27,018 research outputs found

### A common-envelope wind model for Type Ia supernovae (I): binary evolution and birth rate

The single-degenerate (SD) model is one of the principal models for the
progenitors of type Ia supernovae (SNe Ia), but some of the predictions in the
most widely studied version of the SD model, i.e. the optically thick wind
(OTW) model, have not been confirmed by observations. Here, we propose a new
version of the SD model in which a common envelope (CE) is assumed to form when
the mass-transfer rate between a carbon-oxygen white dwarf (CO WD) and its
companion exceeds a critical accretion rate. The WD may gradually increase its
mass at the base of the CE. Due to the large nuclear luminosity for stable
hydrogen burning, the CE may expand to giant dimensions and will lose mass from
the surface of the CE by a CE wind (CEW). Because of the low CE density, the
binary system will avoid a fast spiral-in phase and finally re-emerge from the
CE phase. Our model may share the virtues of the OTW model but avoid some of
its shortcomings. We performed binary stellar evolution calculations for more
than 1100 close WD + MS binaries. Compared with the OTW model, the parameter
space for SNe Ia from our CEW model extends to more massive companions and less
massive WDs. Correspondingly, the Galactic birth rate from the CEW model is
higher than that from the OTW model by $\sim$30\%. Finally, we discuss the
uncertainties of the CEW model and the differences between our CEW model and
the OTW model.Comment: 28 pages, 24 figures, accepted for publication in MNRA

### Capital and macroeconomic instability in a discrete-time model with forward-looking interest rate rules

The authors establish the necessary and sufficient conditions for local real determinacy in a discrete-time production economy with monopolistic competition and a quadratic price adjustment cost under forward-looking policy rules, for the case where capital is in exogenously fixed supply and the case with endogenous capital accumulation. Using these conditions, they show that (i) indeterminacy is more likely to occur with a greater share of payment to capital in value-added production cost; (ii) indeterminacy can be more or less likely to occur with constant capital than with variable capital; (iii) indeterminacy is more likely to occur when prices are modelled as jump variables than as predetermined variables; (iv) indeterminacy is less likely to occur with a greater degree of steady-state monopolistic distortions; and (v) indeterminacy is less likely to occur with a greater degree of price stickiness or with a higher steady-state inflation rate. In contrast to some existing research, the authors' analysis indicates that capital tends to lead to macroeconomic instability by affecting firms' pricing behavior in product markets rather than households' arbitrage activity in asset markets even under forward-looking policy rules.Capital ; Interest rates

### Quantitative test of a quantum theory for the resistive transition in a superconducting single-walled carbon nanotube bundle

The phenomenon of superconductivity depends on the coherence of the phase of
the superconducting order parameter. The resistive transition in
quasi-one-dimensional (quasi-1D) superconductors is broad because of a large
phase fluctuation. We show that the resistive transition of a superconducting
single-walled carbon nanotube bundle is in quantitative agreement with the
Langer-Ambegaokar-McCumber-Halperin (LAMH) theory. We also demonstrate that the
resistive transition below T^*_c = 0.89T_c0 is simply proportional to exp
[-(3\beta T^*_c/T)(1-T/T^*_c)^3/2], where the barrier height has the same form
as that predicted by the LAMH theory and T_c0 is the mean field superconducting
transition temperature.Comment: 4 pages, 3 figure

### Solving Dirac equations on a 3D lattice with inverse Hamiltonian and spectral methods

A new method to solve the Dirac equation on a 3D lattice is proposed, in
which the variational collapse problem is avoided by the inverse Hamiltonian
method and the fermion doubling problem is avoided by performing spatial
derivatives in momentum space with the help of the discrete Fourier transform,
i.e., the spectral method. This method is demonstrated in solving the Dirac
equation for a given spherical potential in 3D lattice space. In comparison
with the results obtained by the shooting method, the differences in single
particle energy are smaller than $10^{-4}$~MeV, and the densities are almost
identical, which demonstrates the high accuracy of the present method. The
results obtained by applying this method without any modification to solve the
Dirac equations for an axial deformed, non-axial deformed, and octupole
deformed potential are provided and discussed.Comment: 18 pages, 6 figure

### Matter loops corrected modified gravity in Palatini formulation

Recently, corrections to the standard Einstein-Hilbert action are proposed to
explain the current cosmic acceleration in stead of introducing dark energy. In
the Palatini formulation of those modified gravity models, there is an
important observation due to Arkani-Hamed: matter loops will give rise to a
correction to the modified gravity action proportional to the Ricci scalar of
the metric. In the presence of such term, we show that the current forms of
modified gravity models in Palatini formulation, specifically, the 1/R gravity
and $\ln R$ gravity, will have phantoms. Then we study the possible
instabilities due to the presence of phantom fields. We show that the strong
instability in the metric formulation of 1/R gravity indicated by Dolgov and
Kawasaki will not appear and the decay timescales for the phantom fields may be
long enough for the theories to make sense as effective field theory . On the
other hand, if we change the sign of the modification terms to eliminate the
phantoms, some other inconsistencies will arise for the various versions of the
modified gravity models. Finally, we comment on the universal property of the
Palatini formulation of the matter loops corrected modified gravity models and
its implications.Comment: 11 pages, 1 figures, References adde

### An Efficient Method for GPS Multipath Mitigation Using the Teager-Kaiser-Operator-based MEDLL

An efficient method for GPS multipath mitigation is proposed. The motivation for this proposed method is to integrate the Teager-Kaiser Operator (TKO) with the Multipath Estimating Delay Lock Loop (MEDLL) module to mitigate the GPS multipath efficiently. The general implementation process of the proposed method is that we first utilize the TKO to operate on the received signal’s Auto-Correlation Function (ACF) to get an initial estimate of the multipaths. Then we transfer the initial estimated results to the MEDLL module for a further estimation. Finally, with a few iterations which are less than those of the original MEDLL algorithm, we can get a more accurate estimate of the Line-Of-Sight (LOS) signal, and thus the goal of the GPS multipath mitigation is achieved. The simulation results show that compared to the original MEDLL algorithm, the proposed method can reduce the computation load and the hardware and/or software consumption of the MEDLL module, meanwhile, without decreasing the algorithm accuracy

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