5,956 research outputs found
Environmental Impact on the Southeast Limb of the Cygnus Loop
We analyze observations from the Chandra X-ray Observatory of the southeast
knot of the Cygnus Loop supernova remnant. In this region, the blast wave
propagates through an inhomogeneous environment. Extrinsic differences and
subsequent multiple projections along the line of sight rather than intrinsic
shock variations, such as fluid instabilities, account for the apparent
complexity of the images. Interactions between the supernova blast wave and
density enhancements of a large interstellar cloud can produce the
morphological and spectral characteristics. Most of the X-ray flux arises in
such interactions, not in the diffuse interior of the supernova remnant.
Additional observations at optical and radio wavelengths support this account
of the existing interstellar medium and its role in shaping the Cygnus Loop,
and they demonstrate that the southeast knot is not a small cloud that the
blast wave has engulfed. These data are consistent with rapid equilibration of
electron and ion temperatures behind the shock front, and the current blast
wave velocity v_{bw} approx 330 km/s. Most of this area does not show strong
evidence for non-equilibrium ionization conditions, which may be a consequence
of the high densities of the bright emission regions.Comment: To appear in ApJ, April 1, 200
Toward High-Precision Astrometry with WFPC2. I. Deriving an Accurate PSF
The first step toward doing high-precision astrometry is the measurement of
individual stars in individual images, a step that is fraught with dangers when
the images are undersampled. The key to avoiding systematic positional error in
undersampled images is to determine an extremely accurate point-spread function
(PSF). We apply the concept of the {\it effective} PSF, and show that in images
that consist of pixels it is the ePSF, rather than the often-used instrumental
PSF, that embodies the information from which accurate star positions and
magnitudes can be derived. We show how, in a rich star field, one can use the
information from dithered exposures to derive an extremely accurate effective
PSF by iterating between the PSF itself and the star positions that we measure
with it. We also give a simple but effective procedure for representing spatial
variations of the HST PSF. With such attention to the PSF, we find that we are
able to measure the position of a single reasonably bright star in a single
image with a precision of 0.02 pixel (2 mas in WF frames, 1 mas in PC), but
with a systematic accuracy better than 0.002 pixel (0.2 mas in WF, 0.1 mas in
PC), so that multiple observations can reliably be combined to improve the
accuracy by .Comment: 33 pp. text + 15 figs.; accepted by PAS
Instabilities and propagation of neutrino magnetohydrodynamic waves in arbitrary direction
In a previous work [16], a new model was introduced, taking into account the
role of the Fermi weak force due to neutrinos coupled to magnetohydrodynamic
plasmas. The resulting neutrino-magnetohydrodynamics was investigated in a
particular geometry associated with the magnetosonic wave, where the ambient
magnetic field and the wavevector are perpendicular. The corresponding fast,
short wavelength neutrino beam instability was then obtained in the context of
supernova parameters. The present communication generalizes these results,
allowing for arbitrary direction of wave propagation, including fast and slow
magnetohydrodynamic waves and the intermediate cases of oblique angles. The
numerical estimates of the neutrino-plasma instabilities are derived in extreme
astrophysical environments where dense neutrino beams exist
Evolution of the Dark Matter Distribution at the Galactic Center
Annihilation radiation from neutralino dark matter at the Galactic center
(GC) would be greatly enhanced if the dark matter were strongly clustered
around the supermassive black hole (SBH). The existence of a dark-matter
"spike" is made plausible by the observed, steeply-rising stellar density near
the GC SBH. Here the time-dependent equations describing gravitational
interaction of the dark matter particles with the stars are solved. Scattering
of dark matter particles by stars would substantially lower the dark matter
density near the GC SBH over 10^10 yr, due both to kinetic heating, and to
capture of dark matter particles by the SBH. This result suggests that
enhancements in the dark matter density around a SBH would be modest whether or
not the host galaxy had experienced the scouring effects of a binary SBH.Comment: 5 pages, 3 figures. Submitted to Physical Review Letter
The energy partitioning of non-thermal particles in a plasma: or the Coulomb logarithm revisited
The charged particle stopping power in a highly ionized and weakly to
moderately coupled plasma has been calculated to leading and next-to-leading
order by Brown, Preston, and Singleton (BPS). After reviewing the main ideas
behind this calculation, we use a Fokker-Planck equation derived by BPS to
compute the electron-ion energy partitioning of a charged particle traversing a
plasma. The motivation for this application is ignition for inertial
confinement fusion -- more energy delivered to the ions means a better chance
of ignition, and conversely. It is therefore important to calculate the
fractional energy loss to electrons and ions as accurately as possible, as this
could have implications for the Laser Megajoule (LMJ) facility in France and
the National Ignition Facility (NIF) in the United States. The traditional
method by which one calculates the electron-ion energy splitting of a charged
particle traversing a plasma involves integrating the stopping power dE/dx.
However, as the charged particle slows down and becomes thermalized into the
background plasma, this method of calculating the electron-ion energy splitting
breaks down. As a result, the method suffers a systematic error of order T/E0,
where T is the plasma temperature and E0 is the initial energy of the charged
particle. In the case of DT fusion, for example, this can lead to uncertainties
as high as 10% or so. The formalism presented here is designed to account for
the thermalization process, and in contrast, it provides results that are
near-exact.Comment: 10 pages, 3 figures, invited talk at the 35th European Physical
Society meeting on plasma physic
Distance-redshift from an optical metric that includes absorption
We show that it is possible to equate the intensity reduction of a light wave
caused by weak absorption with a geometrical reduction in intensity caused by a
"transverse" conformal transformation of the spacetime metric in which the wave
travels. We are consequently able to modify Gordon's optical metric to account
for electromagnetic properties of ponderable material whose properties include
both refraction and absorption. Unlike refraction alone however, including
absorption requires a modification of the optical metric that depends on the
eikonal of the wave itself. We derive the distance-redshift relation from the
modified optical metric for Friedman-Lema\^itre-Robertson-Walker spacetimes
whose cosmic fluid has associated refraction and absorption coefficients. We
then fit the current supernovae data and provide an alternate explanation
(other than dark energy) of the apparent acceleration of the universe.Comment: 2 figure
High-Precision Entropy Values for Spanning Trees in Lattices
Shrock and Wu have given numerical values for the exponential growth rate of
the number of spanning trees in Euclidean lattices. We give a new technique for
numerical evaluation that gives much more precise values, together with
rigorous bounds on the accuracy. In particular, the new values resolve one of
their questions.Comment: 7 pages. Revision mentions alternative approach. Title changed
slightly. 2nd revision corrects first displayed equatio
Young and intermediate-age massive star clusters
An overview of our current understanding of the formation and evolution of
star clusters is given, with main emphasis on high-mass clusters. Clusters form
deeply embedded within dense clouds of molecular gas. Left-over gas is cleared
within a few million years and, depending on the efficiency of star formation,
the clusters may disperse almost immediately or remain gravitationally bound.
Current evidence suggests that a few percent of star formation occurs in
clusters that remain bound, although it is not yet clear if this fraction is
truly universal. Internal two-body relaxation and external shocks will lead to
further, gradual dissolution on timescales of up to a few hundred million years
for low-mass open clusters in the Milky Way, while the most massive clusters (>
10^5 Msun) have lifetimes comparable to or exceeding the age of the Universe.
The low-mass end of the initial cluster mass function is well approximated by a
power-law distribution, dN/dM ~ M^{-2}, but there is mounting evidence that
quiescent spiral discs form relatively few clusters with masses M > 2 x 10^5
Msun. In starburst galaxies and old globular cluster systems, this limit
appears to be higher, at least several x 10^6 Msun. The difference is likely
related to the higher gas densities and pressures in starburst galaxies, which
allow denser, more massive giant molecular clouds to form. Low-mass clusters
may thus trace star formation quite universally, while the more long-lived,
massive clusters appear to form preferentially in the context of violent star
formation.Comment: 21 pages, 3 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 9 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
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