4,578 research outputs found
On the stability of naked singularities
We study the linearised stability of the nakedly singular negative mass
Schwarzschild solution against gravitational perturbations. There is a one
parameter family of possible boundary conditions at the singularity. We give a
precise criterion for stability depending on the boundary condition. We show
that one particular boundary condition is physically preferred and show that
the spacetime is stable with this boundary condition.Comment: 20 pages. 5 figure
Microlensing in phase space II: Correlations analysis
Applications of the phase space approach to the calculation of the
microlensing autocorrelation function are presented. The continuous propagation
equation for a random star field with a Gaussian velocity distribution is
solved in the leading non-trivial approximation using the perturbation
technique. It is shown that microlensing modulations can be important in the
interpretation of optical and shorter-wavelength light curves of pulsars, power
spectra of active galactic nuclei and coherence estimates for quasi-periodic
oscillations of dwarf novae and low-mass X-ray binaries. Extra scatter in the
brightness of type Ia supernovae due to gravitational microlensing is shown to
be of order up to 0.2 stellar magnitudes depending on the extent of the light
curves.Comment: Accepted for publication in MNRAS. 17 pages, 8 figures. The first
part of this little series is available at
http://www.arxiv.org/abs/astro-ph/0604302 . Replaced to add a link to the
first par
Microlensing in phase space I: Continuous propagation of variability moments
A method to calculate the statistical properties of microlensing light curves
is developed. The approach follows works by Deguchi & Watson, Seitz & Schneider
and Neindorf, attempting to clarify the ideas involved and techniques used in
the calculations. The method is then modified to include scattering by multiple
lensing planes along the line of sight and transition to a continuous limit of
this treatment for average quantities is performed leading to a Fokker-Planck
type equation. The equation is solved for a particular model of the random star
field and microlensing effect on the flux temporal variability is extracted.
Applications in astrophysically relevant situations are discussed.Comment: Accepted for publication in MNRAS. 15 pages, 4 figures. The second
part of this little series is available at
http://www.arxiv.org/abs/astro-ph/060419
Thermal conductance of Andreev interferometers
We calculate the thermal conductance of diffusive Andreev
interferometers, which are hybrid loops with one superconducting arm and one
normal-metal arm. The presence of the superconductor suppresses ; however,
unlike a conventional superconductor, does not vanish as the
temperature , but saturates at a finite value that depends on the
resistance of the normal-superconducting interfaces, and their distance from
the path of the temperature gradient. The reduction of is determined
primarily by the suppression of the density of states in the proximity-coupled
normal metal along the path of the temperature gradient. is also a
strongly nonlinear function of the thermal current, as found in recent
experiments.Comment: 5 pages, 4 figure
Colliding Plane Impulsive Gravitational Waves
When two non-interacting plane impulsive gravitational waves undergo a
head-on collision, the vacuum interaction region between the waves after the
collision contains backscattered gravitational radiation from both waves. The
two systems of backscattered waves have each got a family of rays (null
geodesics) associated with them. We demonstrate that if it is assumed that a
parameter exists along each of these families of rays such that the modulus of
the complex shear of each is equal then Einstein's vacuum field equations, with
the appropriate boundary conditions, can be integrated systematically to reveal
the well-known solutions in the interaction region. In so doing the mystery
behind the origin of such solutions is removed. With the use of the field
equations it is suggested that the assumption leading to their integration may
be interpreted physically as implying that the energy densities of the two
backscattered radiation fields are equal. With the use of different boundary
conditions this approach can lead to new collision solutions.Comment: 21 pages, LaTeX2
Stellar Pulsations excited by a scattered mass
We compute the energy spectra of the gravitational signals emitted when a
mass m is scattered by the gravitational field of a star of mass M >> m. We
show that, unlike black holes in similar processes, the quasi-normal modes of
the star are excited, and that the amount of energy emitted in these modes
depends on how close the exciting mass can get to the star.Comment: 23 pages, 6 figures, RevTe
When Celebrities Speak: A Nationwide Twitter Experiment Promoting Vaccination in Indonesia
Celebrity endorsements are often sought to influence public opinion. We ask
whether celebrity endorsement per se has an effect beyond the fact that their
statements are seen by many, and whether on net their statements actually lead
people to change their beliefs. To do so, we conducted a nationwide Twitter
experiment in Indonesia with 46 high-profile celebrities and organizations,
with a total of 7.8 million followers, who agreed to let us randomly tweet or
retweet content promoting immunization from their accounts. Our design exploits
the structure of what information is passed on along a retweet chain on Twitter
to parse reach versus endorsement effects. Endorsements matter: tweets that
users can identify as being originated by a celebrity are far more likely to be
liked or retweeted by users than similar tweets seen by the same users but
without the celebrities' imprimatur. By contrast, explicitly citing sources in
the tweets actually reduces diffusion. By randomizing which celebrities tweeted
when, we find suggestive evidence that overall exposure to the campaign may
influence beliefs about vaccination and knowledge of immunization-seeking
behavior by one's network. Taken together, the findings suggest an important
role for celebrity endorsement.Comment: 55 pages, 13 tables, 6 figure
Collective relaxation of stellar systems revisited
The chaos in stellar systems is studied using the theory of dynamical systems
and the Van Kampen stochastic differential equation approach. The exponential
instability (chaos) of spherical N-body gravitating systems, already known
previously, is confirmed. The characteristic timescale of that instability is
estimated confirming the collective relaxation time obtained by means of the
Maupertuis principle.Comment: A & A (in press), 3 pages, to match the published versio
Light transport in cold atoms and thermal decoherence
By using the coherent backscattering interference effect, we investigate
experimentally and theoretically how coherent transport of light inside a cold
atomic vapour is affected by the residual motion of atomic scatterers. As the
temperature of the atomic cloud increases, the interference contrast
dramatically decreases emphazising the role of motion-induced decoherence for
resonant scatterers even in the sub-Doppler regime of temperature. We derive
analytical expressions for the corresponding coherence time.Comment: 4 pages - submitted to Physical Review Letter
Convective envelopes in rotating OB stars
We study the effects of rotation on the outer convective zones of massive
stars. We examine the effects of rotation on the thermal gradient and on the
Solberg--Hoiland term by analytical developments and by numerical models.
Writing the criterion for convection in rotating envelopes, we show that the
effects of rotation on the thermal gradient are much larger and of opposite
sign to the effect of the Solberg-Hoiland criterion. On the whole, rotation
favors convection in stellar envelopes at the equator and to a smaller extent
at the poles. In a rotating 20 Msun star at 94% of the critical angular
velocity, there are two convective envelopes, with the bigger one having a
thickness of 13.2% of the equatorial radius. In the non-rotating model, the
corresponding convective zone has a thickness of only 4.6% of the radius. The
occurrence of outer convection in massive stars has many consequences.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic
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