71 research outputs found
Degassing cascades in a shear-thinning viscoelastic fluid
International audienceWe report the experimental study of the degassing dynamics through a thin layer of shear-thinning viscoelastic fluid when a constant air flow is imposed at its bottom. The fluid is an aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). Over a large range of parameters, the air is periodically released through a series of successive bubbles, hereafter named cascades. Each cascade is followed by a continuous degassing, lasting for several seconds, corresponding to an open channel crossing the fluid layer. The periodicity between two cascades does not depend on the injected flow rate. Inside one cascade, the properties of the overpressure signal associated with the successive bubbles vary continuously. The pressure threshold above which the fluid starts flowing, fluid deformation and pressure drop due to degassing through the thin fluid layer can be simply described by a Maxwell model
Path integral Monte Carlo and density functional molecular dynamics simulations of warm dense MgSiO3
In order to provide a comprehensive theoretical description of MgSiO at
extreme conditions, we combine results from path integral Monte Carlo (PIMC)
and density functional molecular dynamics simulations (DFT-MD) and generate a
consistent equation of state for this material. We consider a wide range of
temperature and density conditions from 10 to 10 K and from 0.321 to
64.2 gcm (0.1- to 20-fold the ambient density). We study how the L
and K shell electrons are ionized with increasing temperature and pressure. We
derive the shock Hugoniot curve and compare with experimental results. Our
Hugoniot curve is in good agreement with the experiments, and we predict a
broad compression maximum that is dominated by the K shell ionization of all
three nuclei while the peak compression ratio of 4.70 is obtained when the Si
and Mg nuclei are ionized. Finally we analyze the heat capacity and structural
properties of the liquid.Comment: 14 pages, 14 color figures, Physical Review B (accepted, Dec. 2019
The NLTE Barium Abundance in Dwarf Stars in the Metallicity Range of -1 < [Fe/H] < +0.3
We present the results of determination of the barium abundance considering
the non-LTE (NLTE) effects in 172 dwarf stars in the metallicity range of -1<
[Fe/H] <+0.3, assigned to different Galactic substructures by kinematic
criteria. We used a model of the Ba atom with 31 levels of Ba I and 101 levels
of Ba II. The atmosphere models for the investigated stars were computed using
the ATLAS9 code modified by new opacity distribution functions. The NLTE
profiles of the unblended Ba II (4554 A, 5853 A, 6496 A) were computed and then
compared to those observed. The line 6141 A was also used, but with an
allowance for its correlation with the iron line. The average barium abundances
in the thin and thick discs are 0.01 +/- 0.08 and -0.03 +/- 0.07, respectively.
The comparison to the calculations of the Galactic chemical evolution by
Serminato et al. (2009) was conducted. The trend obtained for the Ba abundance
versus [Fe/H] suggests a complex barium production process in the thin and
thick discs
SPADES: a Stellar PArameters DEtermination Software
With the large amounts of spectroscopic data available today and the very
large surveys to come (e.g. Gaia), the need for automatic data analysis
software is unquestionable. We thus developed an automatic spectra analysis
program for the determination of stellar parameters: radial velocity, effective
temperature, surface gravity, micro-turbulence, metallicity and the elemental
abundances of the elements present in the spectral range. Target stars for this
software should include all types of stars. The analysis method relies on a
line by line comparison of the spectrum of a target star to a library of
synthetic spectra. The idea is built on the experience acquired in developing
the TGMET (Katz et al. 1998 and Soubiran et al. 2003) ETOILE (Katz 2001) and
Abbo (Bonifacio & Caffau 2003) softwares. The method is presented and the
performances are illustrated with GIRAFFE-like simulated spectra with high
resolution (R = 25000), with high and low signal to noise ratios (down to SNR=
30). These spectra should be close to what could be targeted by the Gaia-ESO
Survey (GCDS).Comment: 5 pages, SF2A 2011 Poster Proceeding
GYES, a multifibre spectrograph for the CFHT
We have chosen the name of GYES, one of the mythological giants with one
hundred arms, offspring of Gaia and Uranus, for our instrument study of a
multifibre spectrograph for the prime focus of the Canada-France-Hawaii
Telescope. Such an instrument could provide an excellent ground-based
complement for the Gaia mission and a northern complement to the HERMES project
on the AAT. The CFHT is well known for providing a stable prime focus
environment, with a large field of view, which has hosted several imaging
instruments, but has never hosted a multifibre spectrograph. Building upon the
experience gained at GEPI with FLAMES-Giraffe and X-Shooter, we are
investigating the feasibility of a high multiplex spectrograph (about 500
fibres) over a field of view 1 degree in diameter. We are investigating an
instrument with resolution in the range 15000 to 30000, which should provide
accurate chemical abundances for stars down to 16th magnitude and radial
velocities, accurate to 1 km/s for fainter stars. The study is led by
GEPI-Observatoire de Paris with a contribution from Oxford for the study of the
positioner. The financing for the study comes from INSU CSAA and Observatoire
de Paris. The conceptual study will be delivered to CFHT for review by October
1st 2010.Comment: Contributed talk at the Gaia ELSA conference 2010, S\`evres 7-11 June
2010, to be published on the EAS Series, Editors: C. Turon, F. Arenou & F.
Meynadie
- âŠ