1,416 research outputs found
Collapsing strange quark matter in Vaidya geometry
Exact solutions of the gravitational field equations for a mixture of a null
charged strange quark fluid and radiation are obtained in a Vaidya space-time.
The conditions for the formation of a naked singularity are analyzed by
considering the behavior of radial geodesics originating from the central
singularity.Comment: 6 pages, no figure
On the role of pressure anisotropy for relativistic stars admitting conformal motion
We investigate the spacetime of anisotropic stars admitting conformal motion.
The Einstein field equations are solved using different ansatz of the surface
tension. In this investigation, we study two cases in details with the
anisotropy as: [1] [2] where, n, and are arbitrary constants.
The solutions yield expressions of the physical quantities like pressure
gradients and the mass.Comment: 21 pages, accepted for publication in 'Astrophysics and Space
Science
Gamma-Ray Emission From Be/X-Ray Binaries
Be/X-ray binaries are systems formed by a massive Be star and a magnetized
neutron star, usually in an eccentric orbit. The Be star has strong equatorial
winds occasionally forming a circumstellar disk. When the neutron star
intersects the disk the accretion rate dramatically increases and a transient
accretion disk can be formed around the compact object. This disk can last
longer than a single orbit in the case of major outbursts. If the disk rotates
faster than the neutron star, the Cheng-Ruderman mechanism can produce a
current of relativistic protons that would impact onto the disk surface,
producing gamma-rays from neutral pion decays and initiating electromagnetic
cascades inside the disk. In this paper we present calculations of the
evolution of the disk parameters during both major and minor X-ray events, and
we discuss the generation of gamma-ray emission at different energies within a
variety of models that include both screened and unscreened disks.Comment: 14 pages, to appear in: "The multiwavelength approach to unidentified
gamma-ray sources", Eds. K. S. Cheng & G.E. Romero, Kluwer Academic Publisher
(Astrophysics and Space Sciences Journal). The present version has two
additional figures respect to the version to be published in the journa
Contribution of pulsars to the gamma-ray background and their observation with the space telescopes GLAST and AGILE
Luminosities and uxes of the expected population of galactic gamma-ray
pulsars become foreseeable if physical distributions at birth and evolutive
history are assigned. In this work we estimate the contribution of pulsar uxes
to the gamma-ray background, which has been measured by the EGRET experiment on
board of the CGRO. For pulsar luminosities we select some of the most important
gamma-ray emission models, taking into account both polar cap and outer gap
scenarios. We nd that this contribution strongly depends upon controversial
neutron star birth properties. A comparison between our simulation results and
EGRET data is presented for each model, nding an average contribution of about
10%. In addition, we perform the calculation of the number of new gamma-ray
pulsars detectable by GLAST and AGILE, showing a remarkable di erence between
the two classes of models. Finally, we suggest some improvements in the
numerical code, including more sophisticated galactic m odels and di erent
populations of pulsars like binaries, milliseconds, anomalous pulsars and
magnetars.Comment: 6 pages, 6 figures, to be published in the Proceedings of the 6th
International Symposium ''Frontiers of Fundamental and Computational
Physics'' (FFP6), Udine (Italy), Sep. 26-29, 200
Cooling Properties of Cloudy Bag Strange Stars
As the chiral symmetry is widely recognized as an important driver of the
strong interaction dynamics, current strange stars models based on MIT bag
models do not obey such symmetry. We investigate properties of bare strange
stars using the Cloudy Bag Model, in which a pion cloud coupled to the
quark-confining bag is introduced such that chiral symmetry is conserved. We
find that in this model the decay of pions is a very efficient cooling way. In
fact it can carry out most the thermal energy in a few milliseconds and
directly convert them into 100MeV photons via pion decay. This may be a very
efficient -ray burst mechanism. Furthermore, the cooling behavior may
provide a possible way to distinguish a compact object between a neutron star,
MIT strange star and Cloudy Bag strange star in observations.Comment: 23 pages, 14 figures, accepted by Astroparticle Physics, abstract
appeared here has been shortene
Protection Against Cardiac Injury by Small Ca\u3csup\u3e2 +\u3c/sup\u3e-Sensitive K\u3csup\u3e+\u3c/sup\u3e Channels Identified in Guinea Pig Cardiac Inner Mitochondrial Membrane
We tested if small conductance, Ca2 +âsensitive K+ channels (SKCa) precondition hearts against ischemia reperfusion (IR) injury by improving mitochondrial (m) bioenergetics, if O2âderived free radicals are required to initiate protection via SKCa channels, and, importantly, if SKCa channels are present in cardiac cell inner mitochondrial membrane (IMM). NADH and FAD, superoxide (O2â), and m[Ca2 +] were measured in guinea pig isolated hearts by fluorescence spectrophotometry. SKCa and IKCa channel opener DCEBIO (DCEB) was given for 10 min and ended 20 min before IR. Either TBAP, a dismutator of O2â, NS8593, an antagonist of SKCa isoforms, or other KCa and KATP channel antagonists, were given before DCEB and before ischemia. DCEB treatment resulted in a 2-fold increase in LV pressure on reperfusion and a 2.5 fold decrease in infarct size vs. non-treated hearts associated with reduced O2â and m[Ca2 +], and more normalized NADH and FAD during IR. Only NS8593 and TBAP antagonized protection by DCEB. Localization of SKCa channels to mitochondria and IMM was evidenced by a) identification of purified mSKCa protein by Western blotting, immuno-histochemical staining, confocal microscopy, and immuno-gold electron microscopy, b) 2-D gel electrophoresis and mass spectroscopy of IMM protein, c) [Ca2 +]âdependence of mSKCa channels in planar lipid bilayers, and d) matrix K+ influx induced by DCEB and blocked by SKCa antagonist UCL1684. This study shows that 1) SKCa channels are located and functional in IMM, 2) mSKCa channel opening by DCEB leads to protection that is O2âdependent, and 3) protection by DCEB is evident beginning during ischemia
Product Groups, Discrete Symmetries, and Grand Unification
We study grand unified theories based on an SU(5)xSU(5) gauge group in which
the GUT scale, M_{GUT}, is the VEV of an exact or approximate modulus, and in
which fast proton decay is avoided through a combination of a large triplet
mass and small triplet couplings. These features are achieved by discrete
symmetries. In many of our models, M_{GUT} is generated naturally by the
balance of higher dimension terms that lift the GUT modulus potential, and soft
supersymmetry breaking masses. The theories often lead to interesting patterns
of quark and lepton masses. We also discuss some distinctions between grand
unified theories and string unification.Comment: 23 pages; no figures; revtex
Mitochondrial Matrix K\u3csup\u3e+\u3c/sup\u3e Flux Independent of Large-conductance Ca\u3csup\u3e2+\u3c/sup\u3e-activated K\u3csup\u3e+\u3c/sup\u3e Channel Opening
Large-conductance Ca2+-activated K+ channels (BKCa) in the inner mitochondrial membrane may play a role in protecting against cardiac ischemia-reperfusion injury. NS1619 (30 ÎŒM), an activator of BKCa channels, was shown to increase respiration and to stimulate reactive oxygen species generation in isolated cardiac mitochondria energized with succinate. Here, we tested effects of NS1619 to alter matrix K+, H+, and swelling in mitochondria isolated from guinea pig hearts. We found that 30 ÎŒM NS1619 did not change matrix K+, H+, and swelling, but that 50 and 100 ÎŒM NS1619 caused a concentration-dependent increase in matrix K+ influx (PBFI fluorescence) only when quinine was present to block K+/H+ exchange (KHE); this was accompanied by increased mitochondrial matrix volume (light scattering). Matrix pH (BCECF fluorescence) was decreased slightly by 50 and 100 ÎŒM NS1619 but markedly more so when quinine was present. NS1619 (100 ÎŒM) caused a significant leak in lipid bilayers, and this was enhanced in the presence of quinine. The K+ ionophore valinomycin (0.25 nM), which like NS1619 increased matrix volume and increased K+ influx in the presence of quinine, caused matrix alkalinization followed by acidification when quinine was absent, and only alkalinization when quinine was present. If K+ is exchanged instantly by H+ through activated KHE, then matrix K+ influx should stimulate H+ influx through KHE and cause matrix acidification. Our results indicate that KHE is not activated immediately by NS1619-induced K+ influx that NS1619 induces matrix K+ and H+ influx through a nonspecific transport mechanism, and that enhancement with quinine is not due to the blocking of KHE, but to a nonspecific effect of quinine to enhance current leak by NS1619
Phenomenological Consequences of Singlet Neutrinos
In this paper, we study the phenomenology of right-handed neutrino
isosinglets. We consider the general situation where the neutrino masses are
not necessarily given by , where and are the Dirac and
Majorana mass terms respectively. The consequent mixing between the light and
heavy neutrinos is then not suppressed, and we treat it as an independent
parameter in the analysis. It turns out that conversion is an important
experiment in placing limits on the heavy mass scale () and the mixing.
Mixings among light neutrinos are constrained by neutrinoless double beta
decay, as well as by solar and atmospheric neutrino experiments. Detailed
one-loop calculations for lepton number violating vertices are provided.Comment: Revtex file,TRI-PP-94-1,VPI-IHEP-94-1, 23 pages, a compressed for 8
figures is appende
The pseudo scalar form factor of the nucleon, the sigma-like term, and the amplitude for charged pion electro-production near threshold
The pseudo scalar form factor, which represents the pseudo scalar quark
density distribution due to finite quark masses on the nucleon, is shown to
manifest itself with the induced pseudo scalar form factor in the
amplitude for the charged pion electro-production. Both form factors show their
own peculiar momentum dependence. Under the approximation on which the
Goldberg-Treimann relation holds, a sum of both form factors' contributions
accounts for the t-channel contribution in the charged pion electro-production
near threshold.Comment: 10 page
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