454 research outputs found
Une sonde photométrique pour l'analyse in situ : Principe, méthode, premiers essais
Certains composés dissous ne sont pas stables une fois prélevés hors de leur milieu. Pour éviter que l'information ne se perde entre le prélèvement et l'analyse, il est nécessaire d'effectuer cette dernière in situ. La solution que nous présentons, consiste à développer une réaction colorimétrique en profondeur; la cellule photométrique est immergée et reliée à un spectrophotomètre en surface, par 2 fibres optiques (fig. 1a, b, c). Cependant, lors d'un essai préliminaire, nous avons observé que, dans le circuit de mélange de la sonde, les proportions entre réactif et échantillon ne sont pas constantes. Ces variations de débits sont corrigées par des mesures à deux longueurs d'onde (λ1 et λ2)* et par l'adjonction d'un colorant auxiliaire ne perturbant pas la réaction calorimétrique. L'étalonnage se fait directement sur la cellule photométrique : dans un diagramme Absorbance à λ1 = f (Absorbance à λ2) (fig. 2), on place une droite d'étalonnage et des points particuliers. Les règles de mélange sont vérifiées indépendamment de toute réaction chimique avec différentes solutions d'hélianthine dans un tampon à pH 7 et du rouge de chlorophénol à la place du réactif (fig. 4 et 5). En outre nous utilisons le rouge de chlorophénol, jaune sous forme acide, comme colorant auxiliaire pour le dosage du fer total dans un premier essai in situ (lac d'Aydat, Puy de Dôme, France). Les résultats sont comparés à ceux obtenus par prélèvements et analyses au laboratoire (fig. 6). L'accord est satisfaisant. L'incorporation au système présenté, d'une pompe osmotique devrait permettre, avec cet appareillage simple, des mesures pendant plusieurs mois sans intervention.Various dissolved compounds are mot stables in surface conditions. We realized a prototype to collect chemical data related to redox sensitive species without any contact with the atmospheric oxygen.The principle of this probe for in situ measurements is to produce colorimetric reaction in depth. A photometric cell and a horizontal coiled glass tube for fluid mixing are immersed and connected with two optic fibers to a spectrophotometer on boat (fig. la, b, c). Reagent is injected continuously from surface and sample is sucked up with a peristaltic pump through a tubular filter.Every species which can be analysed by colorimetric method should be determined, in deep river or in lake, with this simple equipment.Nevertheless, during preliminary trial, we detected a lack of reproducibility in the mixing ratio of the sample with the complexing agent. The problem is solved by adding an auxiliary dye with reagent and measuring optical densities at two different wavelengths (λ1 and λ2). In the system, with a the sample proportion, absorbance A at λ is expressed as :A=ɛe∙l∙Ce∙α+ɛr∙l∙Cr∙(1-α)We suggest to calibrate directly the cell of the probe and work in a calibration graph. It is built with first, marking on an A1λ1=f(A2λ2)graph (fig. 2), the « pivot » point (P) (when α= 0), second, plotting the « calibration curve »A2=A1 (ɛe2/ɛe1)(when α= 1), third, plotting the different S1 (A1i, A2i) measured from standards. Therefore, if sample signal Re at two wavelengths is plotted in this graph, by joining P and Re, the straight line intersects with calibration curve at C. On this curve, interpolation of C between two standards determine the concentration of the analyte.Experimental verification of the mixing rule has been clone independently of chemical reaction, with different heliantine solutions in pH 7 buffer as samples and chlorophenol red as reagent, bath in laboratory and at 15 meters depth (Beffes lake, France) (fig. 4 and 5).Furthermore, chlorophenol red, previously tested, is used as auxiliary dye for total iron measurement, in Aydat lake (Puy de Dôme, France) for a first in situ trial.Results are compared to those got from oceanographic bottle sampling and laboratory analysis (fig. 6). Data from the probe are in good agreement with data from the laboratory method.Next development of this chemical sensor will consist in adding to the system an osmotic pump which should allow measurements without intervention during several months
Kinematics of Black Hole X-ray Binary GRS 1915+105
The space velocity of a stellar black hole encodes the history of its
formation and evolution. Here we measure the 3-dimensional motion of the
microquasar GRS 1915+105, using a decade of astrometry with the NRAO Very Long
Baseline Array, together with the published radial velocity. The velocity in
the Galactic Plane deviates from circular rotation by 53-80 +_ 8 km/s, where
the range covers any specific distance from 6-12 kpc. Perpendicular to the
plane, the velocity is only 10 +_ 4 km/s. The peculiar velocity is minimized at
a distance 9-10 kpc, and is then nearly in the radial direction towards the
Galactic Center. We discuss mechanisms for the origin of the peculiar velocity,
and conclude that it is most likely a consequence of Galactic velocity
diffusion on this old binary, rather than the result of a supernova kick during
the formation of the 14 Mo black hole. Finally, a brief comparison is made with
4 other BH binaries whose kinematics are well determined.Comment: 16 pages, 4 figures. ApJ accepte
Astrometric Effects of a Stochastic Gravitational Wave Background
A stochastic gravitational wave background causes the apparent positions of
distant sources to fluctuate, with angular deflections of order the
characteristic strain amplitude of the gravitational waves. These fluctuations
may be detectable with high precision astrometry, as first suggested by
Braginsky et al. in 1990. Several researchers have made order of magnitude
estimates of the upper limits obtainable on the gravitational wave spectrum
\Omega_gw(f), at frequencies of order f ~ 1 yr^-1, both for the future
space-based optical interferometry missions GAIA and SIM, and for VLBI
interferometry in radio wavelengths with the SKA. For GAIA, tracking N ~ 10^6
quasars over a time of T ~ 1 yr with an angular accuracy of \Delta \theta ~ 10
\mu as would yield a sensitivity level of \Omega_gw ~ (\Delta \theta)^2/(N T^2
H_0^2) ~ 10^-6, which would be comparable with pulsar timing. In this paper we
take a first step toward firming up these estimates by computing in detail the
statistical properties of the angular deflections caused by a stochastic
background. We compute analytically the two point correlation function of the
deflections on the sphere, and the spectrum as a function of frequency and
angular scale. The fluctuations are concentrated at low frequencies (for a
scale invariant stochastic background), and at large angular scales, starting
with the quadrupole. The magnetic-type and electric-type pieces of the
fluctuations have equal amounts of power.Comment: 23 pages, 2 figures, references added and minor text correction
Detectability of Weakly Interacting Massive Particles in the Sagittarius Dwarf Tidal Stream
Tidal streams of the Sagittarius dwarf spheroidal galaxy (Sgr) may be
showering dark matter onto the solar system and contributing approx (0.3--23)%
of the local density of our Galactic Halo. If the Sagittarius galaxy contains
WIMP dark matter, the extra contribution from the stream gives rise to a
step-like feature in the energy recoil spectrum in direct dark matter
detection. For our best estimate of stream velocity (300 km/sec) and direction
(the plane containing the Sgr dwarf and its debris), the count rate is maximum
on June 28 and minimum on December 27 (for most recoil energies), and the
location of the step oscillates yearly with a phase opposite to that of the
count rate. In the CDMS experiment, for 60 GeV WIMPs, the location of the step
oscillates between 35 and 42 keV, and for the most favorable stream density,
the stream should be detectable at the 11 sigma level in four years of data
with 10 keV energy bins. Planned large detectors like XENON, CryoArray and the
directional detector DRIFT may also be able to identify the Sgr stream.Comment: 26 pages, 4 figure
The effect of the motion of the Sun on the light-time in interplanetary relativistic experiments
In 2002 a measurement of the effect of solar gravity upon the phase of
coherent microwave beams passing near the Sun has been carried out with the
Cassini mission, allowing a very accurate measurement of the PPN parameter
. The data have been analyzed with NASA's Orbit Determination Program
(ODP) in the Barycentric Celestial Reference System, in which the Sun moves
around the centre of mass of the solar system with a velocity of
about 10 m/sec; the question arises, what correction this implies for the
predicted phase shift. After a review of the way the ODP works, we set the
problem in the framework of Lorentz (and Galilean) transformations and evaluate
the correction; it is several orders of magnitude below our experimental
accuracy. We also discuss a recent paper \cite{kopeikin07}, which claims wrong
and much larger corrections, and clarify the reasons for the discrepancy.Comment: Final version accepted by Classical and Quantum Gravity (8 Jan. 2008
Tidal friction in close-in satellites and exoplanets. The Darwin theory re-visited
This report is a review of Darwin's classical theory of bodily tides in which
we present the analytical expressions for the orbital and rotational evolution
of the bodies and for the energy dissipation rates due to their tidal
interaction. General formulas are given which do not depend on any assumption
linking the tidal lags to the frequencies of the corresponding tidal waves
(except that equal frequency harmonics are assumed to span equal lags).
Emphasis is given to the cases of companions having reached one of the two
possible final states: (1) the super-synchronous stationary rotation resulting
from the vanishing of the average tidal torque; (2) the capture into a 1:1
spin-orbit resonance (true synchronization). In these cases, the energy
dissipation is controlled by the tidal harmonic with period equal to the
orbital period (instead of the semi-diurnal tide) and the singularity due to
the vanishing of the geometric phase lag does not exist. It is also shown that
the true synchronization with non-zero eccentricity is only possible if an
extra torque exists opposite to the tidal torque. The theory is developed
assuming that this additional torque is produced by an equatorial permanent
asymmetry in the companion. The results are model-dependent and the theory is
developed only to the second degree in eccentricity and inclination
(obliquity). It can easily be extended to higher orders, but formal accuracy
will not be a real improvement as long as the physics of the processes leading
to tidal lags is not better known.Comment: 30 pages, 7 figures, corrected typo
The first accurate parallax distance to a black hole
Using astrometric VLBI observations, we have determined the parallax of the
black hole X-ray binary V404 Cyg to be 0.418 +/- 0.024 milliarcseconds,
corresponding to a distance of 2.39 +/- 0.14 kpc, significantly lower than the
previously accepted value. This model-independent estimate is the most accurate
distance to a Galactic stellar-mass black hole measured to date. With this new
distance, we confirm that the source was not super-Eddington during its 1989
outburst. The fitted distance and proper motion imply that the black hole in
this system likely formed in a supernova, with the peculiar velocity being
consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred
to exist in this system is less than 1.4 AU at 22 GHz. Astrometric observations
of a larger sample of such systems would provide useful insights into the
formation and properties of accreting stellar-mass black holes.Comment: Accepted for publication in ApJ Letters. 6 pages, 2 figure
Gaia Data Processing Architecture
Gaia is ESA's ambitious space astrometry mission the main objective of which
is to astrometrically and spectro-photometrically map 1000 Million celestial
objects (mostly in our galaxy) with unprecedented accuracy. The announcement of
opportunity for the data processing will be issued by ESA late in 2006. The
Gaia Data Processing and Analysis Consortium (DPAC) has been formed recently
and is preparing an answer. The satellite will downlink close to 100 TB of raw
telemetry data over 5 years. To achieve its required accuracy of a few 10s of
Microarcsecond astrometry, a highly involved processing of this data is
required.
In addition to the main astrometric instrument Gaia will host a Radial
Velocity instrument, two low-resolution dispersers for multi-color photometry
and two Star Mappers. Gaia is a flying Giga Pixel camera. The various
instruments each require relatively complex processing while at the same time
being interdependent. We describe the overall composition of the DPAC and the
envisaged overall architecture of the Gaia data processing system. We shall
delve further into the core processing - one of the nine, so-called,
coordination units comprising the Gaia processing system.Comment: 10 Pages, 2 figures. To appear in ADASS XVI Proceeding
Tidal torques. A critical review of some techniques
We point out that the MacDonald formula for body-tide torques is valid only
in the zeroth order of e/Q, while its time-average is valid in the first order.
So the formula cannot be used for analysis in higher orders of e/Q. This
necessitates corrections in the theory of tidal despinning and libration
damping.
We prove that when the inclination is low and phase lags are linear in
frequency, the Kaula series is equivalent to a corrected version of the
MacDonald method. The correction to MacDonald's approach would be to set the
phase lag of the integral bulge proportional to the instantaneous frequency.
The equivalence of descriptions gets violated by a nonlinear
frequency-dependence of the lag.
We explain that both the MacDonald- and Darwin-torque-based derivations of
the popular formula for the tidal despinning rate are limited to low
inclinations and to the phase lags being linear in frequency. The
Darwin-torque-based derivation, though, is general enough to accommodate both a
finite inclination and the actual rheology.
Although rheologies with Q scaling as the frequency to a positive power make
the torque diverge at a zero frequency, this reveals not the impossible nature
of the rheology, but a flaw in mathematics, i.e., a common misassumption that
damping merely provides lags to the terms of the Fourier series for the tidal
potential. A hydrodynamical treatment (Darwin 1879) had demonstrated that the
magnitudes of the terms, too, get changed. Reinstating of this detail tames the
infinities and rehabilitates the "impossible" scaling law (which happens to be
the actual law the terrestrial planets obey at low frequencies).Comment: arXiv admin note: sections 4 and 9 of this paper contain substantial
text overlap with arXiv:0712.105
GAIA: Composition, Formation and Evolution of the Galaxy
The GAIA astrometric mission has recently been approved as one of the next
two `cornerstones' of ESA's science programme, with a launch date target of not
later than mid-2012. GAIA will provide positional and radial velocity
measurements with the accuracies needed to produce a stereoscopic and kinematic
census of about one billion stars throughout our Galaxy (and into the Local
Group), amounting to about 1 per cent of the Galactic stellar population.
GAIA's main scientific goal is to clarify the origin and history of our Galaxy,
from a quantitative census of the stellar populations. It will advance
questions such as when the stars in our Galaxy formed, when and how it was
assembled, and its distribution of dark matter. The survey aims for
completeness to V=20 mag, with accuracies of about 10 microarcsec at 15 mag.
Combined with astrophysical information for each star, provided by on-board
multi-colour photometry and (limited) spectroscopy, these data will have the
precision necessary to quantify the early formation, and subsequent dynamical,
chemical and star formation evolution of our Galaxy. Additional products
include detection and orbital classification of tens of thousands of
extra-Solar planetary systems, and a comprehensive survey of some 10^5-10^6
minor bodies in our Solar System, through galaxies in the nearby Universe, to
some 500,000 distant quasars. It will provide a number of stringent new tests
of general relativity and cosmology. The complete satellite system was
evaluated as part of a detailed technology study, including a detailed payload
design, corresponding accuracy assesments, and results from a prototype data
reduction development.Comment: Accepted by A&A: 25 pages, 8 figure
- …