648 research outputs found
Comparaison entre relevés de 25 m2 et 100 m2 pour l’étude de la faune de la strate herbacée
Au cours de l’étude des peuplements animaux d’une savane de Côte d’ivoire par la méthode des carrés de ramassage, il a semblé utile de comparer deux surfaces usuelles : 25 m2 et 100 m2. Dans ce but, trois séries de relevés comprenant chacune un quadrat de 100 m2 et quatre de 25 m2 ont été réalisées en une seule matinée dans un même milieu. Il est apparu que les récoltes sur 25 m2 sont plus minutieuses et, en considérant les groupes zoologiques globalement, pas plus hétérogènes que celles sur 100 m2. L’étude spécifique de quelques groupes d’insectes montre qu’une surface de 100 m2 au minimum est nécessaire pour avoir une première idée des principales espèces. L’analyse de ces relevés montre aussi que la variabilité de densité des peuplements est très différente suivant les groupes. On peut supposer que les plus variables seraient de bons indicateurs écologiques.The results obtained by hand-collecting of Arthropods from quadrats of different size (25 m2 and 100 m2) have been compared. Collecting was done in Loudetici savanna on the same day and at the same time. Retter results are obtained using small (25 m2) rather than large (100 m2) quadrats, a greater number of small sized individuals being taken per unit area in the former compared with the latter. Sample variability is no higher in the smaller quadrats, and it is concluded that the best compromise between the cost and variance of daily sampling is attained by sampling from 4 to 6 areas, each of 25 m2
Spin density distribution in a partially magnetized organic quantum magnet
Polarized neutron diffraction experiments on an organic magnetic material
reveal a highly skewed distribution of spin density within the magnetic
molecular unit. The very large magnitude of the observed effect is due to
quantum spin fluctuations. The data are in quantitative agreement with direct
diagonalization results for a model spin Hamiltonian, and provide insight on
the actual microscopic origin of the relevant exchange interactions.Comment: 5 pages 4 figure
Comparaison de plusieurs indices de diversité dans l’étude d’un peuplement de Mantes
The biological diversity of praying Mantids has been analysed to illustrate some classical indices between stability and diversity on the one hand and the numbers of individuals and species on the other. A good description of the Mantid communities can be reached by using only two parameters : the « a », of Fischer for specific richness and the « E » of Margalef for interspecific relations
A deconvolution-based algorithm for crowded field photometry with unknown Point Spread Function
A new method is presented for determining the Point Spread Function (PSF) of
images that lack bright and isolated stars. It is based on the same principles
as the MCS (Magain, Courbin, Sohy, 1998) image deconvolution algorithm. It uses
the information contained in all stellar images to achieve the double task of
reconstructing the PSFs for single or multiple exposures of the same field and
to extract the photometry of all point sources in the field of view. The use of
the full information available allows to construct an accurate PSF. The
possibility to simultaneously consider several exposures makes it very well
suited to the measurement of the light curves of blended point sources from
data that would be very difficult or even impossible to analyse with
traditional PSF fitting techniques. The potential of the method for the
analysis of ground-based and space-based data is tested on artificial images
and illustrated by several examples, including HST/NICMOS images of a lensed
quasar and VLT/ISAAC images of a faint blended Mira star in the halo of the
giant elliptical galaxy NGC5128 (Cen A).Comment: Institutes: (1) Institut d'Astrophysique et de Geophysique,
Universite de Liege, allee du 6 Aout 17, B-4000 Liege, Belgium; (2) Ecole
Polytechnique Federale de Lausanne (EPFL), Laboratoire d'Astrophysique,
Observatoire, CH-1290 Sauverny, Switzerland; (3) Observatoire de Geneve, 51
Chemin des Maillettes, CH-1290 Sauverny, Switzerland. 8 pages, 8 figures.
Accepted for publication in A&
Dust modelling and a dynamical study of comet 41P/Tuttle-Giacobini-Kresak during its 2017 perihelion passage
Thanks to the Rosetta mission, our understanding of comets has greatly
improved. A very good opportunity to apply this knowledge appeared in early
2017 with the appearance of the Jupiter family comet 41P/TGK. We performed an
observational campaign with the TRAPPIST telescopes that covered almost the
entire period of time when the comet was active. In this work we present a
comprehensive study of the evolution of the dust environment of 41P based on
observational data from January to July, 2017. Also, we performed numerical
simulations to constrain its origin and dynamical nature. To model the
observational data set we used a Monte Carlo dust tail model, which allowed us
to derive the dust parameters that best describe its dust environment as a
function of heliocentric distance. In order to study its dynamical evolution,
we completed several experiments to evaluate the degree of stability of its
orbit, its life time in its current region close to Earth, and its future
behaviour. From the dust analysis, we found that comet 41P has a complex
emission pattern that shifted from full isotropic to anisotropic ejection
sometime during February 24-March 14 in 2017, and then from anisotropic to full
isotropic again between June 7-28. During the anisotropic period, the emission
was controlled by two strongly active areas, where one was located in the
southern and one in the northern hemisphere of the nucleus. The total dust mass
loss is estimated to be kg. From the dynamical
simulations we estimate that 3600 yr is the period of time during which
41P will remain in a similar orbit. Taking into account the estimated mass loss
per orbit, after 3600 yr, the nucleus may lose about 30 of its mass.
However, based on its observed dust-to-water mass ratio and its propensity to
outbursts, the lifetime of this comet could be much shorter.Comment: 14 pages, 13 figures. Accepted for its publication in Astronomy &
Astrophysic
Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared
The TRAPPIST-1 planetary system is a favorable target for the atmospheric
characterization of temperate earth-sized exoplanets by means of transmission
spectroscopy with the forthcoming James Webb Space Telescope (JWST). A possible
obstacle to this technique could come from the photospheric heterogeneity of
the host star that could affect planetary signatures in the transit
transmission spectra. To constrain further this possibility, we gathered an
extensive photometric data set of 25 TRAPPIST-1 transits observed in the
near-IR J band (1.2 m) with the UKIRT and the AAT, and in the NB2090 band
(2.1 m) with the VLT during the period 2015-2018. In our analysis of these
data, we used a special strategy aiming to ensure uniformity in our
measurements and robustness in our conclusions. We reach a photometric
precision of (RMS of the residuals), and we detect no significant
temporal variations of transit depths of TRAPPIST-1 b, c, e, and g over the
period of three years. The few transit depths measured for planets d and f hint
towards some level of variability, but more measurements will be required for
confirmation. Our depth measurements for planets b and c disagree with the
stellar contamination spectra originating from the possible existence of bright
spots of temperature 4500 K. We report updated transmission spectra for the six
inner planets of the system which are globally flat for planets b and g and
some structures are seen for planets c, d, e, and f.Comment: accepted for publication in MNRA
The Sun as a planet-host star : proxies from SDO images for HARPS radial-velocity variations
RDH gratefully acknowledges STFC studentship grant number ST/J500744/1, and a grant from the John Templeton Foundation. ACC and RF acknowledge support from STFC consolidated grants numbers ST/J001651/1 and ST/M001296/1. JL acknowledges support from NASA Origins of the Solar System grant No. NNX13AH79G and from STFC grant ST/M001296/1.The Sun is the only star whose surface can be directly resolved at high resolution, and therefore constitutes an excellent test case to explore the physical origin of stellar radial-velocity (RV) variability. We present HARPS observations of sunlight scattered off the bright asteroid 4/Vesta, from which we deduced the Sun's activity-driven RV variations. In parallel, the Helioseismic and Magnetic Imager instrument on board the Solar Dynamics Observatory provided us with simultaneous high spatial resolution magnetograms, Dopplergrams and continuum images of the Sun in the Fe i 6173 Å line. We determine the RV modulation arising from the suppression of granular blueshift in magnetized regions and the flux imbalance induced by dark spots and bright faculae. The rms velocity amplitudes of these contributions are 2.40 and 0.41 m s−1, respectively, which confirms that the inhibition of convection is the dominant source of activity-induced RV variations at play, in accordance with previous studies. We find the Doppler imbalances of spot and plage regions to be only weakly anticorrelated. Light curves can thus only give incomplete predictions of convective blueshift suppression. We must instead seek proxies that track the plage coverage on the visible stellar hemisphere directly. The chromospheric flux index R′HK derived from the HARPS spectra performs poorly in this respect, possibly because of the differences in limb brightening/darkening in the chromosphere and photosphere. We also find that the activity-driven RV variations of the Sun are strongly correlated with its full-disc magnetic flux density, which may become a useful proxy for activity-related RV noise.PostprintPeer reviewe
Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
Since the discovery of the transiting super-Earth CoRoT-7b, several
investigations have yielded different results for the number and masses of
planets present in the system, mainly owing to the star's high level of
activity. We re-observed CoRoT-7 in January 2012 with both HARPS and CoRoT, so
that we now have the benefit of simultaneous radial-velocity and photometric
data. This allows us to use the off-transit variations in the star's light
curve to estimate the radial-velocity variations induced by the suppression of
convective blueshift and the flux blocked by starspots. To account for
activity-related effects in the radial-velocities which do not have a
photometric signature, we also include an additional activity term in the
radial-velocity model, which we treat as a Gaussian process with the same
covariance properties (and hence the same frequency structure) as the light
curve. Our model was incorporated into a Monte Carlo Markov Chain in order to
make a precise determination of the orbits of CoRoT-7b and CoRoT-7c. We measure
the masses of planets b and c to be 4.73 +/- 0.95 Mearth and 13.56 +/- 1.08
Mearth, respectively. The density of CoRoT-7b is (6.61 +/- 1.72)(Rp/1.58
Rearth)^(-3) g.cm^(-3), which is compatible with a rocky composition. We search
for evidence of an additional planet d, identified by previous authors with a
period close to 9 days. We are not able to confirm the existence of a planet
with this orbital period, which is close to the second harmonic of the stellar
rotation at around 7.9 days. Using Bayesian model selection we find that a
model with two planets plus activity-induced variations is most favoured.Comment: Accepted 2014 July 2. Received 2014 June 30; in original form 2013
May 30 (17 pages, 9 figures
Discovery and characterization of WASP-6b, an inflated sub-Jupiter mass planet transiting a solar-type star
We report the discovery of WASP-6b, an inflated sub-Jupiter mass planet transiting every 3.3610060^{\rm + 0.0000022 }_ days a mildly metal-poor solar-type star of magnitude V = 11.9. A combined analysis of the WASP photometry, high-precision followup transit photometry and radial velocities yield a planetary mass M_{\rm p} = 0.503^_ and radius R_{\rm p} = 1.224^_ , resulting in a density . The mass and radius for the host star are M_\ast = 0.88^_ and R_\ast = 0.870^_ . The non-zero orbital eccentricity e = 0.054^{\rm +0.018}_ that we measure suggests that the planet underwent a massive tidal heating ~1 Gyr ago that could have contributed to its inflated radius. High-precision radial velocities obtained during a transit allow us to measure a sky-projected angle between the stellar spin and orbital axis \beta = 11^_ deg. In addition to similar published measurements, this result favors a dominant migration mechanism based on tidal interactions with a protoplanetary disk
SPECULOOS exoplanet search and its prototype on TRAPPIST
One of the most significant goals of modern science is establishing whether
life exists around other suns. The most direct path towards its achievement is
the detection and atmospheric characterization of terrestrial exoplanets with
potentially habitable surface conditions. The nearest ultracool dwarfs (UCDs),
i.e. very-low-mass stars and brown dwarfs with effective temperatures lower
than 2700 K, represent a unique opportunity to reach this goal within the next
decade. The potential of the transit method for detecting potentially habitable
Earth-sized planets around these objects is drastically increased compared to
Earth-Sun analogs. Furthermore, only a terrestrial planet transiting a nearby
UCD would be amenable for a thorough atmospheric characterization, including
the search for possible biosignatures, with near-future facilities such as the
James Webb Space Telescope. In this chapter, we first describe the physical
properties of UCDs as well as the unique potential they offer for the detection
of potentially habitable Earth-sized planets suitable for atmospheric
characterization. Then, we present the SPECULOOS ground-based transit survey,
that will search for Earth-sized planets transiting the nearest UCDs, as well
as its prototype survey on the TRAPPIST telescopes. We conclude by discussing
the prospects offered by the recent detection by this prototype survey of a
system of seven temperate Earth-sized planets transiting a nearby UCD,
TRAPPIST-1.Comment: Submitted as a chapter in the "Handbook of Exoplanets" (editors: H.
Deeg & J.A. Belmonte; Section Editor: N. Narita). 16 pages, 4 figure
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