8,731 research outputs found
The X-ray R Aquarii: A Two-sided Jet and Central Source
We report Chandra ACIS-S3 x-ray imaging and spectroscopy of the R Aquarii
binary system that show a spatially resolved two-sided jet and an unresolved
central source. This is the first published report of such an x-ray jet seen in
an evolved stellar system comprised of ~2-3 solar masses. At E < 1 keV, the
x-ray jet extends both to the northeast and southwest relative to the central
binary system. At 1 < E < 7.1 keV, R Aqr is a point-like source centered on the
star system. While both 3.5-cm radio continuum emission and x-ray emission
appear coincident in projection and have maximum intensities at ~7.5" northeast
of the central binary system, the next strongest x-ray component is located
\~30" southwest of the central binary system and has no radio continuum
counterpart. The x-ray jets are likely shock heated in the recent past, and are
not in thermal equilibrium. The strongest southwest x-ray jet component may
have been shocked recently since there is no relic radio emission as expected
from an older shock. At the position of the central binary, we detect x-ray
emission below 1.6 keV consistent with blackbody emission at T ~2 x 10^6 K. At
the central star there is also a prominent 6.4 keV feature, a possible
fluorescence or collisionally excited Fe K-alpha line from an accretion disk or
from the wind of the giant star. For this excitation to occur, there must be an
unseen hard source of x-rays or particles in the immediate vicinity of the hot
star. Such a source would be hidden from view by the surrounding edge-on
accretion disk.Comment: PS, 20 pages, including 3 figures PNG, JPG - accepted for publication
in ApJ Letters. Subject headings: stars: individual (R Aquarii) -- binaries:
symbiotic -- circumstellar matter -- stars: white dwarfs -- stars: winds,
outflows -- radio continuum: stars -- x-rays: genera
A Seat at the Table: Minority Representation and County Governing Boards
This study focuses on minority representation on county governing boards to determine the extent of minority representation, and then to provide explanation for the exiting patterns in its representation. The dependent variable used in this paper is a count variable employing a Zero-Inflated Negative Binomial model. The results indicate that minority populations, counties located in the South, partisan elections, the size of county governing boards and urban counties have positive effects on increased minority representation, while at-large voting districts have a negative effect. Furthermore, it advances the need for greater research on county governing boards, county governments in general and a new agenda for the future study of minority representation on local governing bodies
Bark-Tissue Thickness of Coastal Western Hemlock in British Columbia
Bark-tissue thicknesses of coastal western hemlock are reported. Variation in these characteristics is considered between sites, trees, and height positions. Total bark thickness did not vary significantly with site, averaging 7.4 mm, 12.2 mm, and 12.6 mm for the top, middle, and butt height positions, respectively. However, the relative contribution of the individual tissues to the total thickness did vary with site. The thickness of all bark characteristics varied with height, being least at the top position, but differing very little between middle and butt positions
VETA-I x ray test analysis
This interim report presents some definitive results from our analysis of the VETA-I x-ray testing data. It also provides a description of the hardware and software used in the conduct of the VETA-I x-ray test program performed at the MSFC x-ray Calibration Facility (XRCF). These test results also serve to supply data and information to include in the TRW final report required by DPD 692, DR XC04. To provide an authoritative compendium of results, we have taken nine papers as published in the SPIE Symposium, 'Grazing Incidence X-ray/EUV Optics for Astronomy and Projection Lithography' and have reproduced them as the content of this report
Surface diffusion coefficients by thermodynamic integration: Cu on Cu(100)
The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated
using thermodynamic integration within the transition state theory. The results
are found to be in excellent agreement with the essentially exact values from
molecular-dynamics simulations. The activation energy and related entropy are
shown to be effectively independent of temperature, thus establishing the
validity of the Arrhenius law over a wide range of temperatures. Our study
demonstrates the equivalence of diffusion rates calculated using thermodynamic
integration within the transition state theory and direct molecular-dynamics
simulations.Comment: 4 pages (revtex), two figures (postscript
Fostering Application Opportunites for the NASA Soil Moisture Active Passive (SMAP) Mission
The NASA Soil Moisture Active Passive (SMAP) Mission will provide global observations of soil moisture and freeze/thaw state from space. We outline how priority applications contributed to the SMAP mission measurement requirements and how the SMAP mission plans to foster applications and applied science
Aquaculture and eelgrass Zostera marina interactions in temperate ecosystems
This paper reviews the impacts of shellfish and finfish aquaculture on eelgrassZostera marina, the most widely distributed seagrass species in the northern hemisphere. Shellfish aquaculture can have positive, neutral, and negative effects on eelgrass. Positive interactions can be generated by the filtering activity of cultured bivalves, which may improve water quality and reduce epiphyte loads, and shellfish biodeposits may provide more nutrients to eelgrass and other vegetation. However, negative responses are more commonly reported and can be causedby shading and sedimentation. These negative effects tend to occur directly under and immediately surrounding shellfish farms and rapidly diminish with increasing distance. In contrast to shellfish aquaculture, only one field study has investigated the effects of finfish aquaculture on eelgrass in a temperate setting, and the results were inconclusive. However, many studies have investigated the effects of Mediterranean finfish farms on 2 other species of seagrass (Posidoniaoceanica and Cymodocea nodosa). These studies reported clear negative interactions, which have been linked to increased nutrient concentrations, sulphides, sedimentation, epiphyte loads, and grazing pressure. It is unknown if these studies are relevant for finfish aquaculture in temperate regions due to differences in environmental conditions, and because the studies focused on differ-ent species of seagrass. Thus, further study in a temperate setting is warranted. We conclude by highlighting key research gaps that could help regulators establish unambiguous operational and siting guidelines that minimize the potential for negative interactions between aquaculture and eelgrass
The TWA 3 Young Triple System: Orbits, Disks, Evolution
We have characterized the spectroscopic orbit of the TWA 3A binary and
provide preliminary families of probable solutions for the TWA 3A visual orbit
as well as for the wide TWA 3A--B orbit. TWA 3 is a hierarchical triple located
at 34 pc in the 10 Myr old TW Hya association. The wide component
separation is 1."55; the close pair was first identified as a possible binary
almost 20 years ago. We initially identified the 35-day period orbital solution
using high-resolution infrared spectroscopy which angularly resolved the A and
B components. We then refined the preliminary orbit by combining the infrared
data with a re-analysis of our high-resolution optical spectroscopy. The
orbital period from the combined spectroscopic solution is 35 days, the
eccentricity is 0.63, and the mass ratio is 0.84; although this
high mass ratio would suggest that optical spectroscopy alone should be
sufficient to identify the orbital solution, the presence of the tertiary B
component likely introduced confusion in the blended optical spectra. Using
millimeter imaging from the literature, we also estimate the inclinations of
the stellar orbital planes with respect to the TWA 3A circumbinary disk
inclination and find that all three planes are likely misaligned by at least
30 degrees. The TWA 3A spectroscopic binary components have spectral
types of M4.0 and M4.5; TWA 3B is an M3. We speculate that the system formed as
a triple, is bound, and that its properties were shaped by dynamical
interactions between the inclined orbits and disk.Comment: Accepted to Ap
Magnetic Surfaces in Stationary Axisymmetric General Relativity
In this paper a new method is derived for constructing electromagnetic
surface sources for stationary axisymmetric electrovac spacetimes endowed with
non-smooth or even discontinuous
Ernst potentials. This can be viewed as a generalization of some classical
potential theory results, since lack of continuity of the potential is related
to dipole density and lack of smoothness, to monopole density. In particular
this approach is useful for constructing the dipole source for the magnetic
field. This formalism involves solving a linear elliptic differential equation
with boundary conditions at infinity. As an example, two different models of
surface densities for the Kerr-Newman electrovac spacetime are derived.Comment: 15 page
Integral Equations for Heat Kernel in Compound Media
By making use of the potentials of the heat conduction equation the integral
equations are derived which determine the heat kernel for the Laplace operator
in the case of compound media. In each of the media the parameter
acquires a certain constant value. At the interface of the media the
conditions are imposed which demand the continuity of the `temperature' and the
`heat flows'. The integration in the equations is spread out only over the
interface of the media. As a result the dimension of the initial problem is
reduced by 1. The perturbation series for the integral equations derived are
nothing else as the multiple scattering expansions for the relevant heat
kernels. Thus a rigorous derivation of these expansions is given. In the one
dimensional case the integral equations at hand are solved explicitly (Abel
equations) and the exact expressions for the regarding heat kernels are
obtained for diverse matching conditions. Derivation of the asymptotic
expansion of the integrated heat kernel for a compound media is considered by
making use of the perturbation series for the integral equations obtained. The
method proposed is also applicable to the configurations when the same medium
is divided, by a smooth compact surface, into internal and external regions, or
when only the region inside (or outside) this surface is considered with
appropriate boundary conditions.Comment: 26 pages, no figures, no tables, REVTeX4; two items are added into
the Reference List; a new section is added, a version that will be published
in J. Math. Phy
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