171 research outputs found
Merging stellar-mass binary black holes
The LIGO and Virgo detectors have recently directly observed gravitational
waves from several mergers of pairs of stellar-mass black holes, as well as
from one merging pair of neutron stars. These observations raise the hope that
compact object mergers could be used as a probe of stellar and binary
evolution, and perhaps of stellar dynamics. This colloquium-style article
summarizes the existing observations, describes theoretical predictions for
formation channels of merging stellar-mass black-hole binaries along with their
rates and observable properties, and presents some of the prospects for
gravitational-wave astronomy.Comment: Colloquium-style article solicited by Reviews of Modern Physics;
comments appreciate
Self-Similar Force-Free Wind From an Accretion Disk
We consider a self-similar force-free wind flowing out of an infinitely thin
disk located in the equatorial plane. On the disk plane, we assume that the
magnetic stream function scales as , where is the
cylindrical radius. We also assume that the azimuthal velocity in the disk is
constant: , where is a constant. For each choice of the
parameters and , we find an infinite number of solutions that are
physically well-behaved and have fluid velocity throughout the domain
of interest. Among these solutions, we show via physical arguments and
time-dependent numerical simulations that the minimum-torque solution, i.e.,
the solution with the smallest amount of toroidal field, is the one picked by a
real system. For , the Lorentz factor of the outflow increases
along a field line as \gamma \approx M(z/\Rfp)^{(2-\nu)/2} \approx R/R_{\rm
A}, where \Rfp is the radius of the foot-point of the field line on the disk
and R_{\rm A}=\Rfp/M is the cylindrical radius at which the field line
crosses the Alfven surface or the light cylinder. For , the Lorentz
factor follows the same scaling for z/\Rfp < M^{-1/(1-\nu)}, but at larger
distances it grows more slowly: \gamma \approx (z/\Rfp)^{\nu/2}. For either
regime of , the dependence of on shows that the rotation of
the disk plays a strong role in jet acceleration. On the other hand, the
poloidal shape of a field line is given by z/\Rfp \approx
(R/\Rfp)^{2/(2-\nu)} and is independent of . Thus rotation has neither a
collimating nor a decollimating effect on field lines, suggesting that
relativistic astrophysical jets are not collimated by the rotational winding up
of the magnetic field.Comment: 21 pages, 15 figures, accepted to MNRA
Wave Damping by Magnetohydrodynamic Turbulence and Its Effect on Cosmic-Ray Propagation in the Interstellar Medium
Cosmic rays scatter off magnetic irregularities (Alfvén waves) with which they are resonant, that is, waves of wavelength comparable to their gyroradii. These waves may be generated either by the cosmic rays themselves, if they stream faster than the Alfvén speed, or by sources of MHD turbulence. Waves excited by streaming cosmic rays are ideally shaped for scattering, whereas the scattering efficiency of MHD turbulence is severely diminished by its anisotropy. We show that MHD turbulence has an indirect effect on cosmic-ray propagation by acting as a damping mechanism for cosmic-ray-generated waves. The hot ("coronal") phase of the interstellar medium is the best candidate location for cosmic-ray confinement by scattering from self-generated waves. We relate the streaming velocity of cosmic rays to the rate of turbulent dissipation in this medium for the case in which turbulent damping is the dominant damping mechanism. We conclude that cosmic rays with up to 10^2 GeV could not stream much faster than the Alfvén speed but 10^6 GeV cosmic rays would stream unimpeded by self-generated waves, unless the coronal gas were remarkably turbulence-free
Spoke formation under moving plasma clouds
Goertz and Morfill (1983) propose that spokes on Saturn's rings form under
radially moving plasma clouds produced by meteoroid impacts. We demonstrate
that the speed at which a plasma cloud can move relative to the ring material
is bounded from above by the difference between the Keplerian and corotation
velocities. The radial orientation of new spokes requires radial speeds that
are at least an order of magnitude larger than this upper limit, thus the model
advanced by Goertz and Morfill fails to make radial spokes.Comment: 15 pages, 2 figures, Icarus in pres
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Integrated Arable Farming Systems and their potential uptake in the UK
Integrated Arable Farming Systems are examined from the perspective of the farmer considering the use of such techniques, and data are presented which suggest that the uptake of the approach may expose the manager to a greater degree of risk. Observations are made about the possible uptake of such systems in the UK and the implications this may have for agricultural and environmental policy in general
Optical Identification of Close White Dwarf Binaries in the LISA Era
The Laser Interferometer Space Antenna (LISA) is expected to detect close
white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000
binaries will be spectrally resolved at frequencies > 3 mHz, and their
positions on the sky will be determined to an accuracy ranging from a few tens
of arcminutes to a degree or more. Due to the small binary separation, the
optical light curves of >~ 30% of these CWDBs are expected to show eclipses,
giving a unique signature for identification in follow-up studies of the LISA
error boxes. While the precise optical location improves binary parameter
determination with LISA data, the optical light curve captures additional
physics of the binary, including the individual sizes of the stars in terms of
the orbital separation. To optically identify a substantial fraction of CWDBs
and thus localize them very accurately, a rapid monitoring campaign is
required, capable of imaging a square degree or more in a reasonable time, at
intervals of 10--100 seconds, to magnitudes between 20 and 25. While the
detectable fraction can be up to many tens of percent of the total resolved
LISA CWDBs, the exact fraction is uncertain due to unknowns related to the
white dwarf spatial distribution, and potentially interesting physics, such as
induced tidal heating of the WDs due to their small orbital separation.Comment: 4 pages, 2 figure
Finding White Dwarfs with Transit Searches
We make predictions for the rate of discovery of eclipsing white dwarf-main
sequence (WD-MS) binaries in terrestrial-planet transit searches, taking the
planned Kepler and Eddington missions as examples. We use a population
synthesis model to characterize the Galactic WD-MS population, and we find
that, despite increased noise due to stellar variability compared with the
typical planetary case, discovery of >100 non-accreting, eclipsing WD-MS
systems is likely using Kepler and Eddington, with periods of 2-20 days and
transit amplitudes of |delta m|~0.0003-0.00003 magnitudes. Follow-up
observations of these systems could accurately test the theoretical white dwarf
mass-radius relation or theories of binary star evolution.Comment: 5 pages, 3 figures, one table, accepted for publication in ApJ. Minor
changes to Galactic model and to discusion section; conclusions unchange
Occultation Searches for Kuiper Belt Objects
The occultation of background stellar sources by foreground Kuiper Belt
Objects (KBOs) can be used to survey physical properties of the KBO population.
We discuss statistics related to a KBO occultation survey, such as the event
duration distribution, and suggest that occultation searches can be effectively
used to probe the KBO size distribution below 10 km. In particular, we suggest
that occultation surveys may be best suited to search for a turnover radius in
the KBO size distribution due to collisions between small-size objects. For
occultation surveys that monitor stellar sources near the ecliptic over a few
square degrees, with time sampling intervals of order 0.1 sec and sensitivity
to flux variations of a few percent or more, a turnover radius between 0.1 and
1.0 km can be probed. While occultation surveys will probe the low-radius limit
and imaging surveys will detect KBOs of size 100 km or more, statistics of
objects with sizes in the intermediate range of around 1 km to 100 km will
likely remain unattainable.Comment: 4 pages, 3 figures; ApJL submitte
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