4,551 research outputs found
An Anomalous UV Extension in NGC6251
Deep U-band FOC images of the nuclear region of NGC6251 have revealed a
region of extended emission which is most probably radiation scattered from a
continuum source in the nucleus. This radiation lies interior to a dust ring,
is nearly perpendicular to the radio jet axis, and is seen primarily in the FOC
U and b filters. The extension has a low observed polarization(), and
is unlikely to arise from line emission. We know of no other examples similar
to what we have found in NGC 6251, and we offer some tentative explanations.
The nuclear morphology shows clear similarities to that seen in the nucleus of
NGC 4261 except for the extended U-band radiation.Comment: 14 pages AAStex format + 4 figures; accepted for publication in ApJ
Letter
Generality and singular terms in the proposition: a comparison between Bradley and various modern logicians
It is argued that propositions are general, which
is taken to mean that there are no such things as
'singular terms' in the sense that certain parts of
sentences are connected in a direct and simple one-to-one
way with isolable bits of the world. Rather, it is
suggested, the kinds of expressions usually called
singular terms can be used to refer to individual things
because of their respective meanings. An expression's
possessing a meaning is understood univocally for both
singular and general terms: it means that the expression implies certain characteristics which anything must
have before the expression can properly be applied to
it. The claim that the proposition is general is under¬
stood, therefore, as the claim that any part of a
proposition which purports to pick out and individuate
some item does so through devices common to the rest
of the proposition, and thus cannot be thought of as
non-general any more than the predicative component can
be.Bradley first articulated this view, and that he
did is the chief burden of Part I. In Part II this view
is supported through an examination of the three kinds
of singular terms: proper names, demonstratives, and
definite descriptions. In each case it is argued that
the expressions involved are general, as a consequence
of their possessing moaning
Response to Comment on `Undamped electrostatic plasma waves' [Phys. Plasmas 19, 092103 (2012)]
Numerical and experimental evidence is given for the occurrence of the
plateau states and concomitant corner modes proposed in \cite{valentini12}. It
is argued that these states provide a better description of reality for small
amplitude off-dispersion disturbances than the conventional
Bernstein-Greene-Kruskal or cnoidal states such as those proposed in
\cite{comment
Oxygen isotope equilibrium between muscovite and water
Oxygen isotopes have been equilibrated between muscovite and aqueous alkali chloride solution and between paragonite and alkali chloride solution in the temperature range of 400°–650°C at 1 and 1.5 kb fluid pressure. Isotopic equilibrium was inferred from the fact that compatible fractionation factors were obtained using 3 different chemical reactions to produce the mica: (1) muscovite or paragonite was prepared by reacting natural kaolinite with 2–3 molal KCl or NaCl solutions; (2) muscovite was crystallized in pure water from a gel; and (3) synthetic paragonite was reacted with 2–3 molal KCl solution, producing muscovite by an alkali ion exchange reaction. The 1 M modification of the mica was made in every experiment. In several cases the extent of oxygen exchange was traced by running companion equilibrations in solutions of unusually low O^(18)/O^(16) ratio. No isotopic fractionation was discernible between muscovite and paragonite in the temperature range studied. Per mille fractionations between muscovite and water are given by the expression 10^3 In α = 2.38(10^6T^(−2)) − 3.89. These data can be combined with the results of other laboratory equilibration studies to establish a set of calibrated oxygen isotope geothermometers. Analogous to the alkali feldspar systems previously reported, the direct relationship between cation and oxygen isotope exchange suggests that some type of solution-redeposition mechanism operated during muscovite-paragonite transformations in aqueous solutions. Also, the extensive oxygen isotope exchange (with the solution) that accompanies the formation of muscovite from kaolinite implies a breakdown of the kaolinite structure. This notion does not concur with hypotheses based on rate studies and X-ray measurements that the unaltered kaolinite structure is partially inherited by the mica
Where are the Baryons?
New, high resolution, large-scale, cosmological hydrodynamic galaxy formation
simulations of a standard cold dark matter model (with a cosmological constant)
are utilized to predict the distribution of baryons at the present and at
moderate redshift. It is found that the average temperature of baryons is an
increasing function of time, with most of the baryons at the present time
having a temperature in the range 10^{5-7} K. Thus, not only is the universe
dominated by dark matter, but more than one half of the normal matter is yet to
be detected. Detection of this warm/hot gas poses an observational challenge,
requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmic
X-ray background, apparent warm components in hot clusters due to both
intrinsic warm intra-cluster gas and warm inter-cluster gas projected onto
clusters along the line of sight, absorption lines in X-ray and UV quasar
spectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emission
lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density
\lya absorption lines. We estimate that approximately 1/4 of the
extragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from the
warm/hot gas, half of it coming from and three-quarters from ,
so the source regions should be identifiable on deep optical images.Comment: ApJ in press, revised (fig 3 is in jpg). Whole paper including
fig3.ps can be obtained at
"http://astro.princeton.edu/~cen/PAPERS_TO_APPEAR/64
The First CO Map of a Low Surface Brightness Galaxy
Using the Owens Valley Radio Observatory Millimeter-Wavelength Array (OVRO)
we have obtained the first CO map of a low surface brightness (LSB) galaxy. The
studied galaxy, UGC 01922, was chosen for these observations because both of
its previous CO detection with the IRAM 30m telescope and its classification as
a Malin 1 `cousin' - an LSB galaxy with M_HI > 10^10 Msol. The OVRO map
detected approximately 65% of the CO(1-0) flux found earlier with the single
dish measurements, giving a detected gas mass equivalent to M_H2 = 1.1X10^9
Msol. The integrated gas peak lies at the center of the galaxy and coincides
with both the optical and 1.4 GHz continuum emission peaks. The molecular gas
extends well beyond the OVRO beam size (~4'' or 3 kpc), covering ~25% of the
optical bulge. In all, perhaps the most remarkable aspect of this map is its
unexceptional appearance. Given that it took over ten years to successfully
detect molecular gas in any low surface brightness system, it is surprising
that the appearance and distribution of UGC 01922's CO is similar to what would
be expected for a high surface brightness galaxy in the same morphological
class.Comment: 5 pages, including 3 figures and 3 tables. also available online at
http://www.gb.nrao.edu/~koneil. Accepted by ApJ
Undamped electrostatic plasma waves
Electrostatic waves in a collision-free unmagnetized plasma of electrons with
fixed ions are investigated for electron equilibrium velocity distribution
functions that deviate slightly from Maxwellian. Of interest are undamped waves
that are the small amplitude limit of nonlinear excitations, such as electron
acoustic waves (EAWs). A deviation consisting of a small plateau, a region with
zero velocity derivative over a width that is a very small fraction of the
electron thermal speed, is shown to give rise to new undamped modes, which here
are named {\it corner modes}. The presence of the plateau turns off Landau
damping and allows oscillations with phase speeds within the plateau. These
undamped waves are obtained in a wide region of the plane
( being the real part of the wave frequency and the
wavenumber), away from the well-known `thumb curve' for Langmuir waves and EAWs
based on the Maxwellian. Results of nonlinear Vlasov-Poisson simulations that
corroborate the existence of these modes are described. It is also shown that
deviations caused by fattening the tail of the distribution shift roots off of
the thumb curve toward lower -values and chopping the tail shifts them
toward higher -values. In addition, a rule of thumb is obtained for
assessing how the existence of a plateau shifts roots off of the thumb curve.
Suggestions are made for interpreting experimental observations of
electrostatic waves, such as recent ones in nonneutral plasmas.Comment: 11 pages, 10 figure
The Energy of a Plasma in the Classical Limit
When \lambda_{T} << d_{T}, where \lambda_{T} is the de Broglie wavelength and
d_{T}, the distance of closest approach of thermal electrons, a classical
analysis of the energy of a plasma can be made. In all the classical analysis
made until now, it was assumed that the frequency of the fluctuations \omega <<
T (k_{B}=\hbar=1). Using the fluctuation-dissipation theorem, we evaluate the
energy of a plasma, allowing the frequency of the fluctuations to be arbitrary.
We find that the energy density is appreciably larger than previously thought
for many interesting plasmas, such as the plasma of the Universe before the
recombination era.Comment: 10 pages, 2 figures, accepted for publication in Phys.Rev.Let
Medical device design in context: a model of user–device interaction and consequences
The practice of evaluating interaction with devices is embedded in disciplines such as human-computer interaction and cognitive ergonomics, including concepts such as affordances, error analysis, skill, rule and knowledge based behaviour and decision making biases. This paper considers the way in which the approach that has been routinely applied to displays and control design within the control and transport domains can be transferred to the context of medical devices. The importance of considering the context in which medical devices are used and implemented is presented, and the need for a systems approach to medical device design is emphasised. Five case studies from medical device control and display design are presented as an aide to developing an understanding of the relationship between device design and resultant behaviours. On the basis of these case studies, four types of mediating factors (catalysts, enablers, facilitators and enhancers) are proposed and a model to describe the link between device design, user, context and consequences is presented
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