3,347 research outputs found
The structure of radiative shock waves. III. The model grid for partially ionized hydrogen gas
The grid of the models of radiative shock waves propagating through partially
ionized hydrogen gas with temperature 3000K <= T_1 <= 8000K and density
10^{-12} gm/cm^3 <= \rho_1 <= 10^{-9}gm/cm^3 is computed for shock velocities
20 km/s <= U_1 <= 90 km/s. The fraction of the total energy of the shock wave
irreversibly lost due to radiation flux ranges from 0.3 to 0.8 for 20 km/s <=
U_1 <= 70 km/s. The postshock gas is compressed mostly due to radiative cooling
in the hydrogen recombination zone and final compression ratios are within 1
<\rho_N/\rho_1 \lesssim 10^2, depending mostly on the shock velocity U_1. The
preshock gas temperature affects the shock wave structure due to the
equilibrium ionization of the unperturbed hydrogen gas, since the rates of
postshock relaxation processes are very sensitive to the number density of
hydrogen ions ahead the discontinuous jump. Both the increase of the preshock
gas temperature and the decrease of the preshock gas density lead to lower
postshock compression ratios. The width of the shock wave decreases with
increasing upstream velocity while the postshock gas is still partially ionized
and increases as soon as the hydrogen is fully ionized. All shock wave models
exhibit stronger upstream radiation flux emerging from the preshock outer
boundary in comparison with downstream radiation flux emerging in the opposite
direction from the postshock outer boundary. The difference between these
fluxes depends on the shock velocity and ranges from 1% to 16% for 20 km/s <=
U_1 <= 60 km/s. The monochromatic radiation flux transported in hydrogen lines
significantly exceeds the flux of the background continuum and all shock wave
models demonstrate the hydrogen lines in emission.Comment: 11 pages, 11 figures, LaTeX, to appear in A
Envelope tomography of long-period variable stars: I. The Schwarzschild mechanism and the Balmer emission lines
This paper is the first one in a series devoted to the study of the dynamics
of the atmospheres of long-period variable stars. Results from a two-month-long
monitoring of the Mira variables RT Cyg and X Oph around maximum light with the
ELODIE spectrograph at the Haute-Provence Observatory are presented. The
monitoring covers phases 0.80 to 1.16 for RT Cyg and phases 0.83 to 1.04 for X
Oph. The cross-correlation profile of the spectrum of RT Cyg with a K0 III mask
confirms that the absorption lines of RT Cyg in the optical domain appear
double around maximum light. No line doubling was found in the optical spectrum
of X Oph around maximum light, indicating that this feature is not common to
all long-period variables. This paper also presents the application to RT Cyg
of a new tomographic technique deriving the velocity field across the
atmosphere by cross-correlating the optical spectrum with numerical masks
constructed from synthetic spectra and probing layers of increasing depths.
This technique reveals that both the temporal evolution of the line doubling,
and its variation with depth in the atmosphere of RT Cyg, are consistent with
the ``Schwarzschild scenario''. This scenario relates the temporal evolution of
the red and blue peaks of the double absorption lines to the progression of a
shock wave in the atmosphere. The temporal evolution of the Balmer Halpha, H
beta, Hgamma and Hdelta emission lines around maximum light is also presented
for RT Cyg and X Oph. The velocity variations of Halpha and of the absorption
lines are discussed in the framework of two competing models for the formation
of Balmer emission lines in long-period variable stars.Comment: 11 pages, 8 figures, Latex, accepted for publication in Astronomy and
Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/Html/ps.htm
Inventory and new records of Polychaete species from the Cap Bon peninsula , North East coast of Tunisia, Western Mediterranean Sea
An inventory of polychaete species is presented from the north-east coast of Tunisia with an historic review of the previous literature from Tunisian coasts. Altogether 40 families, 146 genera, and 238 species are currently known from the area, of which 86 taxa, 4 families (Chrysopetalidae, Pilargidae, Protodrilidae and Saccocirridae) and 40 genera (Saccocirrus, Protodrilus, Parathelepus, Thelepus, Petta, Isolda, Brada, Tharyx, Paraprionospio, Jasmineira, Hypsicomus, Euchone, Pseudobranchiomma, Laonome, Galathowenia, Lugia, Pseudomystides, Protomystides, Pirakia, Mysta, Eurysyllis, Parapionosyllis, Streptosyllis, Paraehlersia, Sigambra,
Ancistrosyllis, Kefersteinia, Chrysopetalum, Bhawania, Fimbriosthenelais, Subadyte, Panthalis, Dorvillea, Scalibregma, Paradoneis, Cirrophorus, Metasychis, Websterinereis, Euniphysa and Mastobranchus) are new additions to the polychaete fauna of Tunisia. The list, which provides a synthesis of the regional taxonomic work, including coastal areas from Sidi Daoud to the area of Menzel Hurr (Cap Bon Peninsula, Western Mediterranean Sea), can serve as a baseline for future studies
Tunneling, self-trapping and manipulation of higher modes of a BEC in a double well
We consider an atomic Bose-Einstein condensate trapped in a symmetric
one-dimensional double well potential in the four-mode approximation and show
that the semiclassical dynamics of the two ground state modes can be strongly
influenced by a macroscopic occupation of the two excited modes. In particular,
the addition of the two excited modes already unveils features related to the
effect of dissipation on the condensate. In general, we find a rich dynamics
that includes Rabi oscillations, a mixed Josephson-Rabi regime, self-trapping,
chaotic behavior, and the existence of fixed points. We investigate how the
dynamics of the atoms in the excited modes can be manipulated by controlling
the atomic populations of the ground states.Comment: 12 pages, 5 figure
Fast quasi-adiabatic dynamics
We work out the theory and applications of a fast quasi-adiabatic approach to
speed up slow adiabatic manipulations of quantum systems by driving a control
parameter as near to the adiabatic limit as possible over the entire protocol
duration. Specifically, we show that the population inversion in a two-level
system, the splitting and cotunneling of two-interacting bosons, and the
stirring of a Tonks-Girardeau gas on a ring to achieve mesoscopic
superpositions of many-body rotating and non-rotating states, can be
significantly speeded up.Comment: 5 pages, 6 figure
Recovery and reuse of discontinuous carbon fibres by solvolysis: Realignment and properties of remanufactured materials
Discontinuous carbon fibre tows were recovered after solvolysis of an aeronautic type composite made with RTM6 epoxy resin. A Sohxlet extraction method was used to quantify the organic residue on the fibre tows and showed that less than 3 wt% was remaining on the surface. The recovered tows were therefore reused directly to manufacture a plate with randomly distributed carbon fibres and then three plates with realigned carbon fibres. The latter were then characterised and tested and the results obtained were compared to the material manufactured using the same type of virgin fibres by the same method. The materials made from recycled carbon fibres showed very good properties in comparison to the virgin fibre material, despite the presence of flaws such as quality of the fibre surface after solvolysis, alignment and voids). This is the first time in the open literature that carbon fibres recovered from solvolysis were reused in this way together with characterisation of the resulting materials
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