217 research outputs found
Time and frequency transfer with a microwave link in the ACES/PHARAO mission
The Atomic Clocks Ensemble in Space (ACES/PHARAO mission), which will be
installed on board the International Space Station (ISS), uses a dedicated
two-way Micro-Wave Link (MWL) in order to compare the timescale generated on
board with those provided by many ground stations disseminated on the Earth.
Phase accuracy and stability of this long range link will have a key role in
the success of the ACES/PHARAO experiment. SYRTE laboratory is heavily involved
in the design and development of the data processing software : from
theoretical modelling and numerical simulations to the development of a
software prototype. Our team is working on a wide range of problems that need
to be solved in order to achieve high accuracy in (almost) real time. In this
article we present some key aspects of the measurement, as well as current
status of the software's development.Comment: Proceedings of the European Frequency and Time Forum (EFTF) 2012 held
in Gothenburg, Sweden, April 201
HST's view of the youngest massive stars in the Magellanic Clouds
Accurate physical parameters of newborn massive stars are essential
ingredients to shed light on their formation, which is still an unsolved
problem. The rare class of compact H II regions in the Magellanic Clouds (MCs),
termed ``high-excitation blobs'' (HEBs), presents a unique opportunity to
acquire this information. These objects (~ 4" to 10", ~ 1 to 3 pc, in diameter)
harbor the youngest massive stars of the OB association/molecular cloud
complexes in the MCs accessible through high-resolution near-IR and optical
techniques. We present a brief overview of the results obtained with HST mainly
on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).Comment: 5 pages, to appear in the Proceedings of the 41st ESLAB Symposium
"The Impact of HST on European Astronomy", 29 May to 1 June 2007, ESTEC,
Noordwijk, Netherlands; eds. Guido De Marchi and Duccio Macchett
Starch and oil in the donor cow diet and starch in substrate differently affect the in vitro ruminal biohydrogenation of linoleic and linolenic acids
Trans isomers of fatty acids exhibit different health properties. Among them, trans-10,cis-12 conjugated linoleic acid has negative effects on milk fat production and can affect human health. A shift from the trans-11 to the trans-10 pathway of biohydrogenation (BH) can occur in the rumen of dairy cows receiving high-concentrate diets, especially when the diet is supplemented with highly unsaturated fat sources. The differences of BH patterns between linoleic acid (LeA) and linolenic acid (LnA) in such ruminal conditions remain unknown; thus, the aim of this work was to investigate in vitro the effects of starch and sunflower oil in the diet of the donor cows and starch level in the incubates on the BH patterns and efficiencies of LeA and LnA. The design was a 4 Ă 4 Latin square design with 4 cows, 4 periods, and 4 diets with combinations of 21 or 34% starch and 0 or 5% sunflower oil. The rumen content of each cow during each period was incubated with 4 substrates, combining 2 starch levels and either LeA or LnA addition. Capillary electrophoresis single-strand conformation polymorphism of incubates showed that dietary starch decreased the diversity of the bacterial community and the high-starch plus oil diet modified its structure. High-starch diets poorly affected isomerization and first reduction of LeA and LnA, but decreased the efficiencies of trans-11,cis-15-C18:2 and trans C18:1 reduction. Dietary sunflower oil increased the efficiency of LeA isomerization but decreased the efficiency of trans C18:1 reduction. An interaction between dietary starch and dietary oil resulted in the highest trans-10 isomers production in incubates when the donor cow received the high-starch plus oil diet. The partition between trans-10 and trans-11 isomers was also affected by an interaction between starch level and the fatty acid added to the incubates, showing that the trans-10 shift only occurred with LeA, whereas LnA was mainly hydrogenated via the more usual trans-11 pathway, whatever the starch level in the substrate, although the bacterial communities were not different between LeA and LnA incubates. In LeA incubates, trans-10 isomer production was significantly related to the structure of the bacterial community
Effects of oil and natural or synthetic vitamin E on ruminal and milk fatty acid profiles in cows receiving a high-starch diet
Among trans fatty acids, trans-10,cis-12 CLA has negative effects on cow milk fat production and can affect human health. In high-yielding dairy cows, a shift from the trans-11 to the trans-10 pathway of biohydrogenation (BH) can occur in the rumen of cows receiving high-concentrate diets, especially when the diet is supplemented with unsaturated fat sources. In some but not all experiments, vitamin E has been shown to control this shift. To ascertain the effects of vitamin E on this shift of BH pathway, 2 studies were conducted. The first study explored in vitro the effects of addition of natural (RRR-α-tocopherol acetate) and synthetic (dl-α-tocopherol acetate) vitamin E. Compared with control and synthetic vitamin E, the natural form resulted in a greater trans-10/trans-11 ratio; however, the effect was very low, suggesting that vitamin E was neither a limiting factor for rumen BH nor a modulator of the BH pathway. An in vivo study investigated the effect of natural vitamin E (RRR-α-tocopherol) on this
shift and subsequent milk fat depression. Six rumenfistulated lactating Holstein cows were assigned to a 2
Ă 2 crossover design. Cows received 20-kg DM of a control
diet based on corn silage with 22% of wheat, and after 2 wk of adaptation, the diet was supplemented with 600 g of sunflower oil for 2 more weeks. During the last week of this 4-wk experimental period, cows were divided into 2 groups: an unsupplemented control group and a group receiving 11 g of RRR-α-tocopherol acetate per day. A trans-10 shift of ruminal BH associated with milk fat depression due to oil supplementation of a high-wheat diet was observed, but vitamin E supplementation of dairy cows did not result in a reversal toward a trans-11 BH pathway, and did not restore milk fat content
The stellar environment of SMC N81
We present near infrared JHK imaging of the Small Magellanic Cloud compact H
II region N81 using the ISAAC camera at the ESO Very Large Telescope (Antu).
Our analysis of the stellar environment of this young massive star region
reveals the presence of three new stellar populations in the surrounding field
which are mainly composed of low mass stars. The main population is best fitted
by evolutionary models for about 2 solar mass stars with an age of 1 Gyr. We
argue that these populations are not physically associated with the H II region
N81. Instead they are the result of a number of low mass star forming events
through the depth of the SMC south of its Shapley's wing. The populations can
rather easily be probed due to the low interstellar extinction in that
direction.Comment: 11 pages, 5 figures, accepted for publication in A&
Features of radio-detected Extensive Air Shower with CODALEMA
Some performances of the present CODALEMA experiment, set up to analyse
radio-detected Extensive Air Shower (EAS) events, are presented.
Characteristics of the EAS electric field distribution sampled on a 600~m long
axis are discussed.Comment: 4 pages, 3 figures, proceeding des rencontres de l'astrophysique
francaise, Strasbourg, 200
Resolving the compact HII regions in N160A with HST
Using high-resolution imaging with the Hubble Space Telescope, we study the
Large Magellanic Cloud HII region N160A and uncover several striking features
of this complex massive star-forming site. The two compact high excitation HII
blobs (HEBs) A1 and A2 are for the first time resolved and their stellar
content and morphology is revealed. A1, being of higher excitation, is powered
by a single massive star whose strong wind has created a surrounding bubble. A2
harbors several exciting stars enshrouded inside large quantities of dust. The
whole N160A nebula is energized by three star clusters for which we obtain
photometry and study their color-magnitude diagram. The HII region is
particularly dusty, with extinction values reaching an A_v~2.5 mag in the
visible, and it is separated from the molecular cloud by an outstanding
ionization front. A previously detected infrared young stellar object is also
accurately located with respect to the HII region.Comment: 11 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysics. A version of the paper with higher quality images is available
at http://wwwusr.obspm.fr/~heydari/projects/N16
A clock network for geodesy and fundamental science
Leveraging the unrivaled performance of optical clocks in applications in
fundamental physics beyond the standard model, in geo-sciences, and in
astronomy requires comparing the frequency of distant optical clocks
truthfully. Meeting this requirement, we report on the first comparison and
agreement of fully independent optical clocks separated by 700 km being only
limited by the uncertainties of the clocks themselves. This is achieved by a
phase-coherent optical frequency transfer via a 1415 km long telecom fiber link
that enables substantially better precision than classical means of frequency
transfer. The fractional precision in comparing the optical clocks of three
parts in was reached after only 1000 s averaging time, which is
already 10 times better and more than four orders of magnitude faster than with
any other existing frequency transfer method. The capability of performing high
resolution international clock comparisons paves the way for a redefinition of
the unit of time and an all-optical dissemination of the SI-second.Comment: 14 pages, 3 figures, 1 tabl
Low-excitation blobs in the Magellanic Clouds
Aims : We study an unknown, or very poorly known, interstellar HII component
in the Magellanic Clouds. This is the first study ever devoted to this class of
objects, which we call Low-excitation blobs (LEBs).
Methods : We used low-dispersion spectroscopy carried out at ESO to obtain
emission line intensities of Ha, Hb, and [OIII] (4959+5007) for 15 objects in
the Large Magellanic Cloud and 14 objects in the Small Magellanic Cloud.
Results are displayed in excitation ([oiii]/Hb ratio) versus Hb luminosity
diagrams.
Results : We show the presence of an LEB component in the Magellanic Clouds
and study its relationship with the already known class of high-excitation
blobs (HEBs). The newly found LEBs are lower excitation counterparts of HEBs
and are powered by less massive exciting stars. Further study of LEBs is
expected to provide new pieces of information for a better understanding the
low mass end of the upper initial mass function in the Magellanic Clouds.Comment: Accepted in A&
Why dynamos are prone to reversals
In a recent paper (Phys. Rev. Lett. 94 (2005), 184506; physics/0411050) it
was shown that a simple mean-field dynamo model with a spherically symmetric
helical turbulence parameter alpha can exhibit a number of features which are
typical for Earth's magnetic field reversals. In particular, the model produces
asymmetric reversals, a positive correlation of field strength and interval
length, and a bimodal field distribution. All these features are attributable
to the magnetic field dynamics in the vicinity of an exceptional point of the
spectrum of the non-selfadjoint dynamo operator. The negative slope of the
growth rate curve between the nearby local maximum and the exceptional point
makes the system unstable and drives it to the exceptional point and beyond
into the oscillatory branch where the sign change happens. A weakness of this
reversal model is the apparent necessity to fine-tune the magnetic Reynolds
number and/or the radial profile of alpha. In the present paper, it is shown
that this fine-tuning is not necessary in the case of higher supercriticality
of the dynamo. Numerical examples and physical arguments are compiled to show
that, with increasing magnetic Reynolds number, there is strong tendency for
the exceptional point and the associated local maximum to move close to the
zero growth rate line. Although exemplified again by the spherically symmetric
alpha^2 dynamo model, the main idea of this ''self-tuning'' mechanism of
saturated dynamos into a reversal-prone state seems well transferable to other
dynamos. As a consequence, reversing dynamos might be much more typical and may
occur much more frequently in nature than what could be expected from a purely
kinematic perspective.Comment: 11 pages, 10 figure
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