79 research outputs found
Reproduction expérimentale du syndrome hémorragique des jeunes bovidés bretons par ingestion de fougÚre aigle de la région parisienne
VLBI observations of jupiter with the initial test station of LOFAR and the nancay decametric array
AIMS: To demonstrate and test the capability of the next generation of
low-frequency radio telescopes to perform high resolution observations across
intra-continental baselines. Jupiter's strong burst emission is used to perform
broadband full signal cross-correlations on time intervals of up to hundreds of
milliseconds. METHODS: Broadband VLBI observations at about 20 MHz on a
baseline of ~50000 wavelengths were performed to achieve arcsecond angular
resolution. LOFAR's Initial Test Station (LOFAR/ITS, The Netherlands) and the
Nancay Decametric Array (NDA, France) digitize the measured electric field with
12 bit and 14 bit in a 40 MHz baseband. The fine structure in Jupiter's signal
was used for data synchronization prior to correlation on the time-series data.
RESULTS: Strong emission from Jupiter was detected during snapshots of a few
seconds and detailed features down to microsecond time-scales were identified
in dynamic spectra. Correlations of Jupiter's burst emission returned strong
fringes on 1 ms time-scales over channels as narrow as a hundred kilohertz
bandwidth. CONCLUSIONS: Long baseline interferometry is confirmed at low
frequencies, in spite of phase shifts introduced by variations in ionospheric
propagation characteristics. Phase coherence was preserved over tens to
hundreds of milliseconds with a baseline of ~700 km. No significant variation
with time was found in the correlations and an estimate for the fringe
visibility of 1, suggested that the source was not resolved. The upper limit on
the source region size of Jupiter Io-B S-bursts corresponds to an angular
resolution of ~3 arcsec. Adding remote stations to the LOFAR network at
baselines up to thousand kilometers will provide 10 times higher resolution
down to an arcsecond.Comment: 6 pages, 4 figures. Nigl, A., Zarka, P., Kuijpers, J., Falcke, H.,
Baehren, L., VLBI observations of Jupiter with the Initial Test Station of
LOFAR and the Nancay Decametric Array, A&A, 471, 1099-1104, accepted on
31/05/200
The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant () determination
Recent HST determinations of the expansion's rate of the Universe (the Hubble
constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I
are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996,
Tanvir et al., 1995). The three groups obtain different vales for H_0. We note
that most of this discrepancy stems from the asumption (by both groups) that
the Period-Luminosity relation is independent of metallicity. We come to this
conclusion as a result of our study of the Period-Luminosity relation of 481
Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud
and the Small Magellanic Cloud obtained as a by-product of the EROS
microlensing survey. We find that the derived interstellar absorption
corrections are particularly sensitive to the metallicity and when our result
is applied to recent estimates based on HST Cepheids observations it makes the
low-H_0 values higher and the high-H_0 value lower, bringing those discrepant
estimates into agrement around .Comment: 4 pages, Latex, with 2 .ps accepted for publication astronomy and
astrophysics Letter
EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation
We report the discovery of eleven beat Cepheids in the Small Magellanic
Cloud, using data obtained by the EROS microlensing survey. Four stars are
beating in the fundamental and first overtone mode (F/1OT), seven are beating
in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is
systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the
SMC Cepheids is the same as the LMC one.Comment: 4 pages, Latex file with 4 .ps figures. accepted for publication in A
A Letter
Observational Limits on Machos in the Galactic Halo
We present final results from the first phase of the EROS search for
gravitational microlensing of stars in the Magellanic Clouds by unseen
deflectors (machos: MAssive Compact Halo Objects). The search is sensitive to
events with time scales between 15 minutes and 200 days corresponding to
deflector masses in the range 1.e-7 to a few solar masses. Two events were
observed that are compatible with microlensing by objects of mass of about 0.1
Mo. By comparing the results with the expected number of events for various
models of the Galaxy, we conclude that machos in the mass range [1.e-7, 0.02]
Mo make up less than 20% (95% C.L.) of the Halo dark matter.Comment: 4 pages, 3 Postscript figures, to be published in Astronomy &
Astrophysic
Magnetic coupling of planets and small bodies with a pulsar wind"
We investigate the electromagnetic interaction of a relativistic stellar wind
with a planet or a smaller body in orbit around the star. This may be relevant
to objects orbiting a pulsar, such as PSR B1257+12 and PSR B1620-26 that are
expected to hold a planetary system, or to pulsars with suspected asteroids or
comets. We extend the theory of Alfv\'en wings to relativistic winds. When the
wind is relativistic albeit slower than the total Alfv\'en speed, a system of
electric currents carried by a stationary Alfv\'enic structure is driven by the
planet or by its surroundings. For an Earth-like planet around a "standard" one
second pulsar, the associated current can reach the same magnitude as the
Goldreich-Julian current that powers the pulsar's magnetosphere.Comment: ccepted for publication in "Astronomy and Astrophysics
Modelling the radio pulses of an ultracool dwarf
<b>Context:</b> Recently, unanticipated magnetic activity in ultracool dwarfs (UCDs, spectral classes later than M7) has emerged from a number of radio observations. The highly (up to 100%) circularly polarized nature and high brightness temperature of the emission have been interpreted as requiring an effective amplification mechanism of the high-frequency electromagnetic waves â the electron cyclotron maser instability (ECMI).
<p/><b>Aims:</b> We aim to understand the magnetic topology and the properties of the radio emitting region and associated plasmas in these ultracool dwarfs, interpreting the origin of radio pulses and their radiation mechanism.
<p/><b>Methods:</b> An active region model was built, based on the rotation of the UCD and the ECMI mechanism.
<p/><b>Results:</b> The high degree of variability in the brightness and the diverse profile of pulses can be interpreted in terms of a large-scale hot active region with extended magnetic structure existing in the magnetosphere of TVLM 513-46546. We suggest the time profile of the radio light curve is in the form of power law in the model. Combining the analysis of the data and our simulation, we can determine the loss-cone electrons have a density in the range of 1.25 Ă 105â5 Ă 105 cm-3 and temperature between 107 and 5 Ă 107 K. The active region has a size <1 RJup, while the pulses produced by the ECMI mechanism are from a much more compact region (e.g. ~0.007 RJup). A surface magnetic field strength of â7000 G is predicted.
<p/><b>Conclusions:</b> The active region model is applied to the radio emission from TVLM 513-46546, in which the ECMI mechanism is responsible for the radio bursts from the magnetic tubes and the rotation of the dwarf can modulate the integral of flux with respect to time. The radio emitting region consists of complicated substructures. With this model, we can determine the nature (e.g. size, temperature, density) of the radio emitting region and plasma. The magnetic topology can also be constrained. We compare our predicted X-ray flux with Chandra X-ray observation of TVLM 513-46546. Although the X-ray detection is only marginally significant, our predicted flux is significantly lower than the observed flux. Further multi-wavelength observations will help us better understand the magnetic field structure and plasma behavior on the ultracool dwarf
EROS VARIABLE STARS : FUNDAMENTAL-MODE AND FIRST OVERTONE CEPHEIDS IN THE BAR OF THE LARGE MAGELLANIC CLOUD
We present CCD phase-binned light curves at 490 nm for 97 Cepheid variable
stars in the bar of the LMC. The photometry was obtained as part of the French
EROS project and has excellent phase coverage, permitting accurate
decomposition into Fourier components. We identify as `sinusoidal' or
s-Cepheids those stars with periods less than 5.5 d and small second-harmonic
components. These stars comprise 30% of our sample and most form a
sequence 1 mag brighter than the LMC classical Cepheids in the
period-luminosity diagram. They are also generally bluer and have
lower-amplitude light curves. We infer that the s-Cepheids are first-overtone
pulsators because, when their periods are converted to expected
fundamental-mode values, they obey a common period-luminosity-colour relation
with classical Cepheids. This also confirms the reality of the colour term in
the Cepheid period-luminosity-colour relation. Further, the blue edge of the
classical Cepheid instability strip agrees well with the theoretical
calculations for the fundamental mode made by Chiosi et al. (1993) for the
Hertzsprung-Russell and period-luminosity diagrams, but we find that our
observed s-Cepheids are mag brighter and bluer than the Chiosi et al.\
predictions for the first-overtone. We identify a number of features in plots
of our stars' Fourier-component amplitude ratios and phase differences. These
features have been identified with resonances between different pulsation
modes. In the LMC we find these features seem to occur at periods very similar
to Galactic ones for classical Cepheids, but at different periods for
s-Cepheids. We discover a double-mode Cepheid in the LMC, for which , very similar to
observed ratios for Galactic double-mode Cepheids.Comment: 19 pages, uuencoded compressed PS file, including 14 figures.
Accepted for publication in Astronomy and Astrophysics, February-2-199
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