539 research outputs found
Numerical radiative transfer with state-of-the-art iterative methods made easy
This article presents an on-line tool (rttools.irap.omp.eu) and its
accompanying software ressources for the numerical solution of basic radiation
transfer out of local thermodynamic equilibrium (LTE). State-of-the-art
stationary iterative methods such as Accelerated -Iteration and
Gauss-Seidel schemes, using a short characteristics-based formal solver are
used. We also comment on typical numerical experiments associated to the basic
non-LTE radiation problem. These ressources are intended for the largest use
and benefit, in support to more classical radiation transfer lectures usually
given at the Master level.Comment: 8 pages, 5 figures, accepted for Eur. J. Phys. - see also (and use!)
http://rttools.irap.omp.e
Working across scales in integrated catchment management:lessons learned for adaptive water governance from regional experiences
A new catalogue of observations of the eight major satellites of Saturn (1874-2007)
The original publication in Astronomy & Astrophysics is available at www.aanda.org.International audienceContext : The lastest catalogue of observations includes about 51 000 observations (over 3500 nights) of Saturn's satellites from 1874 to 1989. Since 1989, many observations have been published, often in different formats, based on the publication. Aims : Our new catalogue of observations of the eight major satellites of Saturn includes the observations of the previous catalogues, newly published data and also old observations left out of the previous catalogue. The observations are tabulated in a consistent format. Methods : We give, for each observation, the corrections applied for reduction such as refraction, aberration or phase effects. Furthermore, when it was possible, the instrument and catalogue are also indicated. Results : The new catalogue presents more than 130 000 observations (over 6000 nights) of the eight major satellites of Saturn from 1874 to 2007
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Monocular microsaccades are visual-task related
During visual fixation, we constantly move our eyes. These microscopic eye movements are composed of tremor, drift, and microsaccades. Early studies concluded that microsaccades, like larger saccades, are binocular and conjugate, as expected from Hering's law of equal innervation. Here, we document the existence of monocular microsaccades during both fixation and a discrimination task, reporting the location of the gap in a foveal, low-contrast letter C. Monocular microsaccades differ in frequency, amplitude, and peak velocity from binocular microsaccades. Our analyses show that these differences are robust to different velocity and duration criteria that have been used previously to identify microsaccades. Also, the frequency of monocular microsaccades differs systematically according to the task: monocular microsaccades occur more frequently during fixation than discrimination, the opposite of their binocular equivalents. However, during discrimination, monocular microsaccades occur more often around the discrimination threshold, particularly for each subject's dominant eye and in case of successful discrimination. We suggest that monocular microsaccades play a functional role in the production of fine corrections of eye position and vergence during demanding visual tasks
Estimating the accuracy of satellite ephemerides using the bootstrap method
International audienceContext: The accuracy of predicted orbital positions depends on the quality of the theorical model and of the observations used to fit the model. During the period of observations, this accuracy can be estimated through comparison with observations. Outside this period, the estimation remains difficult. Many methods have been developed for asteroid ephemerides in order to evaluate this accuracy. Aims: This paper introduces a new method to estimate the accuracy of predicted positions at any time, in particular outside the observation period. Methods: This new method is based upon a bootstrap resampling and allows this estimation with minimal assumptions. Results: The method was applied to two of the main Saturnian satellites, Mimas and Titan, and compared with other methods used previously for asteroids. The bootstrap resampling is a robust and practical method for estimating the accuracy of predicted positions
It's a wonderful tail: the mass loss history of Mira
Recent observations of the Mira AB binary system have revealed a surrounding
arc-like structure and a stream of material stretching 2 degrees away in
opposition to the arc. The alignment of the proper motion vector and the
arc-like structure shows the structures to be a bow shock and accompanying
tail. We have successfully hydrodynamically modelled the bow shock and tail as
the interaction between the asymptotic giant branch (AGB) wind launched from
Mira A and the surrounding interstellar medium. Our simulations show that the
wake behind the bow shock is turbulent: this forms periodic density variations
in the tail similar to those observed. We investigate the possiblity of
mass-loss variations, but find that these have limited effect on the tail
structure. The tail is estimated to be approximately 450,000 years old, and is
moving with a velocity close to that of Mira itself. We suggest that the
duration of the high mass-loss phase on the AGB may have been underestimated.
Finally, both the tail curvature and the rebrightening at large distance can be
qualitatively understood if Mira recently entered the Local Bubble. This is
estimated to have occured 17 pc downstream from its current location.Comment: 12 pages, 3 colour figures, accepted by ApJ Part II (Letters
Radiative hydrodynamics simulations of red supergiant stars: II. simulations of convection on Betelgeuse match interferometric observations
Context. The red supergiant (RSG) Betelgeuse is an irregular variable star.
Convection may play an important role in understanding this variability.
Interferometric observations can be interpreted using sophisticated simulations
of stellar convection. Aims. We compare the visibility curves and closure
phases obtained from our 3D simulation of RSG convection with CO5BOLD to
various interferometric observations of Betelgeuse from the optical to the H
band in order to characterize and measure the convection pattern on this star.
Methods. We use 3D radiative-hydrodynamics (RHD) simulation to compute
intensity maps in different filters and we thus derive interferometric
observables using the post-processing radiative transfer code OPTIM3D. The
synthetic visibility curves and closure phases are compared to observations.
Results. We provide a robust detection of the granulation pattern on the
surface of Betelgeuse in the optical and in the H band based on excellent fits
to the observed visibility points and closure phases. Moreover, we determine
that the Betelgeuse surface in the H band is covered by small to medium scale
(5-15 mas) convection-related surface structures and a large (30 mas)
convective cell. In this spectral region, H2O molecules are the main absorbers
and contribute to the small structures and to the position of the first null of
the visibility curve (i.e. the apparent stellar radius).Comment: 11 pages, Accepted for publication on A&
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