104 research outputs found
Response of the Intertropical Convergence Zone to Antarctic Ice Sheet melt
Past cooling events in the Northern Hemisphere have been shown to impact the location of the intertropical convergence zone (ITCZ) and therewith induce a southward shift of tropical precipitation. Here we use highâresolution coupled oceanâatmosphere simulations to show that reasonable past melt rates of the Antarctic Ice Sheet can similarly have led to shifts of the ITCZ, albeit in opposite direction, through largeâscale surface air temperature changes over the Southern Ocean. Through sensitivity experiments employing slightly negative to large positive meltwater fluxes we deduce that meridional shifts of the Hadley cell and therewith the ITCZ are, to a first order, a linear response to Southern Hemisphere highâlatitude surface air temperature changes and Antarctic Ice Sheet melt rates. This highlights the possibility to use past episodes of anomalous melt rates to better constrain a possible future response of low latitude precipitation to continued global warming and a shrinking Antarctic Ice Sheet
Magnetic and vibrational properties of the covalent chain antiferromagnet RbFeS2
Ternary rubidium-iron sulfide, RbFeS2, belongs to a family of quasi-one-dimensional compounds with the general chemical composition AFeCh2 (where A â K, Rb, Cs, Tl; Ch â S, Se). Understanding the magnetic properties of these compounds is a challenge. The controversy concerning the spin-state of the iron ion needs to be resolved to build the proper model of magnetism. Single crystals of RbFeS2 were grown and characterized by powder x-ray diffraction. QD MPMS-5 SQUID magnetometry was used to measure the magnetic susceptibility, and specific heat was measured utilizing QD PPMS-9 setup. Above the transition to three-dimensional antiferromagnetic order at the NĂ©el temperature of TN = 188 K, the susceptibility exhibits unusual quasi-linear increase up to the highest measured temperature of 500 K. The specific heat was measured in the temperature range 1.8 â 300 K. Ab initio phonon dispersion and density-of-states calculations were performed by means of density functional theory (DFT), and the calculated lattice specific heat was subtracted from the measured one giving the magnetic contribution to the specific heat. Our results suggest that the features of the magnetic specific heat are general for the whole family of the covalent chain ternary iron chalcogenides of the AFeCh2 structure and indicate an intermediate S = 3/2 spin state of the iron ion
Scalar-Isovector Delta-Meson in the Relativistic Mean-Field Theory and the Structure of Neutron Stars with a Quark Core
In the framework of the relativistic mean-field theory, we have considered
the equation of state of superdense nuclear matter, taking into account an
effective scalar-isovector delta-meson field. The effect of the delta-meson
field on the characteristics of a Maxwell-type quark phase transition has been
studied. The quark phase is described with the aid of the improved version of
the MIT (Massachusetts Institute of Technology) bag model, in which
interactions between the u, d, s quarks inside the bag are taken into account
in the one-gluon exchange approximation. For different values of the bag
parameter B, series of neutron star models with a quark core have been built.
Stability problems for neutron stars with an infinitesimal quark core are
discussed. An estimate is obtained for the amount of energy released in a
catastrophic transformation of a critical neutron star to a star with a
finite-size quark core.Comment: 7 pages, 2 figures, Talk given at the International Conference
RUSGRAV-13, June 23-28, 2008, PFUR, Mosco
Icebergs in the North Atlantic: Modelling circulation changes and glacio-marine deposition
In order to investigate meltwater events in the North Atlantic, a simple iceberg generation, drift, and melting routine was implemented in a high-resolution OGCM. Starting from the modelled last glacial state, every 25th day cylindrical model icebergs 300 meters high were released at 32 specific points along the coasts. Icebergs launched at the Barents Shelf margin spread a light meltwater lid over the Norwegian and Greenland Seas, shutting down the deep convection and the anti-clockwise circulation in this area. Due to the constraining ocean circulation, the icebergs produce a tongue of relatively cold and fresh water extending eastward from Hudson Strait that must develop at this location, regardless of iceberg origin. From the total amount of freshwater inferred by the icebergs, the thickness of the deposited IRD could be calculated in dependance of iceberg sediment concentration. In this way, typical extent and thickness of Heinrich layers could be reproduced, running the model for 250 years of steady state with constant iceberg meltwater inflow
Trapped and excited w modes of stars with a phase transition and R>=5M
The trapped -modes of stars with a first order phase transition (a density
discontinuity) are computed and the excitation of some of the modes of these
stars by a perturbing shell is investigated. Attention is restricted to odd
parity (``axial'') -modes. With the radius of the star, its mass,
the radius of the inner core and the mass of such core, it is
shown that stars with can have several trapped -modes, as long
as . Excitation of the least damped -mode is confirmed for
a few models. All of these stars can only exist however, for values of the
ratio between the densities of the two phases, greater than . We also
show that stars with a phase transition and a given value of can have far
more trapped modes than a homogeneous single density star with the same value
of , provided both and are smaller than 3. If the
phase transition is very fast, most of the stars with trapped modes are
unstable to radial oscillations. We compute the time of instability, and find
it to be comparable to the damping of the -mode excited in most cases where
-mode excitation is likely. If on the other hand the phase transition is
slow, all the stars are stable to radial oscillations.Comment: To appear in Physical Review
Magnetization of manganite thin films on ferroelectric substrates
© 2016 Elsevier B.V.Here we report the magnetic susceptibility measurements of magnetron sputtered orthorhombic manganite RMnO3 (R=Yb, Gd) thin films deposited on dielectric LaAlO3 and ferroelectric SrTiO3, LiNbO3 substrates. We observed that all of investigated o-RMnO3 films show a splitting in the temperature dependence of ZFC and FC magnetization curves. We found that the substrate can impact on the splitting temperature ZFC-FC curves and absolute value of the magnetization of thin films
Magnetization of manganite thin films on ferroelectric substrates
© 2016 Elsevier B.V.Here we report the magnetic susceptibility measurements of magnetron sputtered orthorhombic manganite RMnO3 (R=Yb, Gd) thin films deposited on dielectric LaAlO3 and ferroelectric SrTiO3, LiNbO3 substrates. We observed that all of investigated o-RMnO3 films show a splitting in the temperature dependence of ZFC and FC magnetization curves. We found that the substrate can impact on the splitting temperature ZFC-FC curves and absolute value of the magnetization of thin films
Vibrational properties and magnetic specific heat of the covalent chain antiferromagnet RbFeS e2
© 2018 American Physical Society. The magnetic specific heat of RbFeSe2 and the spin state of Fe3+ ions in the compound have been studied. Phonon dispersion and phonon density of states (PDOS), element specific and total, were evaluated from first-principles calculations. It is shown that iron atoms in quasi-one-dimensional chains have dramatically different vibrational properties against Rb and Se atoms: the Fe PDOS is mostly concentrated within two Einstein-like optical phonon peaks at high frequencies. Analysis of our Mössbauer data for RbFeSe2, utilizing the calculated Fe PDOS as well as our optical absorption measurements, have shown full agreement with the location of the high-frequency optical-type lattice vibrations within the FeSe4 tetrahedra. The calculated PDOS was utilized to evaluate the lattice contribution to the specific heat. The phonon heat capacity has been used to evaluate the magnetic specific heat of the quasi-one-dimensional antiferromagnetically correlated Fe3+ ion chains in RbFeSe2. An intermediate spin state S=3/2 has been found most closely relevant to our magnetic entropy analysis for Fe3+ ions in RbFeSe2
Energy Release During Disk Accretion onto a Rapidly Rotating Neutron Star
The energy release L_s on the surface of a neutron star (NS) with a weak
magnetic field and the energy release L_d in the surrounding accretion disk
depend on two independent parameters that determine its state (for example,
mass M and cyclic rotation frequency f) and is proportional to the accretion
rate. We derive simple approximation formulas illustrating the dependence of
the efficiency of energy release in an extended disk and in a boundary layer
near the NS surface on the frequency and sense of rotation for various NS
equations of state. Such formulas are obtained for the quadrupole moment of a
NS, for a gap between its surface and a marginally stable orbit, for the
rotation frequency in an equatorial Keplerian orbit and in the marginally
stable circular orbit, and for the rate of NS spinup via disk accretion. In the
case of NS and disk counterrotation, the energy release during accretion can
reach . The sense of NS rotation is a factor that strongly
affects the observed ratio of nuclear energy release during bursts to
gravitational energy release between bursts in X-ray bursters. The possible
existence of binary systems with NS and disk counterrotation in the Galaxy is
discussed. Based on the static criterion for stability, we present a method of
constructing the dependence of gravitational mass M on Kerr rotation parameter
j and on total baryon mass (rest mass) m for a rigidly rotating neutron star.
We show that all global NS characteristics can be expressed in terms of the
function M(j, m) and its derivatives.Comment: 42 pages, 12 figures, to appear in Astronomy Letters, 2000, v.26,
p.69
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