187 research outputs found
Systematic acquisition of reading and writing. An Exploration of Structure in Didactic Elementary Texts for German
A Survey for HO Megamasers in Active Galactic Nuclei -- II. A Comparison of Detected and Undetected Galaxies
A survey for H2O megamaser emission from 354 active galaxies has resulted in
the detection of 10 new sources, making 16 known altogether. The galaxies
surveyed include a distance-limited sample (covering Seyferts and LINERs with
recession velocities < 7000 km s^{-1}) and a magnitude-limited sample (covering
Seyferts and LINERs with m(B) <= 14.5). In order to determine whether the
H2O-detected galaxies are "typical" AGN or have special properties which
facilitate the production of powerful masers, we have accumulated a database of
physical, morphological, and spectroscopic properties of the observed galaxies.
The most significant finding is that H2O megamasers are detected only in
Seyfert 2 and LINER galaxies, not Seyfert 1s. Implications for this finding are
discussed. LINERs are detected at a similar rate to Seyfert 2s, constituting a
strong argument that at least some nuclear LINERs are AGN rather than
starbursts, since starbursts have not been detected as H2O megamasers. We
preferentially detect H2O emission from the nearer galaxies and from those
which are apparently brighter at mid- and far-infrared and centimeter radio
wavelengths. There is also a possible trend for the H2O-detected galaxies to be
more intrinsically luminous in nuclear 6 cm radio emission than the undetected
ones, though these data are incomplete. We find evidence that Seyfert 2s with
very high (N(H) > 10^{24} cm^{-2}) X-ray absorbing columns of gas are more
often detected as H2O maser emitters than Seyfert 2s with lower columns. It may
be that the probability of detecting H2O maser emission in Seyfert galaxies
increases with increasing column of cool gas to the nucleus, from Seyfert 1s
through NLXGs to Seyfert 2s.Comment: 19 pages, 35 figures, 3 of the tables are contained in separate LaTeX
files: Table 1-a, 1-b, and 3. Also, figure captions are contained in a
separate file which must be compiled with plain TeX, not LaTe
Data model issues in the Cherenkov Telescope Array project
The planned Cherenkov Telescope Array (CTA), a future ground-based
Very-High-Energy (VHE) gamma-ray observatory, will be the largest project of
its kind. It aims to provide an order of magnitude increase in sensitivity
compared to currently operating VHE experiments and open access to guest
observers. These features, together with the thirty years lifetime planned for
the installation, impose severe constraints on the data model currently being
developed for the project.
In this contribution we analyze the challenges faced by the CTA data model
development and present the requirements imposed to face them. While the full
data model is still not completed we show the organization of the work, status
of the design, and an overview of the prototyping efforts carried out so far.
We also show examples of specific aspects of the data model currently under
development.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Detection of Excercise-Induced Ischemia by Measurement of NT-proBNP
Electrocardiographic exercise testing is the most widely used non-invasive screening test for coronary artery disease (CAD); however, both positive and negative predictive values for this procedure are hampered by relatively low sensitivity and specificity, leading to significant numbers of false negative and false positive studies. We hypothesized that NT-proBNP, a Neuro hormone secreted by cardiac myocytes in the ventricular wall in response to increased wall stress, would rise as a result of exercise-induced ischemia. If this were true, the enhancement of exercise testing by analysis of this plasma biomarker may offer significant improvement in the diagnostic accuracy of this procedure
Discovery of a bipolar X-ray jet from the T Tauri star DG Tau
We have obtained and analyzed Chandra ACIS-S observations of the strongly
accreting classical T Tauri star DG Tau. Our principal goals are to map the
immediate environment of the star to characterize possible extended X-rays
formed in the jet, and to re-visit the anomalous, doubly absorbed X-ray
spectrum of DG Tau itself. We combine our new ACIS-S data with a data set
previously obtained. The data are superimposed to obtain flux and hardness
images. Separate X-ray spectra are extracted for DG Tau and areas outside its
point spread function. We detect a prominent X-ray jet at a position angle of
PA ~225 deg (tentatively suggested by Guedel et al. 2005), coincident with the
optical jet axis. We also identify a counter jet at PA = 45 deg. The X-ray jets
are detected out to a distance of ~5" from the star, their sources being
extended at the ACIS-S resolution. The jet spectra are soft, with a best-fit
electron temperature of 3.4 MK. We find evidence for excess absorption of the
counter jet. The spectrum of the DG Tau point source shows two components with
largely different temperatures and absorption column densities. The similar
temperatures and small absorbing gas columns of the jet sources and the soft
component of the "stellar" source suggest that these sources are related,
produced either by shocks or by magnetic heating in the jets. Cooling estimates
suggest that the pressure in the hot gas contributes to jet expansion. The hard
"stellar" component, on the other hand, is associated with a stellar corona or
magnetosphere. The excessive photoelectric absorption of this component
suggests the presence of dust-depleted accretion streams above coronal magnetic
fields.Comment: accepted by Astronomy & Astrophysics, 11 pages, 6 figure
The Association of Obesity with Walking Independent of Knee Pain: The Multicenter Osteoarthritis Study
Practice guidelines recommend addressing obesity for people with knee OA, however, the association of obesity with walking independent of pain is not known. We investigated this association within the Multicenter Osteoarthritis Study, a cohort of older adults who have or are at high risk of knee OA. Subjects wore a StepWatch to record steps taken over 7 days. We measured knee pain from a visual analogue scale and obesity by BMI. We examined the association of obesity with walking using linear regression adjusting for pain and covariates. Of 1788 subjects, the mean steps/day taken was 8872.9 ± 3543.4. Subjects with a BMI ≥35 took 3355 fewer steps per day independent of knee pain compared with those with a BMI ≤25 (95% CI −3899, −2811). BMI accounted for 9.7% of the variability of walking while knee pain accounted for 2.9%. BMI was associated with walking independent of knee pain
Three-Dimensional Simulations of Jets from Keplerian Disks: Self--Regulatory Stability
We present the extension of previous two-dimensional simulations of the
time-dependent evolution of non-relativistic outflows from the surface of
Keplerian accretion disks, to three dimensions. The accretion disk itself is
taken to provide a set of fixed boundary conditions for the problem. The 3-D
results are consistent with the theory of steady, axisymmetric, centrifugally
driven disk winds up to the Alfv\'en surface of the outflow. Beyond the
Alfv\'en surface however, the jet in 3-D becomes unstable to non-axisymmetric,
Kelvin-Helmholtz instabilities. We show that jets maintain their long-term
stability through a self-limiting process wherein the average Alfv\'enic Mach
number within the jet is maintained to order unity. This is accomplished in at
least two ways. First, poloidal magnetic field is concentrated along the
central axis of the jet forming a ``backbone'' in which the Alfv\'en speed is
sufficiently high to reduce the average jet Alfv\'enic Mach number to unity.
Second, the onset of higher order Kelvin-Helmholtz ``flute'' modes (m \ge 2)
reduce the efficiency with which the jet material is accelerated, and transfer
kinetic energy of the outflow into the stretched, poloidal field lines of the
distorted jet. This too has the effect of increasing the Alfv\'en speed, and
thus reducing the Alfv\'enic Mach number. The jet is able to survive the onset
of the more destructive m=1 mode in this way. Our simulations also show that
jets can acquire corkscrew, or wobbling types of geometries in this relatively
stable end-state, depending on the nature of the perturbations upon them.
Finally, we suggest that jets go into alternating periods of low and high
activity as the disappearance of unstable modes in the sub-Alfv\'enic regime
enables another cycle of acceleration to super-Alfv\'enic speeds.Comment: 57 pages, 22 figures, submitted to Ap
Evidence for an X-Ray Jet in DG Tau A?
We present evidence for an X-ray jet in the T Tau star DG Tau A based on
Chandra ACIS data. DG Tau A, a jet-driving classical T Tau star with a flat
infrared spectrum, reveals an unusual X-ray spectrum that requires two thermal
components with different intervening absorption column densities. The softer
component shows a low temperature of T \approx 2.9 MK, and its absorption is
compatible with the stellar optical extinction (hydrogen column density N_H
\approx 5x10^21 cm^-2). In contrast, the harder component reveals a temperature
(22 MK) characteristic for active T Tau stars but its emission is more strongly
absorbed (N_H \approx 2.8x10^22 cm^-2). Furthermore, the high-resolution ACIS-S
image reveals a weak excess of soft (0.5-2 keV) counts at distances of 2-4"
from the star precisely along the optical jet, with a suggestive concentration
at 4" where a bow-shock-like structure has previously been identified in
optical line observations. The energy distribution of these photons is similar
to those of the stellar soft component. We interpret the soft spectral
component as originating from shocks at the base of the jet, with shock heating
continuing out to a distance of at least 500 AU along the jet, whereas the hard
component is most likely coronal/magnetospheric as in other young stellar
systems.Comment: Accepted for publication in ApJ Letters, 12 pages, 2 figure
The VLA Survey of the Chandra Deep Field South. IV. Source Population
We present a detailed analysis of 256 radio sources from our deep (flux
density limit of 42 microJy at the field centre at 1.4 GHz) Chandra Deep Field
South 1.4 and 5 GHz VLA survey. The radio population is studied by using a
wealth of multi-wavelength information in the radio, optical, and X-ray bands.
The availability of redshifts for ~ 80% of the sources in our complete sample
allows us to derive reliable luminosity estimates for the majority of the
objects. X-ray data, including upper limits, for all our sources turn out to be
a key factor in establishing the nature of faint radio sources. Due to the
faint optical levels probed by this study, we have uncovered a population of
distant Active Galactic Nuclei (AGN) systematically missing from many previous
studies of sub-millijansky radio source identifications. We find that, while
the well-known flattening of the radio number counts below 1 mJy is mostly due
to star forming galaxies, these sources and AGN make up an approximately equal
fraction of the sub-millijansky sky, contrary to some previous results. The AGN
include radio galaxies, mostly of the low-power, Fanaroff-Riley I type, and a
significant radio-quiet component, which amounts to approximately one fifth of
the total sample. The ratio of radio to optical luminosity depends more on
radio luminosity, rather than being due to optical absorption.Comment: 13 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Have proto-planetary discs formed planets?
It has recently been noted that many discs around T Tauri stars appear to
comprise only a few Jupiter-masses of gas and dust. Using millimetre surveys of
discs within six local star-formation regions, we confirm this result, and find
that only a few percent of young stars have enough circumstellar material to
build gas giant planets, in standard core accretion models. Since the frequency
of observed exo-planets is greater than this, there is a `missing mass'
problem. As alternatives to simply adjusting the conversion of dust-flux to
disc mass, we investigate three other classes of solution. Migration of planets
could hypothetically sweep up the disc mass reservoir more efficiently, but
trends in multi-planet systems do not support such a model, and theoretical
models suggest that the gas accretion timescale is too short for migration to
sweep the disc. Enhanced inner-disc mass reservoirs are possible, agreeing with
predictions of disc evolution through self-gravity, but not adding to
millimetre dust-flux as the inner disc is optically thick. Finally, the
incidence of massive discs is shown to be higher at the {\it proto}stellar
stages, Classes 0 and I, where discs substantial enough to form planets via
core accretion are abundant enough to match the frequency of exo-planets.
Gravitational instability may also operate in the Class 0 epoch, where half the
objects have potentially unstable discs of \ga30 % of the stellar mass.
However, recent calculations indicate that forming gas giants inside 50 AU by
instability is unlikely, even in such massive discs. Overall, the results
presented suggest that the canonically 'proto-planetary' discs of Class II T
Tauri stars {\bf have globally low masses in dust observable at millimetre
wavelengths, and conversion to larger bodies (anywhere from small rocks up to
planetary cores) must already have occurred.}Comment: Accepted for publication in MNRAS (main journal
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