3,298 research outputs found
Biases in Expansion Distances of Novae Arising from the Prolate Geometry of Nova Shells
(abridged) Expansion distances (or expansion parallaxes) for classical novae
are based on comparing a measurement of the shell expansion velocity,
multiplied by the time since outburst, with some measure of the angular size of
the shell. We review and formalize this method in the case of prolate
spheroidal shells. We present expressions for the maximum line-of-sight
velocity from a complete, expanding shell and for its projected major and minor
axes, in terms of the intrinsic axis ratio and the inclination of the polar
axis to the line of sight. For six distinct definitions of ``angular size'', we
tabulate the error in distance that is introduced under the assumption of
spherical symmetry (i.e., without correcting for inclination and axis ratio).
The errors can be significant and systematic, affecting studies of novae
whether considered individually or statistically. Each of the six estimators
overpredicts the distance when the polar axis is close to the line of sight,
and most underpredict the distance when the polar axis is close to the plane of
the sky. The straight mean of the projected semimajor and semiminor axes gives
the least distance bias for an ensemble of randomly oriented prolate shells.
The best individual expansion distances, however, result from a full
spatio-kinematic modeling of the nova shell. We discuss several practical
complications that affect expansion distance measurements of real nova shells.
Nova shell expansion distances be based on velocity and angular size
measurements made contemporaneously if possible, and the same ions and
transitions should be used for the imaging and velocity measurements. We
emphasize the need for complete and explicit reporting of measurement
procedures and results, regardless of the specific method used.Comment: 21 pages, LaTeX, uses aasms4.sty, to be published in Publ. Astron.
Soc. of the Pacific, May 200
Temperature dependence of nonlinear auto-oscillator linewidths: Application to spin-torque nano-oscillators
The temperature dependence of the generation linewidth for an auto-oscillator
with a nonlinear frequency shift is calculated. It is shown that the frequency
nonlinearity creates a finite correlation time, tau, for the phase
fluctuations. In the low-temperature limit in which the spectral linewidth is
smaller than 1/tau, the line shape is approximately Lorentzian and the
linewidth is linear in temperature. In the opposite high-temperature limit in
which the linewidth is larger than 1/tau, the nonlinearity leads to an apparent
"inhomogeneous broadening" of the line, which becomes Gaussian in shape and has
a square-root dependence on temperature. The results are illustrated for the
spin-torque nano-oscillator.Comment: 4 pages, 1 figur
Kinetic-scale magnetic turbulence and finite Larmor radius effects at Mercury
We use a nonstationary generalization of the higher-order structure function
technique to investigate statistical properties of the magnetic field
fluctuations recorded by MESSENGER spacecraft during its first flyby
(01/14/2008) through the near Mercury's space environment, with the emphasis on
key boundary regions participating in the solar wind -- magnetosphere
interaction. Our analysis shows, for the first time, that kinetic-scale
fluctuations play a significant role in the Mercury's magnetosphere up to the
largest resolvable time scale ~20 s imposed by the signal nonstationarity,
suggesting that turbulence at this planet is largely controlled by finite
Larmor radius effects. In particular, we report the presence of a highly
turbulent and extended foreshock system filled with packets of ULF
oscillations, broad-band intermittent fluctuations in the magnetosheath,
ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail,
and kinetic-scale fluctuations in the inner current sheet encountered at the
outbound (dawn-side) magnetopause. Overall, our measurements indicate that the
Hermean magnetosphere, as well as the surrounding region, are strongly affected
by non-MHD effects introduced by finite sizes of cyclotron orbits of the
constituting ion species. Physical mechanisms of these effects and their
potentially critical impact on the structure and dynamics of Mercury's magnetic
field remain to be understood.Comment: 46 pages, 5 figures, 2 table
The Ground State of the ``Frozen'' Electron Phase in Two-Dimensional Narrow-Band Conductors with a Long-Range Interelectron Repulsion. Stripe Formation and Effective Lowering of Dimension
In narrow-band conductors a weakly screened Coulomb interelectron repulsion
can supress narrow-band electrons' hopping, resulting in formation of a
``frozen'' electron phase which differs principally from any known macroscopic
self-localized electron state including the Wigner crystal. In a zero-bandwidth
limit the ``frozen'' electron phase is a classical lattice system with a
long-range interparticle repulsion. The ground state of such systems has been
considered in the case of two dimensions for an isotropic pair potential of the
mutual particle repulsion. It has been shown that particle ordering into
stripes and effective lowering of dimension universally resides in the ground
state for any physically reasonable pair potential and for any geometry of the
conductor lattice. On the basis of this fact a rigorous general procedure to
fully describe the ground state has been formulated. Arguments have been
adduced that charge ordering in High-T_c superconductors testifies to presence
of a ``frozen'' electron phase in these systems.Comment: 5 pages, LaTeX 2.09, 1 figure in external PostScript files. To appear
in Phys.Rev B Rapid Communication
Lineshape distortion in a nonlinear auto-oscillator near generation threshold: Application to spin-torque nano-oscillators
The lineshape in an auto-oscillator with a large nonlinear frequency shift in
the presence of thermal noise is calculated. Near the generation threshold,
this lineshape becomes strongly non-Lorentzian, broadened, and asymmetric. A
Lorentzian lineshape is recovered far below and far above threshold, which
suggests that lineshape distortions provide a signature of the generation
threshold. The theory developed adequately describes the observed behavior of a
strongly nonlinear spin-torque nano-oscillator.Comment: 4 pages, 3 figure
The Origin of Soft X-rays in DQ Herculis
DQ Herculis (Nova Herculis 1934) is a deeply eclipsing cataclysmic variable
containing a magnetic white dwarf primary. The accretion disk is thought to
block our line of sight to the white dwarf at all orbital phases due to its
extreme inclination angle. Nevertheless, soft X-rays were detected from DQ Her
with ROSAT PSPC. To probe the origin of these soft X-rays, we have performed
Chandra ACIS observations. We confirm that DQ Her is an X-ray source. The bulk
of the X-rays are from a point-like source and exhibit a shallow partial
eclipse. We interpret this as due to scattering of the unseen central X-ray
source, probably in an accretion disk wind. At the same time, we observe what
appear to be weak extended X-ray features around DQ Her, which we interpret as
an X-ray emitting knot in the nova shell.Comment: 18 pages including 4 figures, accepted for publication in
Astrphyisical Journa
Stochastic theory of spin-transfer oscillator linewidths
We present a stochastic theory of linewidths for magnetization oscillations
in spin-valve structures driven by spin-polarized currents. Starting from a
nonlinear oscillator model derived from spin-wave theory, we derive Langevin
equations for amplitude and phase fluctuations due to the presence of thermal
noise. We find that the spectral linewidths are inversely proportional to the
spin-wave intensities with a lower bound that is determined purely by
modulations in the oscillation frequencies. Reasonable quantitative agreement
with recent experimental results from spin-valve nanopillars is demonstrated.Comment: Submitted to Physical Review
- …