1,550 research outputs found
Two-point motional Stark effect diagnostic for Madison Symmetric Torus
A high-precision spectral motional Stark effect (MSE) diagnostic provides internal magnetic field measurements for Madison Symmetric Torus (MST) plasmas. Currently, MST uses two spatial views-on the magnetic axis and on the midminor (off-axis) radius, the latter added recently. A new analysis scheme has been developed to infer both the pitch angle and the magnitude of the magnetic field from MSE spectra. Systematic errors are reduced by using atomic data from atomic data and analysis structure in the fit. Reconstructed current density and safety factor profiles are more strongly and globally constrained with the addition of the off-axis radius measurement than with the on-axis one only
Nonmonotonic Trust Management for P2P Applications
Community decisions about access control in virtual communities are
non-monotonic in nature. This means that they cannot be expressed in current,
monotonic trust management languages such as the family of Role Based Trust
Management languages (RT). To solve this problem we propose RT-, which adds a
restricted form of negation to the standard RT language, thus admitting a
controlled form of non-monotonicity. The semantics of RT- is discussed and
presented in terms of the well-founded semantics for Logic Programs. Finally we
discuss how chain discovery can be accomplished for RT-.Comment: This paper appears in the proceedings of the 1st International
Workshop on Security and Trust Management (STM 2005). To appear in ENTC
Improved V II log() Values, Hyperfine Structure Constants, and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937
New experimental absolute atomic transition probabilities are reported for
203 lines of V II. Branching fractions are measured from spectra recorded using
a Fourier transform spectrometer and an echelle spectrometer. The branching
fractions are normalized with radiative lifetime measurements to determine the
new transition probabilities. Generally good agreement is found between this
work and previously reported V II transition probabilities. Use of two
spectrometers, independent radiometric calibration methods, and independent
data analysis routines enables a reduction in systematic uncertainties, in
particular those due to optical depth errors. In addition, new hyperfine
structure constants are measured for selected levels by least squares fitting
line profiles in the FTS spectra. The new V II data are applied to high
resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to
determine new, more accurate V abundances. Lines covering a range of wavelength
and excitation potential are used to search for non-LTE effects. Very good
agreement is found between our new solar photospheric V abundance, log
{\epsilon}(V) = 3.95 from 15 V II lines, and the solar-system meteoritic value.
In HD 84937, we derive [V/H] = -2.08 from 68 lines, leading to a value of
[V/Fe] = 0.24.Comment: 32 pages, 7 tables (3 machine-readable), 8 figures; accepted for
publication in ApJ
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Improved V I Log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937
New emission branching fraction measurements for 836 lines of the first spectrum of vanadium (V I) are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1 m Fourier transform spectrometer (FTS) and a high-resolution echelle spectrometer. The branching fractions are combined with recently published radiative lifetimes from laser-induced fluorescence measurements to determine accurate absolute atomic transition probabilities for the 836 lines. The FTS data are also used to extract new hyperfine structure A coefficients for 26 levels of neutral vanadium. These new laboratory data are applied to determine the V abundance in the Sun and metal-poor star HD 84937, yielding log epsilon(V) = 3.956 +/- 0.004 (sigma = 0.037) based on 93 V I lines and log epsilon(V) = 1.89 +/- 0.03 (sigma = 0.07) based on nine Vi lines, respectively, using the Holweger-Muller 1D model. These new V I abundance values for the Sun and HD 84937 agree well with our earlier determinations based upon V II.NASA NNX10AN93GNSF AST-1211055, AST-1211585Astronom
Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197
Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE
J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission
(XMM-Newton) are used to study its spectrum and pulsed light curve as the
source fades from outburst. The decay is consistent with an exponential of time
constant 300 days, but not a power law as predicted in some models of sudden
deep crustal heating events. All spectra are well fitted by a blackbody plus a
steep power law, a problematic model that is commonly fitted to anomalous X-ray
pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better
motivated physically in view of the faint optical/IR fluxes, the X-ray pulse
shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting
areas that are less than or equal to the surface area of a neutron star. The
fitted temperatures remained the same while the flux declined by 46%, which can
be interpreted as a decrease in area of the emitting regions. The pulsar
continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s
s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic
field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with
both the outburst and quiescent X-ray luminosities being powered by magnetic
field decay, i.e., XTE J1810-197 is a magnetar.Comment: 10 pages, 5 figures, accepted by Ap.
Anémonas anillo, una visión general (Cnidaria, Anthozoa, Actiniaria)
The present observations represent a new record of species of Actiniaria that are able to attach to a gorgonian branch by surrounding it with a solid piece of tissue junction and a significant reduction of the coelenteron. Ring sea anemones are provisionally placed in the Actinostolidae. They seem to be more common at depths between 85 and 1500 m., and according to our observations they are specialized to exclusively colonize on some species of gorgonians or pennatulaceans. Parasitism best describes the relationship between ring sea anemones and their hosts. We recognize five different species of ring sea anemones, one described by Hiles (1899) as Peronanthus verrucellae, and the others named by us provisionally as Peronanthus sp1, sp2, sp3 & sp4. The strategy displayed by ring sea anemones has several advantages, such as placement economy (see above), a better attachment against any current action, exploitation of food resources inaccessible to most other Actiniarians, and it allows them avoidance of habitat competition. The impossibility to colonize any other substrate but a certain group of gorgonians and a reduction of the gastric cavity are the main disadvantages detected in the ring sea anemones’ way of life. Apparently, this group of sea anemones is widespread through the Pacific Ocean.Un nuevo grupo de especies de actiniarios que desarrollan un nuevo sistema para afianzarse alrededor de los ejes de gorgonias y pennatuláceos es estudiado por nosotros en el presente trabajo. Se trata de un anillo de tejido que rodea los ejes y queda perfectamente sellado por medio de uniones del tejido produciendo una reducción del celenterón. Anémonas anillo es como las hemos denominado y provisionalmente han sido incluidas dentro de la familia Actinostolidae. Parece que son comunes en profundidades comprendidas entre los 85 y los 1500 metros, colonizando solamente determinadas especies de gorgonias y de pennatuláceos. El parasitismo es la relación que mejor describe lo que acontece entre las anémonas y los octocorales que la portan. Hay, al menos, cinco especies diferentes de anémonas anillo, la primera fue descrita por Hiles (1899) como Peronanthus verrucellae, las otras especies las denominamos provisionalmente como Peronanthus sp1, sp2, sp3 y sp4. El modo de vida de estas especies presenta ciertas ventajas como son: una reducción del espacio para asentarse, una mejor fijación frente a fenómenos de corriente, la capacidad para explotar recursos inaccesibles a la mayorÃa de los actiniarios y por último la falta de competencia con otras especies por el hábitat. La imposibilidad de colonizar otros sustratos y la reducción de la cavidad gástrica son las principales desventajas que presenta este modo de vida exhibido por las anémonas anillo. Aparentemente este grupo de anémonas está ampliamente distribuido en el Indo-PacÃfico
Burst-properties as a function of mass accretion rate in GX 3+1
GX 3+1 is a low-mass X-ray binary that is persistently bright since its
discovery in 1964. It was found to be an X-ray burster twenty years ago proving
that the compact object in this system is a neutron star. The burst rate is so
low that only 18 bursts were reported prior to 1996. The Wide Field Cameras on
BeppoSAX have, through a dedicated monitoring program on the Galactic center
region, increased the number of X-ray bursts from GX 3+1 by 61. Since GX 3+1
exhibits a slow (order of years) modulation in the persistent flux of about
50%, these observations opens up the unique possibility to study burst
properties as a function of mass accretion rate for very low burst rates. This
is the first time that bursts are detected from GX 3+1 in the high state. From
the analysis we learn that all bursts are short with e-folding decay times
smaller than 10 s. Therefore, all bursts are due to unstable helium burning.
Furthermore, the burst rate drops sixfold in a fairly narrow range of 2-20 keV
flux; we discuss possible origins for this.Comment: 9 pages and 7 figures. Accepted for publication in Astronomy &
Astrophysic
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