152 research outputs found
Scaling laws in spherical shell dynamos with free-slip boundaries
Numerical simulations of convection driven rotating spherical shell dynamos
have often been performed with rigid boundary conditions, as is appropriate for
the metallic cores of terrestrial planets. Free-slip boundaries are more
appropriate for dynamos in other astrophysical objects, such as gas-giants or
stars. Using a set of 57 direct numerical simulations, we investigate the
effect of free-slip boundary conditions on the scaling properties of heat flow,
flow velocity and magnetic field strength and compare it with earlier results
for rigid boundaries. We find that the nature of the mechanical boundary
condition has only a minor influence on the scaling laws. We also find that
although dipolar and multipolar dynamos exhibit approximately the same scaling
exponents, there is an offset in the scaling pre-factors for velocity and
magnetic field strength. We argue that the offset can be attributed to the
differences in the zonal flow contribution between dipolar and multipolar
dynamos.Comment: 10 pages, 9 figures, 1 table. To appear in ICARU
Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars
Observations of cool stars reveal dark spot-like features on their surfaces.
Compared to sunspots, starspots can be bigger or cover a larger fraction of the
stellar surface. While sunspots appear only at low latitudes, starspots are
also found in polar regions, in particular on rapidly rotating stars. Sunspots
are believed to result from the eruption of magnetic flux-tubes rising from the
deep interior of the Sun. The strong magnetic field locally reduces convective
heat transport to the solar surface. Such flux-tube models have also been
invoked to explain starspot properties. However, these models use several
simplifications and so far the generation of either sunspots or starspots has
not been demonstrated in a self-consistent simulation of stellar magnetic
convection. Here we show that direct numerical simulations of a distributed
dynamo operating in a density-stratified rotating spherical shell can
spontaneously generate cool spots. Convection in the interior of the model
produces a large scale magnetic field which interacts with near surface
granular convection leading to strong concentrations of magnetic flux and
formation of starspots. Prerequisites for the formation of sizeable
high-latitude spots in the model are sufficiently strong density stratification
and rapid rotation. Our model presents an alternate mechanism for starspot
formation by distributed dynamo action.Comment: 14 pages; Important additions in version 2; To appear in A&
Mercury or Mercury Free Restorations in Oral Cavity
Amalgam is basically a concoction of metals that has been used as a potent filling material in dentistry for the last 150 years. Amalgam usually consists of silver, mercury, tin and copper. Dental amalgam is a material used to fill cavities of tooth. Over the years, amalgam has become a topic of concern because it contains mercury. Mercury is a naturally occurring metal in the environment. Mercury exists as a liquid in room temperature but when heated, it becomes a gas. Flexibility of amalgam as a filling material is due Mercury. An alloy powder, a compound that is soft in nature when mixed with mercury makes it enough to mix and condense into the tooth. It hardens quickly and offers strong resistance to the forces of biting and chewing. There are studies reported on the safety of amalgam fillings. In 2005, European Union launched a comprehensive mercury strategy to reduce use of mercury. In 2008, countries like Norway and Denmark restricted the use of dental amalgam containing mercury. In 2009, this research was evaluated by U.S. Food and Drug Administration (FDA) and found no rationale to limit the use of amalgam. There are certain restorative materials that are available commercially that are mercury free in nature like Gold, Porcelain, Gallium alloys, Composite resin restoratives etc. They offer many advantages over amalgams containing mercury like: seals the dentin from future decay, reinforces remaining tooth structure, provides smooth and bonded margins, conservative and it blends naturally
Magnetic cycles in a dynamo simulation of the fully convective M-star Proxima Centauri
The recent discovery of an Earth-like exoplanet around Proxima Centauri has
shined a spot light on slowly rotating fully convective M-stars. When such
stars rotate rapidly (period days), they are known to generate
very high levels of activity that is powered by a magnetic field much stronger
than the solar magnetic field. Recent theoretical efforts are beginning to
understand the dynamo process that generates such strong magnetic fields.
However, the observational and theoretical landscape remains relatively
uncharted for fully convective M-stars that rotate slowly. Here we present an
anelastic dynamo simulation designed to mimic some of the physical
characteristics of Proxima Centauri, a representative case for slowly rotating
fully convective M-stars. The rotating convection spontaneously generates
differential rotation in the convection zone which drives coherent magnetic
cycles where the axisymmetric magnetic field repeatedly changes polarity at all
latitudes as time progress. The typical length of the `activity' cycle in the
simulation is about nine years, in good agreement with the recently proposed
activity cycle length of about seven years for Proxima Centauri. Comparing our
results with earlier work, we hypothesis that the dynamo mechanism undergoes a
fundamental change in nature as fully convective stars spin down with age.Comment: 8 pages, 4 figures, double column; Accepted in ApJ Letter
Consistent scaling laws in anelastic spherical shell dynamos
Numerical dynamo models always employ parameter values that differ by orders
of magnitude from the values expected in natural objects. However, such models
have been successful in qualitatively reproducing properties of planetary and
stellar dynamos. This qualitative agreement fuels the idea that both numerical
models and astrophysical objects may operate in the same asymptotic regime of
dynamics. This can be tested by exploring the scaling behavior of the models.
For convection-driven incompressible spherical shell dynamos with constant
material properties, scaling laws had been established previously that relate
flow velocity and magnetic field strength to the available power. Here we
analyze 273 direct numerical simulations using the anelastic approximation,
involving also cases with radius-dependent magnetic, thermal and viscous
diffusivities. These better represent conditions in gas giant planets and
low-mass stars compared to Boussinesq models. Our study provides strong support
for the hypothesis that both mean velocity and mean magnetic field strength
scale as a function of power generated by buoyancy forces in the same way for a
wide range of conditions.Comment: 9 pages, 4 figures, 1 table; data used in the paper can be found in
"Dataset.txt" file available in the source; to appear in Ap
Alcoholic Extract of Eclipta alba
As per WHO estimates, 80% of people around the world use medicinal plants for the cure and prevention of various diseases including cancer owing to their easy availability and cost effectiveness. Eclipta alba has long been used in Ayurveda to treat liver diseases, eye ailments, and hair related disorders. The promising medicinal value of E. alba prompted us to study the antioxidant, nontoxic, and anticancer potential of its alcoholic extract. In the current study, we evaluated the in vitro cytotoxic and antioxidant effect of the alcoholic extract of Eclipta alba (AEEA) in multiple cancer cell lines along with control. We have also evaluated its effect on different in vivo toxicity parameters. Here, we found that AEEA was found to be most active in most of the cancer cell lines but it significantly induced apoptosis in human breast cancer cell lines by disrupting mitochondrial membrane potential and DNA damage. Moreover, AEEA treatment inhibited migration in both MCF 7 and MDA-MB-231 cells in a dose dependent manner. Further, AEEA possesses robust in vitro antioxidant activity along with high total phenolic and flavonoid contents. In summary, our results indicate that Eclipta alba has enormous potential in complementary and alternative medicine for the treatment of cancer
Post-outburst evolution of bonafide FUor V2493 Cyg: A Spectro-photometric monitoring
We present here the results of eight years of our near-simultaneous
optical/near-infrared spectro-photometric monitoring of bonafide FUor candidate
`V2493 Cyg' starting from 2013 September to 2021 June. During our optical
monitoring period (between October 16, 2015 and December 30, 2019), the V2493
Cyg is slowly dimming with an average dimming rate of 26.6 5.6
mmag/yr in V band. Our optical photometric colors show a significant reddening
of the source post the second outburst pointing towards a gradual expansion of
the emitting region post the second outburst. The mid infra-red colors, on the
contrary, exhibits a blueing trend which can be attributed to the brightening
of the disc due to the outburst. Our spectroscopic monitoring shows a dramatic
variation of the H line as it transitioned from absorption feature to
the emission feature and back. Such transition can possibly be explained by the
variation in the wind structure in combination with accretion. Combining our
time evolution spectra of the Ca II infra-red triplet lines with the previously
published spectra of V2493 Cyg, we find that the accretion region has
stabilised compared to the early days of the outburst. The evolution of the O I
7773 \AA~ line also points towards the stabilization of the
circumstellar disc post the second outburst.Comment: 34 pages, 12 figures, 6 tables, accepted for publication in Ap
Challenges and opportunities in mixed method data collection on mental health issues of health care workers during COVID-19 pandemic in India
Background: The present paper describes the key challenges and opportunities of mixed method telephonic data collection for mental health research using field notes and the experiences of the investigators in a multicenter study in ten sites of India. The study was conducted in public and private hospitals to understand the mental health status, social stigma and coping strategies of different healthcare personnel during the COVID-19 pandemic in India.Methods: Qualitative and quantitative interviews were conducted telephonically. The experiences of data collection were noted as a field notes/diary by the data collectors and principal investigators.Results: The interviewers reported challenges such as network issues, lack of transfer of visual cues and sensitive content of data. Although the telephonic interviews present various challenges in mixed method data collection, it can be used as an alternative to face-to-face data collection using available technology.Conclusions: It is important that the investigators are well trained keeping these challenges in mind so that their capacity is built to deal with these challenges and good quality data is obtained
Reduced cortical thickness in patients with acute-on-chronic liver failure due to non-alcoholic etiology
Background:
Acute-on-chronic liver failure (ACLF) is a form of liver disease with high short-term mortality. ACLF offers considerable potential to affect the cortical areas by significant tissue injury due to loss of neurons and other supporting cells. We measured changes in cortical thickness and metabolites profile in ACLF patients following treatment, and compared it with those of age matched healthy volunteers.
Methods:
For the cortical thickness analysis we performed whole brain high resolution T1-weighted magnetic resonance imaging (MRI) on 15 ACLF and 10 healthy volunteers at 3T clinical MR scanner. Proton MR Spectroscopy (1H MRS) was also performed to measure level of altered metabolites. Out of 15 ACLF patients 10 survived and underwent
follow-up study after clinical recovery at 3 weeks. FreeSurfer program was used to quantify cortical thickness and LC- Model software was used to quantify absolute metabolites concentrations. Neuropsychological (NP) test was performed to assess the cognitive performance in follow-up ACLF patients compared to controls.
Results:
Significantly reduced cortical thicknesses in multiple brain sites, and significantly decreased N-acetyl
aspartate (NAA), myo-inositol (mI) and significantly increased glutamate/glutamine (glx) metabolites were observed in ACLF compared to those of controls at baseline study. Follow-up patients showed significant recovery in cortical thickness and Glx level, while NAA and mI were partially recovered compared to baseline study. When compared to controls, follow-up patients still showed reduced cortical thickness and altered metabolites level. Follow-up patients had abnormal neuropsychological (NP) scores compared to controls.
Conclusions:
Neuronal loss as suggested by the reduced NAA, decreased cellular density due to increased cerebral hyperammonemia as supported by the increased glx level, and increased proinflammatory cytokines and free radicals may account for the reduced cortical thickness in ACLF patients. Presence of reduced cortical thickness, altered
metabolites and abnormal NP test scores in post recovery subjects as compared to those of controls is associated
with incomplete clinical recovery. The current imaging protocol can be easily implemented in clinical settings to evaluate and monitor brain tissue changes in patients with ACLF during the course of treatment
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