10,101 research outputs found
Sarnak’s saturation problem for complete intersections
We study almost prime solutions of systems of Diophantine equations
in the Birch setting. Previous work shows that there exist integer solutions of size
B with each component having no prime divisors below B
1/u
, where u equals
c0n
3/2
, n is the number of variables and c0 is a constant depending on the degree
and the number of equations. We improve the polynomial growth n
3/2
to the
logarithmic (log n)(log log n)
−1
. Our main new ingredients are the generalization of
the Brudern–Fouvry vector sieve in any dimension and the incorporation of smooth ¨
weights into the Davenport–Birch version of the circle method
Ultraviolet and X-ray detection of the 56 Peg system (KO 2p + WD)
Both IUE short and long wavelength exposures of the 56 Peg system are discussed. This mild barium star has an X-ray luminosity of 3 x 10 to the 31st power ergs/1, comparable to the rapidly rotating RS CVn binary systems, yet lies in a region of the HR diagram where stellar X-rays are generally not observed. This cool, bright giant is not a rapid rotator and the key to understanding its emission lies in the recent discovery of its white dwarf companion. Accretion onto the white dwarf of approximately 0.1% of the stellar wind of the primary is sufficient to power an X-ray source of the observed luminosity. Reprocessing of the X-rays in the cool dense stellar wind explains the origin of the UV emission line spectrum, and may explain the time varying asymmetry of the Mg 2 kappa line profile that is observed. Graphs which show observed fluxes and wavelengths are discussed
3C 295, a cluster and its cooling flow at z=0.46
We present ROSAT HRI data of the distant and X-ray luminous (L_x(bol)=2.6^
{+0.4}_{-0.2} 10^{45}erg/sec) cluster of galaxies 3C 295. We fit both a
one-dimensional and a two-dimensional isothermal beta-model to the data, the
latter one taking into account the effects of the point spread function (PSF).
For the error analysis of the parameters of the two-dimensional model we
introduce a Monte-Carlo technique. Applying a substructure analysis, by
subtracting a cluster model from the data, we find no evidence for a merger,
but we see a decrement in emission South-East of the center of the cluster,
which might be due to absorption. We confirm previous results by Henry &
Henriksen(1986) that 3C 295 hosts a cooling flow. The equations for the simple
and idealized cooling flow analysis presented here are solely based on the
isothermal beta-model, which fits the data very well, including the center of
the cluster. We determine a cooling flow radius of 60-120kpc and mass accretion
rates of dot{M}=400-900 Msun/y, depending on the applied model and temperature
profile. We also investigate the effects of the ROSAT PSF on our estimate of
dot{M}, which tends to lead to a small overestimate of this quantity if not
taken into account. This increase of dot{M} (10-25%) can be explained by a
shallower gravitational potential inferred by the broader overall profile
caused by the PSF, which diminishes the efficiency of mass accretion. We also
determine the total mass of the cluster using the hydrostatic approach. At a
radius of 2.1 Mpc, we estimate the total mass of the cluster (M{tot}) to be
(9.2 +/- 2.7) 10^{14}Msun. For the gas to total mass ratio we get M{gas}/M{tot}
=0.17-0.31, in very good agreement with the results for other clusters of
galaxies, giving strong evidence for a low density universe.Comment: 26 pages, 7 figures, accepted for publication in Ap
Virgo Galaxies with Long One-Sided HI Tails
In a new HI imaging survey of Virgo galaxies (VIVA: VLA Imaging of Virgo
galaxies in Atomic gas), we find 7 spiral galaxies with long HI tails. The
morphology varies but all the tails are extended well beyond the optical radii
on one side. These galaxies are found in intermediate-low density regions
(0.6-1 Mpc in projection from M87). The tails are all pointing roughly away
from M87, suggesting that these tails may have been created by a global cluster
mechanism. While the tidal effects of the cluster potential are too small, a
rough estimate suggests that simple ram-pressure stripping indeed could have
formed the tails in all but two cases. At least three systems show HI
truncation to within the stellar disk, providing evidence for a gas-gas
interaction. Although most of these galaxies do not appear disturbed optically,
some have close neighbors, suggesting that tidal interactions may have moved
gas outwards making it more susceptible to the ICM ram-pressure or viscosity.
Indeed, a simulation study of one of the tail galaxies, NGC 4654, suggests that
the galaxy is most likely affected by the combined effect of a gravitational
interaction and ram-pressure stripping. We conclude that these one-sided HI
tail galaxies have recently arrived in the cluster, falling in on highly radial
orbits. It appears that galaxies begin to lose their gas already at
intermediate distances from the cluster center through ram-pressure or
turbulent viscous stripping and tidal interactions with neighbours, or a
combination of both.Comment: 4 pages, 3 figures (including 1 plate), accepted for accepted for
publication in ApJ Letter (vol. 659, L115), a version with full resolution
Plate 1 is available from
http://www.astro.umass.edu/~achung/astro-ph/viva_tail.pd
Measuring quality of life in the speaker with dysarthria: reliability and validity of the european portuguese version of the QoL-DyS
Background: The impact of acquired dysarthria on the person’s life and social participation is well recognized and is the key to the process of rehabilitation. Evaluation of the effectiveness of an intervention that addresses this impact is a challenge for clinicians and researchers. Objectives: The purpose of this study was to evaluate the validity and reliability of the European Portuguese version of the Quality of Life in the Speaker with Dysarthria (QoL-DyS). Methods: This research was conducted in three phases: (i) cultural and linguistic adaptation; (ii) feasibility and reliability; and (iii) validity. The sample was composed of 105 subjects with dysarthria and 103 healthy subjects. The QAD (Quick Assessment for Dysarthria), PEAT10 (Portuguese Eating Assessment Tool), and EQ-5D (EuroQol five-dimension scale) were used for validity and reliability. Results and Conclusion: The QoL-DyS correlated positively with the QAD, PEAT-10, and EQ5D. Cronbach’s ? was 0.973, and it remained excellent when any item was deleted. The QoL-DyS score mean difference between the non-dysarthric cohort and the dysarthric cohort was also significant. Confirmatory factor analysis did not validate the original 40-item scale but a 33-item scale maintaining the four domains of the original version. A significantly higher perception of QoL was found in the non-dysarthric group. The results indicate that the European Portuguese version of the QoL-DyS is a reliable and valid tool to assess QoL in patients with dysarthria and may be used for screening in clinical practice and in research.info:eu-repo/semantics/acceptedVersio
Problems for MOND in Clusters and the Ly-alpha Forest
The observed dynamics of gas and stars on galactic and larger scales cannot
be accounted for by self-gravity, indicating that there are large quantities of
unseen matter, or that gravity is non-Newtonian in these regimes. Milgrom's
MOdified Newtonian Dynamics (MOND) postulates that Newton's laws are modified
at very low acceleration, and can account for the rotation curves of galaxies
and some other astrophysical observations, without dark matter. Here we apply
MOND to two independent physical systems: Ly-alpha absorbers and galaxy
clusters. While physically distinct, both are simple hydrodynamical systems
with characteristic accelerations in the MOND regime. We find that Ly-alpha
absorbers are somewhat smaller than in Newtonian gravity with dark matter, but
the result depends crucially on the (unknown) background acceleration field in
which they are embedded. In clusters MOND appears to explain the observed
(baryonic) mass-temperature relation. However, given observed gas density and
enclosed mass profiles and the assumption of hydrostatic equilibrium, MOND
predicts radial temperature profiles which disagree badly with observations. We
show this explicitly for the Virgo, Abell 2199 and Coma clusters, but the
results are general, and seem very difficult to avoid. If this discrepancy is
to be resolved by positing additional (presumably baryonic) dark matter, then
this dark matter must have ~1-3 times the cluster gas mass within 1 Mpc. This
result strongly disfavors MOND as an alternative to dark matter (Abridged).Comment: Revised version. Important caveat in Ly-alpha calculation discussed;
conclusions weakened. Coma cluster and calculation of dark matter mass
required by MOND added; cluster conclusions strengthened. 11 EmulateApJ pages
with 3 embedded figures. Accepted by Ap
Disrupted seasonal clockwork in the population dynamics of a freshwater copepod by climate warming
Life history responses are expected to accompany climate warming, yet little is known how long-term effects of climate and environmental change affect the seasonal dynamics of planktonic organisms. We used an historical data set from Lake Washington (U.S.A.) to quantify population responses of a calanoid copepod (Leptodiaptomus ashlandi) to long-term changes in temperature and resource availability and explore potential mechanisms for the responses. Increasing water temperatures (annual mean increase of 1.5 degrees C in the upper 10-m water volume) and longer stratification periods (about 4 weeks) were observed between 1962 and 2005, coincident with a pronounced decline in Leptodiaptomus densities. However, production was maintained because of an increase in the production to biomass ratio and a life cycle shift in Leptodiaptomus from an annual to a 6-month cycle. Cross-wavelet analyses demonstrated that the annual thermal forcing of copepod recruitment observed during the first two decades of the study weakened substantially, leading to more stochastic population dynamics during the past two decades. This shift from one to two generations per year was most likely produced by a longer and warmer growing period combined with changing fluctuations in resource (phytoplankton) availability. Climate change can lead to higher-frequency voltinism in ectothermic organisms and to temporal reorganization of their population dynamics
Patchy Reconnection in a Y-Type Current Sheet
We study the evolution of the magnetic field in a Y-type current sheet
subject to a brief, localized magnetic reconnection event. The reconnection
produces up- and down-flowing reconnected flux tubes which rapidly decelerate
when they hit the Y-lines and underlying magnetic arcade loops at the ends of
the current sheet. This localized reconnection outflow followed by a rapid
deceleration reproduces the observed behavior of post-CME downflowing coronal
voids. These simulations support the hypothesis that these observed coronal
downflows are the retraction of magnetic fields reconnected in localized
patches in the high corona.Comment: 4 pages, 3 figure
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