippocampal neurogenesis
Altres ajuts: Cancer Prevention and Research Institute of Texas (CPRIT), Core Facility Support Award (CPRIT-RP180672, R1313, 1R01GM138781-01); NIH (CA125123, RR024574); Eunice Kennedy Shriver National Institute of Child Health & Human Development of the NIH (P50HD103555); BCM start-up funds; Albert and Margaret Alkek Foundation; McNair Medical Institute; Robert and Janice McNair Foundation; BCM Seed Funding (1P20CA221731-01A1); National Institute of General Medical Sciences (R01 GM120033); Cynthia and Antony Petrello Endowment; Mark A. Wallace Endowment; McKnight Foundation; Dana Foundation; BCM Computational and Integrative Biomedical Research Center seed grant.Neural stem cells, the source of newborn neurons in the adult hippocampus, are intimately involved in learning and memory, mood, and stress response. Despite considerable progress in understanding the biology of neural stem cells and neurogenesis, regulating the neural stem cell population precisely has remained elusive because we have lacked the specific targets to stimulate their proliferation and neurogenesis. The orphan nuclear receptor TLX/NR2E1 governs neural stem and progenitor cell self-renewal and proliferation, but the precise mechanism by which it accomplishes this is not well understood because its endogenous ligand is not known. Here, we identify oleic acid (18:1ω9 monounsaturated fatty acid) as such a ligand. We first show that oleic acid is critical for neural stem cell survival. Next, we demonstrate that it binds to TLX to convert it from a transcriptional repressor to a transcriptional activator of cell-cycle and neurogenesis genes, which in turn increases neural stem cell mitotic activity and drives hippocampal neurogenesis in mice. Interestingly, oleic acid-activated TLX strongly up-regulates cell cycle genes while only modestly up-regulating neurogenic genes. We propose a model in which sufficient quantities of this endogenous ligand must bind to TLX to trigger the switch to proliferation and drive the progeny toward neuronal lineage. Oleic acid thus serves as a metabolic regulator of TLX activity that can be used to selectively target neural stem cells, paving the way for future therapeutic manipulations to counteract pathogenic impairments of neurogenesis
On the pulmonary toxicity of oxygen : III. The induction of oxygen dependency by oxygen use
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Experimental and Molecular Pathology 89 (2010): 36-45, doi:10.1016/j.yexmp.2010.05.004.Oxygen is central to the development of neonatal lung injury. The increase in oxygen exposure of
the neonatal lung during the onset of extrauterine air breathing is an order of magnitude, from a
range of 10-12 to 110-120 Torr. The contributions of oxygen and the volume and pressure
relationships of ventilatory support to lung injury are not easily distinguished in the clinical setting.
Sequential changes in inspired air or 100% oxygen were studied in 536 newborn rabbits without
ventilatory support. Bilateral cervical vagotomies (BCV) were performed at 24 hours post natal to
induce ventilatory distress which eventuates in hyaline membrane disease. The sequences applied
yielded evidence for an induced state of oxygen dependency from oxygen use which was reflected
in differences in survival and the extent of pulmonary injury. The median survival for animals kept
in air throughout was 3 hours. Oxygen before vagotomy or during the first 3 hours afterwards
extended the survival significantly but produced more extensive, more severe, and more rapid lung
lesions. Returning animals to air after prior oxygen exposure reduced the number of survivors past
10 hours and shortened the maximum survival in those groups. These features indicate the
development of a dependency of the defense mechanisms on the availability of oxygen at the higher
level for metabolic and possibly other aspects of the pulmonary and systemic response to injury,
beyond the usual physiological need. Subset analysis revealed additive and latent effects of
oxygen and demonstrated a remarkable rapidity in onset of severe lesions under some
circumstances, illustrating the toxicity of oxygen per se.John A. Hartford Foundation,
New York, N.Y. 10022-171
The SCIDOTS Project: Evidence of benefits of an integrated tobacco cessation intervention in tuberculosis care on treatment outcomes
<p>Abstract</p> <p>Background</p> <p>There is substantial evidence to support the association between tuberculosis (TB) and tobacco smoking and that the smoking-related immunological abnormalities in TB are reversible within six weeks of cessation. Therefore, connecting TB and tobacco cessation interventions may produce significant benefits and positively impact TB treatment outcomes. However, no study has extensively documented the evidence of benefits of such integration. SCIDOTS Project is a study from the context of a developing nation aimed to determine this.</p> <p>Methods</p> <p>An integrated TB-tobacco intervention was provided by trained TB directly observed therapy short-course (DOTS) providers at five chest clinics in Malaysia. The study was a prospective non-randomized controlled intervention using quasi-experimental design. Using Transtheoretical Model approach, 120 eligible participants who were current smokers at the time of TB diagnosis were assigned to either of two treatment groups: conventional TB DOTS plus smoking cessation intervention (integrated intervention or SCIDOTS group) or conventional TB DOTS alone (comparison or DOTS group). At baseline, newly diagnosed TB patients considering quitting smoking within the next 30 days were placed in the integrated intervention group, while those who were contemplating quitting were assigned to the comparison group. Eleven sessions of individualized cognitive behavioral therapy with or without nicotine replacement therapy were provided to each participant in the integrated intervention group. The impacts of the novel approach on biochemically validated smoking cessation and TB treatment outcomes were measured periodically as appropriate.</p> <p>Results</p> <p>A linear effect on both 7-day point prevalence abstinence and continuous abstinence was observed over time in the intervention group. At the end of 6 months, patients who received the integrated intervention had significantly higher rate of success in quitting smoking when compared with those who received the conventional TB treatment alone (77.5% vs. 8.7%; p < 0.001). Furthermore, at the end of TB treatment (6 months or later), there were significantly higher rates of treatment default (15.2% vs. 2.5%; p = 0.019) and treatment failure (6.5% vs. 0%; p = 0.019) in the DOTS group than in the SCIDOTS group.</p> <p>Conclusion</p> <p>This study provides evidence that connecting TB-tobacco treatment strategy is significant among TB patients who are smokers. The findings suggest that the integrated approach may be beneficial and confer advantages on short-term outcomes and possibly on future lung health of TB patients who quit smoking. This study may have important implications on health policy and clinical practice related to TB management among tobacco users.</p
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