4,272 research outputs found
A Certain Mongrel Court: Congress\u27s Past Power and Present Potential to Reinforce the Supreme Court
Mass transfer in eccentric binaries: the new Oil-on-Water SPH technique
To measure the onset of mass transfer in eccentric binaries we have developed
a two-phase SPH technique. Mass transfer is important in the evolution of close
binaries, and a key issue is to determine the separation at which mass transfer
begins. The circular case is well understood and can be treated through the use
of the Roche formalism. To treat the eccentric case we use a newly-developed
two phase system. The body of the donor star is made up from high-mass "water"
particles, whilst the atmosphere is modelled with low-mass "oil" particles.
Both sets of particles take part fully in SPH interactions. To test the
technique we model circular mass-transfer binaries containing a 0.6 Msun donor
star and a 1 Msun white dwarf; such binaries are thought to form cataclysmic
variable (CV) systems. We find that we can reproduce a reasonable CV
mass-transfer rate, and that our extended atmosphere gives a separation that is
too large by aproximately 16%, although its pressure scale height is
considerably exaggerated. We use the technique to measure the semi-major axis
required for the onset of mass transfer in binaries with a mass ratio of q=0.6
and a range of eccentricities. Comparing to the value obtained by considering
the instantaneous Roche lobe at pericentre we find that the radius of the star
required for mass transfer to begin decreases systematically with increasing
eccentricity.Comment: 9 pages, 8 figures, accepted by MNRA
On the enigmatic X-ray Source V1408 Aql (=4U 1957+11)
Models for the characteristically soft X-ray spectrum of the compact X-ray
source V1498 Aql (=4U 1957+11) have ranged from optically thick Comptonization
to multicolor accretion disk models. We critically examine the X-ray spectrum
of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics
(ASCA) data, archival Roentgensatellit (ROSAT) data, and recent Rossi X-Ray
Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray
spectral models to these data, we favor an interpretation of the X-ray spectrum
as being due to an accretion disk viewed at large inclination angles. Evidence
for this hypothesis includes long term (117 day, 235 day, 352 day)
periodicities seen by the RXTE All Sky Monitor (ASM), which we interpret as
being due to a warped precessing disk, and a 1 keV feature in the ASCA data,
which we interpret as being the blend of L fluorescence features from a disk
atmosphere or wind. We also present timing analysis of the RXTE data and find
upper limits of 4% for the root mean square (rms) variability between
f=0.001-16 Hz. The situation of whether the compact object is a black hole or
neutron star is still ambiguous; however, it now seems more likely that an
X-ray emitting, warped accretion disk is an important component of this system.Comment: High Frequency Power Spectrum corrected for unflagged `data dropouts'
(described in Appendix) and correct upper limits for variability presented.
All energy spectra and long term variability sections unchanged. Additional
references and acknowledgements added. 13 pages in emulateapj.st
Exploring the role of fire, succession, climate, and weather on landscape dynamics using comparative modelling
An assessment of the relative importance of vegetation change and disturbance as agents of landscape change under current and future climates would (1) provide insight into the controls of landscape dynamics, (2) help inform the design and development o
A toy model of the five-dimensional universe with the cosmological constant
A value of the cosmological constant in a toy model of the five-dimensional
universe is calculated in such a manner that it remains in agreement with both
astronomical observations and the quantum field theory concerning the
zero-point fluctuations of the vacuum. The (negative) cosmological constant is
equal to the inverse of the Planck length squared, which means that in the toy
model the vanishing of the observed value of the cosmological constant is a
consequence of the existence of an energy cutoff exactly at the level of the
Planck scale. In turn, a model for both a virtual and a real
particle-antiparticle pair is proposed which describes properly some energetic
properties of both the vacuum fluctuations and created particles, as well as it
allows one to calculate the discrete "bare" values of an elementary-particle
mass, electric charge and intrinsic angular momentum (spin) at the energy
cutoff. The relationships between the discussed model and some phenomena such
as the Zitterbewegung and the Unruh-Davies effect are briefly analyzed, too.
The proposed model also allows one to derive the Lorentz transformation and the
Maxwell equations while considering the properties of the vacuum filled with
the sea of virtual particles and their antiparticles. Finally, the existence of
a finite value of the vacuum-energy density resulting from the toy model leads
us to the formulation of dimensionless Einstein field equations which can be
derived from the Lagrangian with a dimensionless (naively renormalized)
coupling constant.Comment: 52 pages, 1 figure; a post-final, rewritten version with a number of
new remarks and conclusion
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