1,629 research outputs found
Can a Kasner Universe with a Viscous Cosmological Fluid be Anisotropic?
A Bianchi type -I metric of Kasner form is considered, when the space is
filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the
Kasner metric to be anisotropic provided that the fluid satisfies the
Zel'dovich equation of state, the viscous fluid does not permit the Kasner
metric to be anisotropic at all. In the latter case, we calculate the Kasner
(isotropic) metric expressed by the fluid's density, pressure, and bulk
viscosity, at some chosen instant . The equation of state is also
calculated. The present paper is related to a recent Comment of Cataldo and del
Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the
present authors [Phys. Rev. D {\bf 56}, 3322 (1997)].Comment: 8 pages, LaTeX, no figures. To appear in PR
Visual orbit for the low-mass binary Gliese 22 AC from speckle interferometry
Based on 14 data points obtained with near-infrared speckle interferometry
and covering an almost entire revolution, we present a first visual orbit for
the low-mass binary system Gliese 22 AC. The quality of the orbit is largely
improved with respect to previous astrometric solutions. The dynamical system
mass is 0.592 +- 0.065 solar masses, where the largest part of the error is due
to the Hipparcos parallax. A comparison of this dynamical mass with
mass-luminosity relations on the lower main sequence and theoretical
evolutionary models for low-mass objects shows that both probably underestimate
the masses of M dwarfs. A mass estimate for the companion Gliese 22 C indicates
that this object is a very low-mass star with a mass close to the hydrogen
burning mass limit.Comment: Accepted by Astronomy and Astrophysics, 6 pages, 2 figure
The Density of Coronal Plasma in Active Stellar Coronae
We have analyzed high-resolution X-ray spectra of a sample of 22 active stars
observed with the High Energy Transmission Grating Spectrometer on {\em
Chandra} in order to investigate their coronal plasma density. Densities where
investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII.
While Si XIII lines in all stars of the sample are compatible with the
low-density limit, Mg XI lines betray the presence of high plasma densities ( cm) for most of the sources with higher X-ray luminosity ( erg/s); stars with higher and tend to have higher
densities at high temperatures. Ratios of O VII lines yield much lower
densities of a few cm, indicating that the ``hot'' and
``cool'' plasma resides in physically different structures. Our findings imply
remarkably compact coronal structures, especially for the hotter plasma
emitting the Mg XI lines characterized by coronal surface filling factor,
, ranging from to , while we find
values from a few up to for the cooler plasma emitting the O
VII lines. We find that approaches unity at the same stellar surface
X-ray flux level as solar active regions, suggesting that these stars become
completely covered by active regions. At the same surface flux level,
is seen to increase more sharply with increasing surface flux. These
results appear to support earlier suggestions that hot K plasma in
active coronae arises from flaring activity, and that this flaring activity
increases markedly once the stellar surface becomes covered with active
regions.Comment: 53 pages, 19 figures, accepted for publication in Astrophysical
Journal. A version of the paper with higher quality figures is available from
http://www.astropa.unipa.it/Library/preprint.htm
Cavitation induced by explosion in a model of ideal fluid
We discuss the problem of an explosion in the cubic-quintic superfluid model,
in relation to some experimental observations. We show numerically that an
explosion in such a model might induce a cavitation bubble for large enough
energy. This gives a consistent view for rebound bubbles in superfluid and we
indentify the loss of energy between the successive rebounds as radiated waves.
We compute self-similar solution of the explosion for the early stage, when no
bubbles have been nucleated. The solution also gives the wave number of the
excitations emitted through the shock wave.Comment: 21 pages,13 figures, other comment
The Ethics of Care: Normative Structures and Empirical Implications
In this article I argue that the ethics of care provides us with a novel reading of human relations, and therefore makes possible a fresh approach to several empirical challenges. In order to explore this connection, I discuss some specific normative features of the ethics of care—primarily the comprehension of the moral agent and the concept of care—as these two key elements contribute substantially to a new ethical outlook. Subsequently, I argue that the relational and reciprocal mode of thinking with regard to the moral agent must be extended to our understanding of care. I term this comprehension “mature care”. Citing conflicts of interests as examples, I demonstrate how this conceptualization of care may further advance the ethics of care’s ability to take on empirical challenges. Finally, I discuss political implications that may emanate from the ethics of care and the concept of mature care
Anomaly in the stability limit of liquid helium 3
We propose that the liquid-gas spinodal line of helium 3 reaches a minimum at
0.4 K. This feature is supported by our cavitation measurements. We also show
that it is consistent with extrapolations of sound velocity measurements.
Speedy [J. Phys. Chem. 86, 3002 (1982)] previously proposed this peculiar
behavior for the spinodal of water and related it to a change in sign of the
expansion coefficient alpha, i. e. a line of density maxima. Helium 3 exhibits
such a line at positive pressure. We consider its extrapolation to negative
pressure. Our discussion raises fundamental questions about the sign of alpha
in a Fermi liquid along its spinodal.Comment: 5 pages, 3 figure
Potential Antiviral Options against SARS-CoV-2 Infection
As of June 2020, the number of people infected with severe acute respiratory coronavirus 2 (SARS-CoV-2) continues to skyrocket, with more than 6.7 million cases worldwide. Both the World Health Organization (WHO) and United Nations (UN) has highlighted the need for better control of SARS-CoV-2 infections. However, developing novel virus-specific vaccines, monoclonal antibodies and antiviral drugs against SARS-CoV-2 can be time-consuming and costly. Convalescent sera and safe-in-man broad-spectrum antivirals (BSAAs) are readily available treatment options. Here, we developed a neutralization assay using SARS-CoV-2 strain and Vero-E6 cells. We identified the most potent sera from recovered patients for the treatment of SARS-CoV-2-infected patients. We also screened 136 safe-in-man broad-spectrum antivirals against the SARS-CoV-2 infection in Vero-E6 cells and identified nelfinavir, salinomycin, amodiaquine, obatoclax, emetine and homoharringtonine. We found that a combination of orally available virus-directed nelfinavir and host-directed amodiaquine exhibited the highest synergy. Finally, we developed a website to disseminate the knowledge on available and emerging treatments of COVID-19
Toward a Density Functional Description of Liquid pH(2)
A finite-temperature density functional approach to describe the properties of parahydrogen in the liquid-vapor coexistence region is presented. The first proposed functional is zero-range, where the density-gradient term is adjusted so as to reproduce the surface tension of the liquid-vapor interface at low temperature. The second functional is finite-range and, while it is fitted to reproduce bulk pH(2) properties only, it is shown to yield surface properties in good agreement with experiments. These functionals are used to study the surface thickness of the liquid-vapor interface, the wetting transition of parahydrogen on a planar Rb model surface, and homogeneous cavitation in bulk liquid pH(2)
Structure of the hDmc1-ssDNA filament reveals the principles of its architecture
In eukaryotes, meiotic recombination is a major source of genetic diversity, but its defects in humans lead to abnormalities such as Down's, Klinefelter's and other syndromes. Human Dmc1 (hDmc1), a RecA/Rad51 homologue, is a recombinase that plays a crucial role in faithful chromosome segregation during meiosis. The initial step of homologous recombination occurs when hDmc1 forms a filament on single-stranded (ss) DNA. However the structure of this presynaptic complex filament for hDmc1 remains unknown. To compare hDmc1-ssDNA complexes to those known for the RecA/Rad51 family we have obtained electron microscopy (EM) structures of hDmc1-ssDNA nucleoprotein filaments using single particle approach. The EM maps were analysed by docking crystal structures of Dmc1, Rad51, RadA, RecA and DNA. To fully characterise hDmc1-DNA complexes we have analysed their organisation in the presence of Ca2+, Mg2+, ATP, AMP-PNP, ssDNA and dsDNA. The 3D EM structures of the hDmc1-ssDNA filaments allowed us to elucidate the principles of their internal architecture. Similar to the RecA/Rad51 family, hDmc1 forms helical filaments on ssDNA in two states: extended (active) and compressed (inactive). However, in contrast to the RecA/Rad51 family, and the recently reported structure of hDmc1-double stranded (ds) DNA nucleoprotein filaments, the extended (active) state of the hDmc1 filament formed on ssDNA has nine protomers per helical turn, instead of the conventional six, resulting in one protomer covering two nucleotides instead of three. The control reconstruction of the hDmc1-dsDNA filament revealed 6.4 protein subunits per helical turn indicating that the filament organisation varies depending on the DNA templates. Our structural analysis has also revealed that the N-terminal domain of hDmc1 accomplishes its important role in complex formation through domain swapping between adjacent protomers, thus providing a mechanistic basis for coordinated action of hDmc1 protomers during meiotic recombination
To wet or not to wet: that is the question
Wetting transitions have been predicted and observed to occur for various
combinations of fluids and surfaces. This paper describes the origin of such
transitions, for liquid films on solid surfaces, in terms of the gas-surface
interaction potentials V(r), which depend on the specific adsorption system.
The transitions of light inert gases and H2 molecules on alkali metal surfaces
have been explored extensively and are relatively well understood in terms of
the least attractive adsorption interactions in nature. Much less thoroughly
investigated are wetting transitions of Hg, water, heavy inert gases and other
molecular films. The basic idea is that nonwetting occurs, for energetic
reasons, if the adsorption potential's well-depth D is smaller than, or
comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At
the wetting temperature, Tw, the transition to wetting occurs, for entropic
reasons, when the liquid's surface tension is sufficiently small that the free
energy cost in forming a thick film is sufficiently compensated by the fluid-
surface interaction energy. Guidelines useful for exploring wetting transitions
of other systems are analyzed, in terms of generic criteria involving the
"simple model", which yields results in terms of gas-surface interaction
parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy
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