476 research outputs found
Blood flow in the human fetal descending aorta : a pulsed Doppler study
In 1628 William Harvey introduced his concept ofthe human circulation. Although
a lot of studies concerning the fetal circulation were done before it was not until the
1930s that Barcroft (1934, 1939) and associates performed radiograpbic studies on
the feta! goal and lamb to establish the feta! circulation. Later in 1964 Lind, Stern
and Wegelius used cine-angiographic studies to describe the human fetal circulation.
Volume flow measurements were already carried out in 1884 by Cohnstein and
Zuntz using a stromuhr in an umbilical artery. In 1949 Cooper, Greenfield and
Huggett made their measurements of the umbilical blood flow at different
gestational ages in the fetal sheep using a veneus plethysmograph. A variety of
methods foliowed each other, such as density flowmeter, the velodyne flowmeter,
the cannulated type of electromagnetic flowmeter and the cuff electromagnetic
flowmeter. All these methods had several disadvantages. The major ones are
exteriorisation of the fetus and the acute character of the flow measurements.
Therefore, because of these disadvantages, techniques were developed with the
possibility to study the fetus in chronic preparations after recovery from surgery
(Berman et al, 1975). These chronic experiments however, may still not be
indicative of the normal physiological state.
The introduetion of ultrasonic Doppier techniques opened the possibility to
measure blood flow non-invasively. Satomura (1959) published the first report on
the use of ultrasonic Doppier equipment for the deleetion of human blood flow
velocity. Since then the interest in the non-invasive detection ofhuman blood flow
has been overwhelming. Nevertheless it took until the end of theseventies before
the first reports appeared on the deleetion ofhuman feta! blood flow. FitzGerald et
al (1977) introduced a methad for measurement of human urnhilical blood flow
using continuous wave Doppler. A few years later pulsed Doppier was introduced
for the measurement of urnhilical venous blood flow (Gill et al, 1978) and blood
flow in the feta! deseending aorta (Eik-Nes et al, 1980). Blood flow in the feta!
deseending aorta reflects cardiovascular function which in itself is an important
souree of information about fetal well-being. To appreciate correctly the data
obtained from these non-invasive flow measurements it is important to establish
their reproducibility and to study the possible effects of internal and external
stimuli since the latter may lead to misinterpretation of the results.
The objectives of the present study were:
I. to analyse the pitfalls related to the pulsed Doppier flow velocity measurement
in the lower thoracic part of the feta! deseending aorta. 2. to establish the reprodueibility and normal val u esforblood flow veloeity, vessel
diameter and volume flow in the lower thoraeie part of the feta! deseending aorta
during the third trimester of pregnaney.
3. to analyse the influenee of external stimuli e.g. smoking and short term moderate
exercise on these parameters.
4. to develop a more precise mcthod of recording of the vessel diameter changes in
the lower thoraeie part of the feta! deseending aorta
The Sub-Surface Structure of a Large Sample of Active Regions
We employ ring-diagram analysis to study the sub-surface thermal structure of
active regions. We present results using a large number of active regions over
the course of Solar Cycle 23. We present both traditional inversions of
ring-diagram frequency differences, with a total sample size of 264, and a
statistical study using Principal Component Analysis. We confirm earlier
results on smaller samples that sound speed and adiabatic index are changed
below regions of strong magnetic field. We find that sound speed is decreased
in the region between approximately r=0.99R_sun and r=0.995R_sun (depths of 3Mm
to 7Mm), and increased in the region between r=0.97R_sun and r=0.985R_sun
(depths of 11Mm to 21Mm). The adiabatic index is enhanced in the same deeper
layers that sound-speed enhancement is seen. A weak decrease in adiabatic index
is seen in the shallower layers in many active regions. We find that the
magnitudes of these perturbations depend on the strength of the surface
magnetic field, but we find a great deal of scatter in this relation, implying
other factors may be relevant.Comment: 16 pages, 11 figures, accepted for publication in Solar Physic
The time delay of the quadruple quasar RX J0911.4+0551
We present optical lightcurves of the gravitationally lensed components A
(=A1+A2+A3) and B of the quadruple quasar RX J0911.4+0551 (z = 2.80). The
observations were primarily obtained at the Nordic Optical Telescope between
1997 March and 2001 April and consist of 74 I-band data points for each
component. The data allow the measurement of a time delay of 146 +- 8 days (2
sigma) between A and B, with B as the leading component. This value is
significantly shorter than that predicted from simple models and indicates a
very large external shear. Mass models including the main lens galaxy and the
surrounding massive cluster of galaxies at z = 0.77, responsible for the
external shear, yield H_0 = 71 +- 4 (random, 2 sigma) +- 8 (systematic)
km/s/Mpc. The systematic model uncertainty is governed by the surface-mass
density (convergence) at the location of the multiple images.Comment: 12 pages, 3 figures, ApJL, in press (June 20, 2002
Probing Turbulence with Infrared Observations in OMC1
A statistical analysis is presented of the turbulent velocity structure in
the Orion Molecular Cloud at scales ranging from 70 AU to 30000 AU. Results are
based on IR Fabry-Perot interferometric observations of shock and
photon-excited H2 in the K-band S(1) v=1-0 line at 2.121micron and refer to the
dynamical characteristics of warm perturbed gas. Observations establish that
the Larson size-linewidth relation is obeyed to the smallest scales studied
here extending the range of validity of this relationship by nearly 2 orders of
magnitude. The velocity probability distribution function (PDF) is constructed
showing extended exponential wings, providing evidence of intermittency,
further supported by the skewness and kurtosis of the velocity distribution.
Variance and kurtosis of the PDF of velocity differences are constructed as a
function of lag. The variance shows an approximate power law dependence on lag,
with exponent significantly lower than the Kolmogorov value, and with
deviations below 2000AU which are attributed to outflows and possibly disk
structures associated with low mass star formation within OMC1. The kurtosis
shows strong deviation from a gaussian velocity field, providing evidence of
velocity correlations at small lags. Results agree accurately with
semi-empirical simulations in Eggers & Wang (1998).
In addition, 170 individual H2 emitting clumps have been analysed with sizes
between 500 and 2200 AU. These show considerable diversity with regard to PDFs
and variance functions. Our analysis constitutes the first characterization of
the turbulent velocity field at the scale of star formation and provide a
dataset which models of star-forming regions should aim to reproduce.Comment: 17 pages, 11 figures, to appear in A&A, typos correcte
10,000 Standard Solar Models: a Monte Carlo Simulation
We have evolved 10,000 solar models using 21 input parameters that are
randomly drawn for each model from separate probability distributions for every
parameter. We use the results of these models to determine the theoretical
uncertainties in the predicted surface helium abundance, the profile of the
sound speed versus radius, the profile of the density versus radius, the depth
of the solar convective zone, the eight principal solar neutrino fluxes, and
the fractions of nuclear reactions that occur in the CNO cycle or in the three
branches of the p-p chains. We also determine the correlation coefficients of
the neutrino fluxes for use in analysis of solar neutrino oscillations. Our
calculations include the most accurate available input parameters, including
radiative opacity, equation of state, and nuclear cross sections. We
incorporate both the recently determined heavy element abundances recommended
by Asplund, Grevesse & Sauval (2005) and the older (higher) heavy element
abundances recommended by Grevesse & Sauval (1998). We present best-estimates
of many characteristics of the standard solar model for both sets of
recommended heavy element compositions.Comment: ** John N. Bahcall passed away on August 17, 2005. Manuscript has 60
pages including 10 figure
Is it possible to measure the Lense-Thirring effect on the orbits of the planets in the gravitational field of the Sun?
Here we explore a novel approach in order to try to measure the
post-Newtonian 1/c^2 Lense-Thirring secular effect induced by the
gravitomagnetic field of the Sun on the planetary orbital motion. Due to the
relative smallness of the solar angular momentum J and the large values of the
planetary semimajor axes a, the gravitomagnetic precessions, which affect the
nodes Omega and the perihelia omega and are proportional to J/a^3, are of the
order of 10^-3 arcseconds per century only for, e.g., Mercury. This value lies
just at the edge of the present-day observational sensitivity in reconstructing
the planetary orbits, although future missions to Mercury like Messenger and
BepiColombo could allow to increase it. The major problems come from the main
sources of systematic errors. They are the aliasing classical precessions
induced by the multipolar expansion of the Sun's gravitational potential and
the classical secular N-body precessions which are of the same order of
magnitude or much larger than the Lense-Thirring precessions of interest. This
definitely rules out the possibility of analyzing only one orbital element of,
e.g., Mercury. In order to circumvent these problems, we propose a suitable
linear combination of the orbital residuals of the nodes of Mercury, Venus and
Mars which is, by construction, independent of such classical secular
precessions. A 1-sigma reasonable estimate of the obtainable accuracy yields a
36% error. Since the major role in the proposed combination is played by the
Mercury's node, it could happen that the new, more accurate ephemerides
available in future thanks to the Messenger and BepiColombo missions will offer
an opportunity to improve the present unfavorable situation.Comment: LaTex2e, A&A macros, 6 pages, no figure, 3 tables. Substantial
revision. More realistic conclusions. Estimations of the impact of
BepiColombo presente
An optical time-delay estimate for the double gravitational lens system B1600+434
We present optical I-band light curves of the gravitationally lensed double
QSO B1600+434 from observations obtained at the Nordic Optical Telescope (NOT)
between April 1998 and November 1999. The photometry has been performed by
simultaneous deconvolution of all the data frames, involving a numerical lens
galaxy model. Four methods have been applied to determine the time delay
between the two QSO components, giving a mean estimate of \Delta_t = 51+/-4
days (95% confidence level). This is the fourth optical time delay ever
measured. Adopting a Omega=0.3, Lambda=0 Universe and using the mass model of
Maller et al. (2000), this time-delay estimate yields a Hubble parameter of
H_0=52 (+14, -8) km s^-1 Mpc^-1 (95% confidence level) where the errors include
time-delay as well as model uncertainties. There are time-dependent offsets
between the two (appropriately shifted) light curves that indicate the presence
of external variations due to microlensing.Comment: 15 pages, 4 figures, accepted for publication in Ap
First preliminary tests of the general relativistic gravitomagnetic field of the Sun and new constraints on a Yukawa-like fifth force from planetary data
The general relativistic Lense-Thirring precessions of the perihelia of the
inner planets of the Solar System are about 10^-3 arcseconds per century.
Recent improvements in planetary orbit determination may yield the first
observational evidence of such a tiny effect. Indeed, corrections to the known
perihelion rates of -0.0036 +/- 0.0050, -0.0002 +/- 0.0004 and 0.0001 +/-
0.0005 arcseconds per century were recently estimated by E.V. Pitjeva for
Mercury, the Earth and Mars, respectively, on the basis of the EPM2004
ephemerides and a set of more than 317,000 observations of various kinds. The
predicted relativistic Lense-Thirring precessions for these planets are
-0.0020, -0.0001 and -3 10^-5 arcseconds per century, respectively and are
compatible with the determined perihelia corrections. The relativistic
predictions fit better than the zero-effect hypothesis, especially if a
suitable linear combination of the perihelia of Mercury and the Earth, which a
priori cancels out any possible bias due to the solar quadrupole mass moment,
is considered. However, the experimental errors are still large. Also the
latest data for Mercury processed independently by Fienga et al. with the INPOP
ephemerides yield preliminary insights about the existence of the solar
Lense-Thirring effect. The data from the forthcoming planetary mission
BepiColombo will improve our knowledge of the orbital motion of this planet
and, consequently, the precision of the measurement of the Lense-Thirring
effect. As a by-product of the present analysis, it is also possible to
constrain the strength of a Yukawa-like fifth force to a 10^-12-10^-13 level at
scales of about one Astronomical Unit (10^11 m).Comment: LaTex, 22 pages, 1 figure, 5 tables, 62 references. To appear in
Planetary and Space Scienc
Classical and relativistic long-term time variations of some observables for transiting exoplanets
We analytically work out the long-term, i.e. averaged over one orbital
revolution, time variations of some direct observable quantities Y induced by
classical and general relativistic dynamical perturbations of the two-body
pointlike Newtonian acceleration in the case of transiting exoplanets moving
along elliptic orbits. More specifically, the observables with which we
deal are the transit duration, the radial velocity and the time interval
between primary and secondary eclipses. The dynamical effects considered are
the centrifugal oblateness of both the star and the planet, their tidal bulges
mutually raised on each other, a distant third body X, and general relativity
(both Schwarzschild and Lense-Thirring). We take into account the effects due
to the perturbations of all the Keplerian orbital elements involved in a
consistent and uniform way. First, we explicitly compute their instantaneous
time variations due to the dynamical effects considered and plug them in the
general expression for the instantaneous change of Y; then, we take the overall
average over one orbital revolution of the so-obtained instantaneous rate specialized to the perturbations considered. Instead, somewhat hybrid
expressions can be often found in literature: in them, the secular precession
of, typically, the periastron only is straightforwardly inserted into
instantaneous formulas. Numerical evaluations of the obtained results are given
for a typical star-planet scenario and compared with the expected observational
accuracies over a time span 10 yr long. Our results are, in principle, valid
also for other astronomical scenarios. They may allow, e.g., for designing
various tests of gravitational theories with natural and artificial bodies in
our solar system. (Abridged)Comment: LaTex2e, 19 pages, 5 figures, 2 tables. Some references updated. To
appear in Monthly Notices of the Royal Astronomical Society (MNRAS
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